Pinnacle West Capital (PNW) 424B5Prospectus supplement for primary offering

On June 22, 2017, the Alternative Reference Rates Committee (“ARRC”) convened by the Board of Governors of the Federal Reserve System and the Federal Reserve Bank of New York identified SOFR as the rate that, in the consensus view of the ARRC, represented best practice for use in certain new U.S. dollar derivatives and other financial contracts. SOFR is a broad measure of the cost of borrowing cash overnight collateralized by U.S. Treasury securities, and SOFR has been published by the Federal Reserve Bank of New York since April 2018. The Federal Reserve Bank of New York has also begun publishing historical indicative Secured Overnight Financing Rates from 2014. Investors should not rely on any historical changes or trends in SOFR as an indicator of future changes in SOFR.

The composition and characteristics of SOFR are not the same as those of LIBOR, and SOFR is fundamentally different from LIBOR for two key reasons. First, SOFR is a secured rate, while LIBOR is an unsecured rate. Second, SOFR is an overnight rate, while LIBOR is a forward-looking rate that represents interbank funding over different maturities (e.g., three months). As a result, there can be no assurance that SOFR (including Compounded SOFR) will perform in the same way as LIBOR would have at any time, including, without limitation, as a result of changes in interest and yield rates in the market, market volatility or global or regional economic, financial, political, regulatory, judicial or other events.

The publication of SOFR began in April 2018, and, therefore, it has a very limited history. The future performance of SOFR cannot be predicted based on the limited historical performance. Levels of SOFR going forward may bear little or no relation to the historical actual or historical indicative data. Prior observed patterns, if any, in the behavior of market variables and their relation to SOFR, such as correlations, may change in the future. While some pre-publication historical data have been released by the Federal Reserve Bank of New York, such analysis inherently involves assumptions, estimates and approximations. Because the future performance of SOFR cannot be predicted, no future performance of SOFR may be inferred from any of the historical actual or historical indicative data. Hypothetical or historical performance data are not indicative of, and have no bearing on, the potential performance of SOFR. There can be no assurance that SOFR will be positive.

Since the initial publication of SOFR, daily changes in SOFR have, on occasion, been more volatile than daily changes in other benchmark or market rates, such as U.S. dollar LIBOR. Although changes in Compounded SOFR generally are not expected to be as volatile as changes in daily levels of SOFR, the return on and value of the notes may fluctuate more than floating rate debt securities that are linked to less volatile rates. In addition, the volatility of SOFR has reflected the underlying volatility of the overnight U.S. Treasury repo market. The Federal Reserve Bank of New York has at times conducted operations in the overnight U.S. Treasury repo market in order to help maintain the federal funds rate within a target range. There can be no assurance that the Federal Reserve Bank of New York will continue to conduct such operations in the future, and the duration and extent of any such operations is inherently uncertain. The effect of any such operations, or of the cessation of such operations to the extent they are commenced, is uncertain and could be materially adverse to investors in the notes.

According to the ARRC, SOFR was developed for use in certain U.S. dollar derivatives and other financial contracts as an alternative to U.S. dollar LIBOR in part because it is considered a good representation of general funding conditions in the overnight U.S. Treasury repurchase agreement market.

However, as a rate based on transactions secured by U.S. Treasury securities, it does not measure bank-specific credit risk and, as a result, is less likely to correlate with the unsecured short-term funding costs of banks. This may mean that market participants would not consider SOFR a suitable replacement or successor for all of the purposes for which U.S. dollar LIBOR historically has been used (including, without limitation, as a representation of the unsecured short-term funding costs of banks), which may, in turn, lessen market acceptance of SOFR. Any failure of SOFR to gain market acceptance could adversely affect the return on and value of the notes and the price at which investors can sell the notes in the secondary market.

In addition, if SOFR does not prove to be widely used as a benchmark in securities that are similar or comparable to the notes, the trading price of the notes may be lower than those of securities that are linked to rates that are more widely used. Similarly, market terms for floating rate debt securities linked to SOFR, such as the spread over the base rate reflected in interest rate provisions or the manner of compounding the base rate, may evolve over time, and trading prices of the notes may be lower than those of later-issued SOFR-based debt securities as a result. Investors in the notes may not be able to sell the notes at all or may not be able to sell the notes at prices that will provide them with a yield comparable to similar investments that have a developed secondary market and may consequently suffer from increased pricing volatility and market risk.

For each interest period, the interest rate on the notes is based on Compounded SOFR, which is calculated using the SOFR Index (as defined in this prospectus supplement) published by the Federal Reserve Bank of New York according to the specific formula described under “Description of the Notes — Interest — Compounded SOFR,” not the SOFR rate published on or in respect of a particular date during such interest period or an arithmetic average of SOFR rates during such period. For this and other reasons, the interest rate on the notes during any interest period will not necessarily be the same as the interest rate on other SOFR-linked investments that use an alternative basis to determine the applicable interest rate. Further, if the SOFR rate in respect of a particular date during an interest period is negative, its contribution to the SOFR Index will be less than one, resulting in a reduction to Compounded SOFR used to calculate the interest payable on the notes on the Interest Payment Date for such interest period.

Very limited market precedent exists for securities that use SOFR as the interest rate, and the method for calculating an interest rate based upon SOFR in those precedents varies. In addition, the Federal Reserve Bank of New York only began publishing the SOFR Index on March 2, 2020. Accordingly, the use of the SOFR Index or the specific formula for the Compounded SOFR rate used in the notes may not be widely adopted by other market participants, if at all. If the market adopts a different calculation method, that would likely adversely affect the liquidity and market value of the notes.

The level of Compounded SOFR applicable to a particular interest period and, therefore, the amount of interest payable with respect to such interest period will be determined on the Interest Payment Determination Date for such interest period. Because each such date is near the end of such interest period, you will not know the amount of interest payable with respect to a particular interest period until shortly prior to the related Interest Payment Date, and it may be difficult for you to reliably estimate the amount of interest that will be payable on each such Interest Payment Date. In addition, some investors may be unwilling or unable to trade the notes without changes to their information technology systems. An inability to reliably estimate accrued and unpaid interest as well as potential need for some investors to change their information technology systems could both adversely impact the liquidity and trading price of the notes.

The SOFR Index is published by the Federal Reserve Bank of New York based on data received by it from sources other than us, and we have no control over its methods of calculation, publication schedule, rate revision practices or availability of the SOFR Index at any time. There can be no guarantee, particularly

given its relatively recent introduction, that the SOFR Index will not be discontinued or fundamentally altered in a manner that is materially adverse to the interests of investors in the notes. If the manner in which the SOFR Index is calculated, including the manner in which SOFR is calculated, is changed, that change may result in a reduction in the amount of interest payable on the notes and the trading prices of the notes. In addition, the Federal Reserve Bank of New York may withdraw, modify or amend the published SOFR Index or SOFR data in its sole discretion and without notice. The interest rate for any interest period will not be adjusted for any modifications or amendments to the SOFR Index or SOFR data that the SOFR Administrator (as defined below) may publish after the interest rate for that interest period has been determined.

If we (or our designee (which may be an independent financial advisor or any other designee of ours (any of such entities, a “Designee”)) determines that a Benchmark Transition Event and its related Benchmark Replacement Date (each as defined in this prospectus supplement) have occurred in respect of the SOFR Index, then the interest rate on the notes will no longer be determined by reference to the SOFR Index, but instead will be determined by reference to a different rate, plus a spread adjustment, which is referred to as a “Benchmark Replacement”, as further described under the caption “Description of the Notes — Interest.”

If a particular Benchmark Replacement or Benchmark Replacement Adjustment (as defined in this prospectus supplement) cannot be determined, then the next-available Benchmark Replacement or Benchmark Replacement Adjustment will apply. These replacement rates and adjustments may be selected, recommended or formulated by (i) the Relevant Governmental Body (as defined in this prospectus supplement) (such as the ARRC), (ii) the International Swaps and Derivatives Association (“ISDA”) or (iii) in certain circumstances, us (or our Designee). In addition, the terms of the notes expressly authorize us (or our Designee) to make Benchmark Replacement Conforming Changes (as defined in this prospectus supplement) with respect to, among other things, changes to the definition of “interest period”, the timing and frequency of determining rates and making payments of interest, rounding of amounts or tenors, and other administrative matters. The determination of a Benchmark Replacement, the calculation of the interest rate on the notes by reference to a Benchmark Replacement (including the application of a Benchmark Replacement Adjustment), any implementation of Benchmark Replacement Conforming Changes and any other determinations, decisions or elections that may be made under the terms of the notes in connection with a Benchmark Transition Event could adversely affect the value of the notes, the return on the notes and the price at which you can sell such notes.

The terms of the notes are set forth below:

The notes will mature on       , 2026. The notes are initially limited to $      in aggregate principal amount. However, we may issue additional notes of the same series from time to time, without the consent of the holders of the notes.

The notes will bear interest from and including June      , 2024 to, but excluding, the maturity date at a floating rate determined in the manner described below, payable quarterly in arrears on       ,       ,      and        of each year (each, an “Interest Payment Date”), commencing on     , 2024. The notes will bear interest at an annual rate equal to Compounded SOFR, determined as described below, plus      basis points (the “Margin”).

The record date for interest payable on any Interest Payment Date shall be the close of business on (1) the business day immediately preceding such Interest Payment Date so long as all of the notes remain in book-entry only form or (2) the 15th calendar day immediately preceding such Interest Payment Date if any of the notes do not remain in book-entry only form. See “Description of Pinnacle West Unsecured Debt Securities — Global Securities” in the accompanying prospectus. Interest on the notes will accrue from and including the date of original issuance to but excluding the first Interest Payment Date. Starting on the first Interest Payment Date, interest on the notes will accrue from and including the last Interest Payment Date to which we have paid, or duly provided for the payment of, interest on the notes to but excluding the next succeeding Interest Payment Date. No interest will accrue on the notes for the day that the notes mature. The amount of interest payable for any period will be computed on the basis of a 360-day year and the actual number of days in the Observation Period (as defined below).

If any Interest Payment Date falls on a day that is not a business day, we will make the interest payment on the next succeeding business day unless that business day is in the next succeeding calendar month, in which case (other than in the case of the maturity date) we will make the interest payment on the immediately preceding business day. If an interest payment is made on the next succeeding business day, no interest will accrue as a result of the delay in payment. If the maturity date of the notes falls on a day that is not a business day, the payment due on such date will be postponed to the next succeeding business day, and no further interest will accrue in respect of such postponement.

As further described in this prospectus supplement, on each Interest Payment Determination Date relating to the applicable Interest Payment Date, the calculation agent (as defined below) will calculate the amount of accrued interest payable on the notes by multiplying (i) the outstanding principal amount of the notes by (ii) the product of (a) the interest rate for the relevant interest period multiplied by (b) the quotient of the actual number of calendar days in such Observation Period divided by 360. In no event will the interest on the notes be less than zero.

The term “interest period”, with respect to the notes, means the period commencing on any Interest Payment Date (or, with respect to the initial interest period only, commencing on the date of original issuance) to, but excluding, the next succeeding Interest Payment Date, and, in the case of the last such period, from and including the Interest Payment Date immediately preceding the maturity date to but excluding the maturity date.

The Secured Overnight Financing Rate (SOFR) is published by the Federal Reserve Bank of New York and is intended to be a broad measure of the cost of borrowing cash overnight collateralized by U.S. Treasury securities.

The SOFR Index is published by the Federal Reserve Bank of New York and measures the cumulative impact of compounding SOFR on a unit of investment over time, with the initial value set to 1.00000000 on April 2, 2018, the first value date of SOFR. The SOFR Index value reflects the effect of compounding SOFR each business day and allows the calculation of compounded SOFR averages over custom time periods.

The Federal Reserve Bank of New York notes on its publication page for the SOFR Index that use of the SOFR Index is subject to important limitations, indemnification obligations and disclaimers, including

that the Federal Reserve Bank of New York may alter the methods of calculation, publication schedule, rate revision practices or availability of the SOFR Index at any time without notice. The interest rate for any interest period will not be adjusted for any modifications or amendments to the SOFR Index or SOFR data that the SOFR Administrator may publish after the interest rate for that interest period has been determined.

“Compounded SOFR” will be determined by the calculation agent in accordance with the following formula (and the resulting percentage will be rounded, if necessary, to the nearest one hundred-thousandth of a percentage point):

“SOFR IndexStart” = For periods other than the initial interest period, the SOFR Index value on the preceding Interest Payment Determination Date, and, for the initial interest period, the SOFR Index value two U.S. Government Securities Business Days (as defined below) before the date of original issuance;

“SOFR IndexEnd” = The SOFR Index value on the Interest Payment Determination Date relating to the applicable Interest Payment Date (or, in the final interest period, relating to the maturity date); and

“dc” is the number of calendar days in the relevant Observation Period.

For purposes of determining Compounded SOFR:

“Interest Payment Determination Date” means the date two U.S. Government Securities Business Days before each Interest Payment Date.

“Observation Period” means, in respect of each interest period, the period from, and including, the date that is two U.S. Government Securities Business Days preceding the first date in such interest period to, but excluding, the date that is two U.S. Government Securities Business Days preceding the Interest Payment Date for such interest period (or in the final interest period, preceding the maturity date).

“SOFR Index” means, with respect to any U.S. Government Securities Business Day:

“SOFR” means the daily secured overnight financing rate as provided by the SOFR Administrator on the SOFR Administrator’s Website.

“SOFR Administrator” means the Federal Reserve Bank of New York (or a successor administrator of SOFR).

“SOFR Administrator’s Website” means the website of the Federal Reserve Bank of New York, currently at http://www.newyorkfed.org, or any successor source.

“U.S. Government Securities Business Day” means any day except for a Saturday, a Sunday or a day on which the Securities Industry and Financial Markets Association recommends that the fixed income departments of its members be closed for the entire day for purposes of trading in U.S. government securities.

“Benchmark” means, initially, Compounded SOFR, as such term is defined above; provided that if a Benchmark Transition Event and its related Benchmark Replacement Date have occurred with respect to Compounded SOFR (or the published SOFR Index used in the calculation thereof) or the then-current Benchmark, then “Benchmark” means the applicable Benchmark Replacement.

“Benchmark Replacement” means the first alternative set forth in the order below that can be determined by us (or our Designee) as of the Benchmark Replacement Date:

“Benchmark Replacement Adjustment” means the first alternative set forth in the order below that can be determined by us (or our Designee) as of the Benchmark Replacement Date:

The Benchmark Replacement Adjustment shall not include the Margin specified in this prospectus supplement and such Margin shall be applied to the Benchmark Replacement to determine the interest payable on the notes.

“Benchmark Replacement Conforming Changes” means, with respect to any Benchmark Replacement, any technical, administrative or operational changes (including changes to the definition or interpretation of “interest period”, timing and frequency of determining rates and making payments of interest, rounding of amounts or tenor, and other administrative matters), or any other changes to any other terms or provisions of the notes, in each case that we (or our Designee) decide may be appropriate to reflect the adoption of such Benchmark Replacement in a manner substantially consistent with market practice (or, if we (or our Designee) decide that adoption of any portion of such market practice is not administratively feasible or if we (or our Designee) determine that no market practice for use of the Benchmark Replacement exists, in such other manner as we (or our Designee) determine is reasonably necessary or practicable).

“Benchmark Replacement Date” means the earliest to occur of the following events with respect to the then-current Benchmark:

None of the trustee, paying agent, registrar or calculation agent shall be under any obligation (i) to monitor, determine or verify the unavailability or cessation of SOFR or the SOFR Index, or whether or when there has occurred, or to give notice to any other transaction party of the occurrence of, any Benchmark Transition Event or related Benchmark Replacement Date, (ii) to select, determine or designate any Benchmark Replacement, or other successor or replacement benchmark index, or whether any conditions to the designation of such a rate or index have been satisfied, (iii) to select, determine or designate any Benchmark Replacement Adjustment, or other modifier to any replacement or successor index, or (iv) to determine whether or what Benchmark Replacement Conforming Changes are necessary or advisable, if any, in connection with any of the foregoing, including, but not limited to, adjustments as to any alternative spread thereon, the business day convention, interest determination dates or any other relevant methodology applicable to such substitute or successor benchmark. In connection with the foregoing, each of the trustee, the paying agent, the registrar and the calculation agent shall be entitled to conclusively rely on any determinations made by us or our Designee without independent investigation, and none will have any liability for actions taken at our direction in connection therewith.

None of the trustee, paying agent, registrar or calculation agent shall be liable for any inability, failure or delay on its part to perform any of its duties described in this prospectus supplement and the accompanying prospectus as a result of the unavailability of SOFR, the SOFR Index or other applicable Benchmark Replacement, including as a result of any failure, inability, delay, error or inaccuracy on the part of any other transaction party in providing any direction, instruction, notice or information contemplated by this prospectus supplement and the accompanying prospectus and reasonably required for the performance of such duties. None of the trustee, the paying agent, the registrar or the calculation agent shall be responsible or liable for our actions or omissions or for those of our Designee, or for any failure or delay in the performance by such Designee, nor shall any of the trustee, the paying agent, the registrar or the calculation agent be under any obligation to oversee or monitor our performance or that of such Designee.

We may from time to time, without notice to, or the consent of, the then existing registered holders of the notes, create and issue additional notes equal in rank and having the same maturity, payment terms and other terms as the notes, except for the issue date of the additional notes, the public offering price of the additional notes, the payment of interest accruing prior to the issue date of the additional notes and (under some circumstances) the first payment of interest following the issue date of the additional notes, but we will not issue such additional notes unless the additional notes are fungible with the previously issued notes for U.S. federal income tax purposes or are issued with a separate CUSIP number. These additional notes may be consolidated and form a single series with the notes.

The provisions described in the accompanying prospectus under the caption “Description of Pinnacle West Unsecured Debt Securities — Defeasance and Covenant Defeasance” are applicable to the notes.

So long as any of the notes are outstanding, we will not, directly or indirectly (including through a Subsidiary), create, incur, assume or permit to exist any lien, pledge or security interest on any of the capital stock of APS.

“Subsidiary” means a corporation or any other entity more than 50% of the outstanding voting stock of which is owned, directly or indirectly, by Pinnacle West Capital Corporation or by one or more other Subsidiaries, or by Pinnacle West Capital Corporation and one or more other Subsidiaries. For the purposes of this definition, “voting stock” means stock which ordinarily has voting power for the election of directors, whether at all times or only so long as no senior class of stock has such voting power by reason of any contingency.

The following is a summary of certain material U.S. federal income tax consequences of the purchase, ownership and disposition of the notes. Except where noted, this summary deals only with notes held as capital assets by beneficial owners of the notes who purchase notes in this offering at their issue price, which is the first price at which a substantial amount of the notes is sold to investors, excluding sales to the underwriter or to similar persons acting in the capacity of placement agents or wholesalers. This summary is based upon the provisions of the Internal Revenue Code of 1986, as amended (the “Code”), the Treasury Regulations promulgated thereunder and judicial and administrative rulings and decisions now in effect, all of which are subject to change or differing interpretations, possibly with retroactive effect. This summary does not purport to address all aspects of U.S. federal income taxation that may affect particular investors in light of their individual circumstances (including holders that are directly or indirectly related to us), or certain types of investors subject to special treatment under the U.S. federal income tax laws, such as persons that mark to market their securities, accrual method taxpayers subject to special tax accounting rules as a result of their use of financial statements, entities that are members of the same “expanded group” (as defined in the Treasury Regulations under Section 385 of the Code) as us, financial institutions, regulated investment companies, real estate investment trusts, corporations subject to the accumulated earnings tax, holders subject to the alternative minimum tax, individual retirement and other tax-deferred accounts, tax-exempt organizations, brokers, dealers in securities and commodities, certain former U.S. citizens or long-term residents, life insurance companies, persons that hold notes as part of a hedge against currency or interest rate risks or that hold notes as part of a position in a constructive sale, straddle, conversion transaction or other integrated transaction for U.S. federal income tax purposes, controlled foreign corporations, passive foreign investment companies, persons that acquire their notes in connection with employment or other performance of personal services, partnerships or other pass-through entities and investors in such entities, subsequent purchasers of the notes and U.S. holders (as defined below) whose “functional currency” is not the U.S. dollar or that hold notes through a non-U.S. broker or other non-U.S. intermediary. This summary does not address any aspect of state, local or foreign taxation or any U.S. federal tax other than the income tax.

For purposes of this summary, a “U.S. holder” is a beneficial owner of a note that is, for U.S. federal income tax purposes:

For purposes of this summary, a “non-U.S. holder” is a beneficial owner of a note that is not a U.S. holder or a partnership (including an entity or arrangement treated as a partnership for U.S. federal income tax purposes).

If a partnership (including an entity or arrangement treated as a partnership for U.S. federal income tax purposes) is a beneficial owner of notes, the tax treatment of a partner will generally depend upon the status of the partner and the activities of the partnership. Partnerships that hold notes (and partners in such partnerships) should consult their tax advisors.

We intend to treat the notes as “variable rate debt instruments” ​(“VRDIs”) for United States federal income tax purposes. However, our determination is not binding on the U.S. Internal Revenue Service (the “IRS”). If the IRS takes the position that the notes are instead treated as “contingent payment debt instruments” for United States federal income tax purposes, and if such position is sustained, the tax consequences of purchase, ownership and disposition of the notes may differ materially from those described below. In such circumstances, holders may be required to accrue ordinary interest income at a higher rate than the stated interest rate on the notes and to treat as ordinary income rather than capital gain any gain realized on the taxable disposition of the notes. Holders should consult their own tax advisors regarding the

Subject to the discussions below concerning backup withholding and FATCA (as defined below), payments of interest on a note to a non-U.S. holder generally will not be subject to U.S. federal income or withholding tax, as long as the non-U.S. holder:

does not conduct a trade or business in the United States with respect to which the interest is effectively connected;

does not actually, indirectly or constructively own 10% or more of the total combined voting power of all classes of our stock entitled to vote, within the meaning of Section 871(h)(3) of the Code;

is not a “controlled foreign corporation” with respect to which we are a “related person” within the meaning of Section 881(c)(3)(C) of the Code;

is not a bank whose receipt of the interest is described in Section 881(c)(3)(A) of the Code; and

satisfies the certification requirements described below.

The certification requirements will be satisfied if either (a) the beneficial owner of the note timely certifies, under penalties of perjury, to us or to the person who otherwise would be required to withhold U.S. federal income tax that such owner is a non-U.S. holder and provides its name and address or (b) a custodian, broker, nominee or other intermediary acting as an agent for the beneficial owner (such as a securities clearing organization, bank or other financial institution that holds customers’ securities in the ordinary course of its trade or business) that holds the note in such capacity timely certifies, under penalties of perjury, to the applicable withholding agent that such statement has been received from the beneficial owner of the note by such intermediary, or by any other financial institution between such intermediary and the beneficial owner, and furnishes to the applicable withholding agent a copy thereof. In general, the foregoing certification may be provided on a properly completed IRS Form W-8BEN, IRS Form W-8BEN-E or IRS Form W-8IMY, as applicable.

A non-U.S. holder that is not exempt from tax under the foregoing rules generally will be subject to U.S. federal income tax withholding on payments of interest at a rate of 30% unless:

the interest is effectively connected with a U.S. trade or business conducted by such holder (and, if an applicable income tax treaty so provides, is attributable to a permanent establishment maintained in the United States by the non-U.S. holder), in which case the non-U.S. holder will be subject to U.S. federal income tax on a net basis; or

an applicable income tax treaty provides for a lower rate of, or exemption from, withholding tax.

A non-U.S. holder that is treated as a corporation for U.S. federal income tax purposes and has effectively connected interest income (as described in the first bullet point above) may also, under certain circumstances, be subject to an additional “branch profits tax,” which is generally imposed on a foreign corporation on the deemed repatriation from the United States of effectively connected earnings and profits, at a 30% rate, unless the rate is reduced or the tax is eliminated by an applicable income tax treaty.

To claim the benefit of a lower rate of, or exemption from, withholding tax pursuant to an income tax treaty or to claim exemption from withholding because income is effectively connected with a U.S. trade or business, the non-U.S. holder must timely provide the appropriate, properly executed IRS forms to the applicable withholding agent. Certification to claim income is effectively connected with a U.S. trade or business is generally made on IRS Form W-8ECI. Certification to claim the benefit of a lower rate of, or exemption from, withholding tax pursuant to an income tax treaty is generally made on IRS Form W-8BEN or IRS Form W-8BEN-E.

In general, non-U.S. holders will be required to periodically update their IRS W-8 forms.

Subject to the discussions below concerning backup withholding and FATCA, a non-U.S. holder generally will not be subject to U.S. federal income tax on any gain realized on the sale, exchange, redemption

The notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the European Economic Area (“EEA”). For these purposes, a retail investor means a person who is one (or more) of: (i) a retail client as defined in point (11) of Article 4(1) of Directive 2014/65/EU (as amended, “MiFID II”); (ii) a customer within the meaning of Directive (EU) 2016/97 (as amended), where that customer would not qualify as a professional client as defined in point (10) of Article 4(1) of MiFID II; or (iii) not a qualified investor as defined in Regulation (EU) 2017/1129 (as amended, the “Prospectus Regulation”). Consequently no key information document required by Regulation (EU) No 1286/2014 (as amended, the “PRIIPs Regulation”) for offering or selling the notes or otherwise making them available to retail investors in the EEA has been prepared and therefore offering or selling the notes or otherwise making them available to any retail investor in the EEA may be unlawful under the PRIIPs Regulation. This prospectus supplement and the accompanying prospectus have been prepared on the basis that any offer of notes in any member state of the EEA will be made pursuant to an exemption under the Prospectus Regulation from the requirement to publish a prospectus for offers of notes. This prospectus supplement and the accompanying prospectus are not a prospectus for the purposes of the Prospectus Regulation.

In connection with the offering, the underwriter is not acting for anyone other than us and will not be responsible to anyone other than us for providing the protections afforded to its clients nor for providing advice in relation to this offering.

The above selling restriction is in addition to any other selling restrictions set out below.

The notes are not intended to be offered, sold or otherwise made available to and should not be offered, sold or otherwise made available to any retail investor in the United Kingdom (“UK”). For these purposes, a retail investor means a person who is one (or more) of (i) a retail client, as defined in point (8) of Article 2 of Regulation (EU) 2017/565 as it forms part of domestic law by virtue of the European Union (Withdrawal) Act 2018 (as amended, the “EUWA”); (ii) a customer within the meaning of the provisions of the Financial Services and Markets Act 2000 (as amended, the “FSMA”) and any rules or regulations made under the FSMA to implement Directive (EU) 2016/97 (as amended), where that customer would not qualify as a professional client, as defined in point (8) of Article 2(1) of Regulation (EU) No 600/2014 as it forms part of domestic law by virtue of the EUWA; or (iii) not a qualified investor as defined in Article 2 of Regulation (EU) 2017/1129 (as amended) as it forms part of domestic law by virtue of the EUWA (the “UK Prospectus Regulation”). Consequently no key information document required by Regulation (EU) No 1286/2014 (as amended) as it forms part of domestic law by virtue of the EUWA (the “UK PRIIPs Regulation”) for offering or selling the notes or otherwise making them available to retail investors in the UK has been prepared and therefore offering or selling the notes or otherwise making them available to any retail investor in the UK may be unlawful under the UK PRIIPs Regulation. This prospectus supplement and the accompanying prospectus have been prepared on the basis that any offer of notes in the UK will be made pursuant to an exemption under the UK Prospectus Regulation and the FSMA from the requirement to publish a prospectus for offers of notes. This prospectus supplement and the accompanying prospectus are not a prospectus for the purposes of the UK Prospectus Regulation or the FSMA.

In connection with the offering, the underwriter is not acting for anyone other than us and will not be responsible to anyone other than us for providing the protections afforded to its clients nor for providing advice in relation to this offering.

This prospectus supplement and the accompanying prospectus are for distribution only to persons who (i) have professional experience in matters relating to investments and who qualify as investment professionals within the meaning of Article 19(5) of the Financial Services and Markets Act 2000 (Financial Promotion) Order 2005 (as amended, the “Financial Promotion Order”), (ii) are persons falling within Article 49(2)(a) to (d) (“high net worth companies, unincorporated associations etc.”) of the Financial Promotion Order, (iii) are outside the United Kingdom, or (iv) are persons to whom an invitation or inducement to engage in investment activity (within the meaning of section 21 of the FSMA) in connection with the issue or sale of any securities may otherwise lawfully be communicated or caused to be communicated (all such persons

together being referred to as “relevant persons”). This prospectus supplement and the accompanying prospectus are directed only at relevant persons and must not be acted on or relied on by persons who are not relevant persons. Any investment or investment activity to which this prospectus supplement and the accompanying prospectus relates is available only to relevant persons and will be engaged in only with relevant persons.

The underwriter: (i) has only communicated or caused to be communicated and will only communicate or cause to be communicated an invitation or inducement to engage in investment activity (within the meaning of section 21 of the FSMA) received by it in connection with the issue or sale of the notes in circumstances in which section 21(1) of the FSMA does not apply to it; and (ii) has complied and will comply with all applicable provisions of the FSMA with respect to anything done by it in relation to the notes in, from or otherwise involving the United Kingdom.

The notes may not be publicly offered, directly or indirectly, in Switzerland within the meaning of the Swiss Financial Services Act (“FinSA”), and no application has or will be made to admit the notes to trading on any trading venue (exchange or multilateral trading facility) in Switzerland. Neither this prospectus supplement and the accompanying prospectus nor any other offering or marketing material relating to the notes constitutes a prospectus pursuant to the FinSA, and neither this prospectus supplement and the accompanying prospectus nor any other offering or marketing material relating to the notes may be publicly distributed or otherwise made publicly available in Switzerland.

The underwriter (i) has not offered or sold and will not offer or sell in Hong Kong, by means of any document, any notes other than (a) to “professional investors” as defined in the Securities and Futures Ordinance (Cap. 571) of Hong Kong (the “SFO”) and any rules made thereunder or (b) in other circumstances that do not result in the document being a “prospectus” as defined in the Companies (Winding Up and Miscellaneous Provisions) Ordinance (Cap. 32) of Hong Kong or that do not constitute an offer to the public within the meaning thereof and (ii) has not issued or had in its possession for the purposes of issue, and will not issue or have in its possession for the purposes of issue, whether in Hong Kong or elsewhere, any advertisement, invitation or document relating to the notes, which is directed at, or the contents of which are likely to be accessed or read by, the public of Hong Kong (except if permitted to do so under the securities laws of Hong Kong) other than with respect to the notes that are or are intended to be disposed of only to persons outside Hong Kong or only to “professional investors” as defined in the SFO and any rules made thereunder.

The notes have not been and will not be registered under the Financial Instruments and Exchange Law of Japan (Law No. 25 of 1948, as amended) and, accordingly, will not be offered or sold, directly or indirectly, in Japan, or for the benefit of any Japanese Person or to others for re-offering or resale, directly or indirectly, in Japan or to any Japanese Person, except in compliance with all applicable laws, regulations and ministerial guidelines promulgated by relevant Japanese governmental or regulatory authorities in effect at the relevant time. For the purposes of this paragraph, “Japanese Person” shall mean any person resident in Japan, including any corporation or other entity organized under the laws of Japan.

The underwriter has acknowledged that this prospectus supplement and the accompanying prospectus have not been registered as a prospectus with the Monetary Authority of Singapore. Accordingly, the underwriter has represented, warranted and agreed that it has not offered or sold any notes or caused the notes to be made the subject of an invitation for subscription or purchase and will not offer or sell any notes or cause the notes to be made the subject of an invitation for subscription or purchase, and has not circulated or distributed, nor will it circulate or distribute, this prospectus supplement and the accompanying prospectus or any other document or material in connection with the offer or sale, or invitation for subscription or purchase, of the notes, whether directly or indirectly, to any person in Singapore other than

The following description highlights the general terms of the unsecured debt securities that Pinnacle West may offer. In this description, we will refer to the unsecured debt securities as “debt securities.” When we use the terms “we,” “us,” “our,” and like terms in this description, we are referring to Pinnacle West. When we offer debt securities in the future, the prospectus supplement will explain the particular terms of those securities and the extent to which any of these general provisions will not apply.

We can issue an unlimited amount of debt securities under the indentures listed below. We can issue debt securities from time to time and in one or more series as determined by us. In addition, we can issue debt securities of any series with terms different from the terms of debt securities of any other series and the terms of particular debt securities within any series may differ from each other, all without the consent of the holders of previously issued series of debt securities. If specified in a prospectus supplement relating to an offering of debt securities, from time to time, without notice to, or the consent of, the existing holders of any series of debt securities then outstanding, we may create and issue additional debt securities equal in rank and having the same maturity, payment terms, redemption features, and other terms as the debt securities of such series, except for the issue date of the additional debt securities, the public offering price of the additional debt securities, the payment of interest accruing prior to the issue date of the additional debt securities and (under some circumstances) the first payment of interest following the issue date of the additional debt securities. The additional debt securities may be consolidated and form a single series with previously issued debt securities of the affected series.

The debt securities will be our direct, unsecured obligations. The debt securities may be issued in one or more series under:

Because we are structured as a holding company, all existing and future indebtedness and other liabilities of our subsidiaries will be effectively senior in right of payment to our debt securities, whether senior debt securities or subordinated debt securities. Neither of the above Indentures limits our ability or the ability of our subsidiaries to incur additional indebtedness in the future. The assets and cash flows of our subsidiaries will be available, in the first instance, to service their own debt and other obligations and our ability to have the benefit of their assets and cash flows, particularly in the case of any insolvency or financial distress affecting our subsidiaries, would arise only through our equity ownership interests in our subsidiaries and only after their creditors had been satisfied.

We have summarized the material provisions of the Indentures below. We have filed the senior and subordinated Indentures as exhibits to the registration statement. You should read the Indentures in their entirety, including the definitions, together with this prospectus and the prospectus supplement before you make any investment decision in our debt securities. Although separate Indentures are used for subordinated debt securities and senior debt securities, references to the “Indenture” and the description of the “Indenture” in this section apply to both Indentures, unless otherwise noted.

You should refer to the prospectus supplement used in connection with the offering of any debt securities for information about a series of debt securities, including:

any index or formula from which the amount of principal or any premium or interest may be determined;

any allowance for alternative currencies and determination of value;

whether the debt securities are defeasible under the terms of the Indenture;

whether we are issuing the debt securities as global securities;

any additional or different events of default and any change in the right of the trustee or the holders to declare the principal amount due and payable if there is any default;

any addition to or change in the covenants in the Indenture; and

any other terms.

We may sell the debt securities at a substantial discount below their principal amount. The prospectus supplement may describe special federal income tax considerations that apply to debt securities sold at an original issue discount or to debt securities that are denominated in a currency other than United States dollars.

Unless the applicable prospectus supplement specifies otherwise, we do not intend to list the debt securities on any securities exchange.

Other than the protections described in this prospectus and in the prospectus supplement, holders of debt securities would not be protected by the covenants in the Indenture from a highly-leveraged transaction.

The Indenture relating to the subordinated debt securities states that, unless otherwise provided in a supplemental indenture or a board resolution or officers’ certificate establishing a series of debt securities, the debt securities will be subordinate to all senior debt. This is true whether the senior debt is outstanding as of the date of the Indenture or is incurred afterwards. The balance of the information under this heading assumes that a supplemental indenture or a board resolution results in a series of debt securities being subordinated obligations.

The Indenture states that we cannot make payments of principal, premium, or interest on the subordinated debt if:

the maturity of any senior debt has been accelerated because of a default.

The Indenture provides that we must pay all senior debt in full before the holders of the subordinated debt securities may receive or retain any payment if we make any payment to our creditors or our assets are distributed to our creditors, with certain exceptions, upon any of the following:

winding up;

liquidation;

The Indenture does not limit the aggregate amount of senior debt that we may issue.

Unless otherwise indicated in the applicable prospectus supplement, we may not:

The limitations provided above do not apply to a suit instituted by a holder of a debt security for the enforcement of payment of the principal, premium, or interest on the debt security on or after the applicable due date.

We are required to furnish to the trustee annually a certificate of various officers stating whether or not we are in default in the performance or observance of any of the terms, provisions, and conditions of the Indenture and, if so, specifying all known defaults.

In limited cases, we and the trustee may make modifications and amendments to the Indenture without the consent of the holders of any series of debt securities, including to cure any ambiguity, to correct or

particular record date, this period will be 180 days or any other shorter period that we may specify. The period may be shortened or lengthened, but not beyond 180 days.

The law of the State of New York will govern the Indenture and the debt securities.

will bear a legend regarding the restrictions on exchanges and registration of transfer referred to below and other matters specified in a supplemental indenture to the Indenture.

the dividend periods (or the method of calculation thereof);

the voting rights of the shares;

whether and on what terms the shares of the series will be subject to redemption or repurchase at our option or at the option of the holders thereof;

whether the shares of the series of preferred stock will be listed on a securities exchange;

any special United States federal income tax considerations applicable to the series; and

If the proposed amendment would affect a series of the class, but not the entire class, in one or more of the ways described in the bullets above, then the shares of the affected series will have the right to vote on the amendment as a separate voting group. However, if a proposed amendment that would entitle two or more series of the class to vote as separate voting groups would affect those series in the same or a substantially similar way, the shares of all the series so affected must vote together as a single voting group on the proposed amendment.

Unless the articles of incorporation, Arizona law or the Board of Directors would require a greater vote or unless the articles or Arizona law would require a different quorum, if an amendment to the articles would allow the preferred stock or one or more series of the preferred stock to vote as voting groups, the vote required by each voting group would be:

Arizona law may also require that the preferred stock be entitled to vote on certain other extraordinary transactions.

The holders of our preferred stock will have no preemptive rights. All shares of preferred stock being offered by the applicable prospectus supplement will be fully paid and not liable to further calls or assessment by us. If we should redeem or otherwise reacquire shares of our preferred stock, then these shares will resume the status of authorized and unissued shares of preferred stock undesignated as to series, and will be available for subsequent issuance. There are no restrictions on repurchase or redemption of the preferred stock while there is any arrearage on sinking fund installments except as may be set forth in an applicable prospectus supplement. Payment of dividends on any series of preferred stock may be restricted by loan agreements, indentures and other transactions entered into by us. Any material contractual restrictions on dividend payments that exist at the time of the offer of any preferred stock will be described or incorporated by reference in the applicable prospectus supplement.

We may issue shares of preferred stock either separately or represented by depositary shares. Each depositary share will represent a fractional interest in a share of preferred stock of any series, to be described in an applicable prospectus supplement. In connection with the issuance of any depositary shares, we will enter into a deposit agreement with a bank or trust company, as depositary, which will be named in the applicable prospectus supplement. Depositary shares will be evidenced by depositary receipts issued pursuant to the related deposit agreement. Immediately following issuance of any shares of preferred stock related to the depositary shares, we will deposit such shares of preferred stock with the relevant depositary and will cause the depositary to issue, on its behalf, the related depositary receipts. Subject to the terms of the deposit agreement, each owner of a depositary receipt will be entitled, in proportion to the fractional interest in the share of preferred stock represented by the related depositary share, to all of the designations, powers, rights, preferences and privileges of, and will be subject to all of the qualifications, limitations or restrictions on, the preferred stock represented thereby, including any dividend, voting, redemption, conversion, exchange and liquidation rights. We will include a copy of the form of deposit agreement, including the form of depositary receipt, and any other instrument establishing the terms of any depositary shares that we offer as exhibits to a filing we will make with the SEC in connection with that offering. This summary does not purport to be complete and is subject to, and is qualified in its entirety by, the applicable prospectus supplement.

Pinnacle West may issue, from time to time, shares of its common stock, the general terms and provisions of which are summarized below. When we use the terms “we,” “us,” “our,” and like terms in this description, we are referring to Pinnacle West. This summary does not purport to be complete and is subject to, and is qualified in its entirety by express reference to, the provisions of our articles of incorporation, our bylaws and the applicable prospectus supplement.

Under our articles of incorporation, we have the authority to issue 150,000,000 shares of common stock. Our Board of Directors has significant discretion to determine the timing, circumstances and purposes for which the authorized shares of common stock available for issuance under our articles of incorporation may be issued, including in the context of acquisitions or other strategic transactions.

Subject to any preferential rights of any series of preferred stock, holders of shares of common stock will be entitled to receive dividends on the stock out of assets legally available for distribution when, as and if declared by our Board of Directors. The payment of dividends on the common stock will be a business decision to be made by our Board of Directors from time to time based upon results of our operations and our financial condition and any other factors that our Board of Directors considers relevant. Payment of dividends on the common stock may be restricted by loan agreements, indentures and other transactions entered into by us from time to time. Any material contractual restrictions on dividend payments that exist at the time of the offer of any common stock will be described in the applicable prospectus supplement. In addition, our principal income consists of dividends paid to us by our subsidiaries, primarily APS. APS’s ability to pay dividends could be limited or restricted from time to time by loan agreements, indentures and other transactions or by law or regulatory authorities.

Holders of common stock are entitled to one vote per share on all matters voted on generally by the shareholders. Arizona law provides for cumulative voting for the election of directors. As a result, any shareholder may cumulate his or her votes by casting them all for any one director nominee or by distributing them among two or more nominees. This may make it easier for minority shareholders to elect a director.

Subject to any preferential rights of any series of preferred stock, holders of shares of common stock are entitled to share ratably in our assets legally available for distribution to our shareholders in the event of our liquidation, dissolution or winding up.

Holders of common stock have no preferential, preemptive, conversion or exchange rights. When issued in accordance with our articles of incorporation and law, shares of our common stock being offered by the applicable prospectus supplement will be fully paid and not liable to further calls or assessment by us.

Computershare Trust Company, N. A. is the transfer agent and registrar for the common stock.

Our Board of Directors has the authority, without any further action by our shareholders, to issue from time to time up to 10,000,000 shares of preferred stock, in one or more series, and to fix the designations, preferences, rights, qualifications, limitations and restrictions thereof, including voting rights, dividend rights, dividend rates, conversion rights, terms of redemption, redemption prices, liquidation preferences and the number of shares constituting any series. The issuance of preferred stock with voting rights could have an adverse effect on the voting power of holders of common stock by increasing the number of outstanding shares having voting rights. In addition, if our Board of Directors authorizes preferred stock with conversion rights, the number of shares of common stock outstanding could potentially be increased up to the authorized amount. The issuance of preferred stock could decrease the amount of earnings and assets available for distribution to holders of common stock. Any such issuance could also have the effect of delaying, deterring or preventing a change in control of us. See also “Description of Pinnacle West Preferred Stock” above.

any of our affiliates or associates who at any time within the prior three years was such a beneficial owner.

any sale, lease, exchange, mortgage, pledge, transfer or other disposition of assets to an interested shareholder, representing 10% or more of (i) the aggregate market value of all of our consolidated assets as of the end of the most recent fiscal quarter, (ii) the aggregate market value of all our outstanding shares, or (iii) our consolidated revenues or net income for the four most recent fiscal quarters;

the issuance or transfer of shares of stock having an aggregate market value of 5% or more of the aggregate market value of all of our outstanding shares to an interested shareholder;

Our bylaws provide that any director or the entire Board of Directors may be removed by vote of the shareholders with or without cause, but only at a special meeting called for that purpose, if the votes cast in favor of such removal exceed the votes cast against such removal. However, if less than the entire Board of Directors is to be removed, no one director may be removed if the votes cast against the director’s removal would be sufficient to elect the director if then cumulatively voted at an election of directors.

Our bylaws provide that a director in an uncontested election who receives a greater number of votes cast “withheld” for his or her election than “for” such election must tender his or her resignation to the Corporate Governance Committee of our Board of Directors for consideration. The Corporate Governance Committee will evaluate the director’s tendered resignation, taking into account the best interest of Pinnacle West and its shareholders and will recommend to our Board of Directors whether to accept or reject the resignation. Any director tendering a resignation pursuant to this provision of our bylaws will not participate in any committee or Board of Director consideration of his or her resignation.

Our bylaws grant our Board of Directors the exclusive power to increase the size of our Board of Directors. Any such increase in the size of our Board of Directors, and the filling of any vacancy created thereby, require action by a majority of the whole membership of our Board of Directors as comprised immediately before such increase.

A shareholder must also comply with all applicable laws in proposing business to be conducted and in nominating directors. The notice provisions of our bylaws do not affect rights of shareholders to request inclusion of proposals in our proxy statement pursuant to Rule 14a-8 of the Exchange Act.

The Board of Directors may amend or repeal the corporation’s bylaws unless either: (i) the articles or applicable law reserves this power exclusively to shareholders in whole or in part or (ii) the shareholders in

amending or repealing a particular bylaw provide expressly that the Board may not amend or repeal that bylaw. An Arizona corporation’s shareholders may amend or repeal the corporation’s bylaws even though they may also be amended or repealed by the Board of Directors. Our bylaws may not be amended or repealed without the vote of a majority of the Board of Directors then in office or the affirmative vote of a majority of votes cast on the matter at a meeting of shareholders.

Bank of New York Mellon Trust Company, N.A., successor to JPMorgan Chase Bank, N.A., formerly known as The Chase Manhattan Bank, and us.

We have summarized the material provisions of the Indenture below. We have filed the Indenture as an exhibit to the registration statement. You should read the Indenture in its entirety, including the definitions, together with this prospectus and the prospectus supplement before you make any investment decision in our debt securities.

You should refer to the prospectus supplement used in connection with the offering of any debt securities for information about a series of debt securities, including:

Unless otherwise indicated in the applicable prospectus supplement, we may not:

Upon any such merger, consolidation, or transfer or lease of properties, the successor person will be substituted for us under the Indenture, and, thereafter, except in the case of a lease, we will be relieved of all obligations and covenants under the Indenture and the debt securities.

Each of the following will be an event of default under the Indenture with respect to debt securities of any series:

If an event of default occurs and is continuing, other than an event of default relating to bankruptcy, insolvency, or reorganization, either the trustee or the holders of at least 25% in aggregate principal amount of the outstanding debt securities of the affected series may declare the principal amount of the debt securities of that series to be due and payable immediately. In the case of any debt security that is an original issue discount security, the trustee or the holders of at least 25% in aggregate principal amount of the outstanding debt securities of that series may declare the portion of the principal amount of the debt security specified in the terms of such debt security to be immediately due and payable upon an event of default.

If an event of default involving bankruptcy, insolvency, or reorganization occurs, the principal amount of all the debt securities of the affected series will automatically, and without any action by the trustee or any holder, become immediately due and payable. After any acceleration, but before a judgment or decree based on acceleration, the holders of a majority in aggregate principal amount of the outstanding debt securities of that series may rescind and annul the acceleration if all events of default, other than the non-payment of accelerated principal, have been cured or waived as provided in the Indenture.

The trustee will be under no obligation to exercise any of its rights or powers under the Indenture at the request or direction of any of the holders, unless the holders have offered the trustee reasonable security

or indemnity. The holders of a majority in principal amount of the outstanding debt securities of any series will have the right to direct the time, method, and place of conducting any proceeding for any remedy available to the trustee, or exercising any trust or power conferred on the trustee, with respect to the debt securities of that series, provided that:

No holder of a debt security of any series will have any right to institute any proceeding under the Indenture, or for the appointment of a receiver or a trustee, or for any other remedy under the Indenture, unless:

We are required to furnish to the trustee annually a certificate of various officers stating whether or not we are in default in the performance or observance of any of the terms, provisions, and conditions of the Indenture and, if so, specifying all known defaults.

The law of the State of New York will govern the Indenture and the debt securities.

Some or all of the debt securities of any series may be represented, in whole or in part, by one or more global securities, which will have an aggregate principal amount equal to that of the debt securities they represent. We will register each global security in the name of a depositary or nominee identified in a prospectus supplement and deposit the global security with the depositary or nominee. Each global security will bear a legend regarding the restrictions on exchanges and registration of transfer referred to below and other matters specified in a supplemental indenture to the Indenture.

No global security may be exchanged for debt securities registered, and no transfer of a global security may be registered, in the name of any person other than the depositary for the global security or any nominee of the depositary, unless:

We will register all securities issued in exchange for a global security or any portion of a global security in the names specified by the depositary.

As long as the depositary or its nominee is the registered holder of a global security, the depositary or nominee will be considered the sole owner and holder of the global security and the debt securities that it represents. Except in the limited circumstances referred to above, owners of beneficial interests in a global security will not: