A relatively small portion of our revenue comes from national and regional account customers. These customers typically have multiple sites and contract with us to perform maintenance and repair service. These contracts may also provide for us to perform new or replacement systems installation. We operate a national call center to dispatch technicians to sites requiring service. We perform the majority of this work with our own employees, with the balance being subcontracted to third parties that meet our performance qualifications.
Profile and Management of Our Operations
We manage our 47 operating units based on a variety of factors. Financial measures we emphasize include profitability and use of capital as indicated by cash flow and by other measures of working capital principally involving project cost, billings and receivables. We also monitor selling, general, administrative and indirect project support expense, backlog, workforce size and mix, growth in revenue and profits, variation of actual project cost from original estimate, and overall financial performance in comparison to budget and updated forecasts. Operational factors we emphasize include project selection, estimating, pricing, safety, management and execution practices, labor utilization, training, and the make-up of both existing backlog as well as new business being pursued, in terms of project size, technical application, facility type, end-use customers and industries and location of the work.
Most of our operations compete on a local or regional basis. Attracting and retaining effective operating unit managers is an important factor in our business, particularly in view of the relative uniqueness of each market and operation, the importance of relationships with customers and other market participants, such as architects and consulting engineers, and the high degree of competition and low barriers to entry in most of our markets. Accordingly, we devote considerable attention to operating unit management quality, stability, and contingency planning, including related considerations of compensation and non-competition protection where applicable.
Economic and Industry Factors
As a mechanical and electrical services provider, we operate in the broader nonresidential construction services industry and are affected by trends in this sector. While we do not have operations in all major cities of the United States, we believe our national presence is sufficiently large that we experience trends in demand for and pricing of our services that are consistent with trends in the national nonresidential construction sector. As a result, we monitor the views of major construction sector forecasters along with macroeconomic factors they believe drive the sector, including trends in gross domestic product, interest rates, business investment, employment, demographics and the fiscal condition of federal, state and local governments.
Spending decisions for building construction, renovation and system replacement are generally made on a project basis, usually with some degree of discretion as to when and if projects proceed. With larger amounts of capital, time, and discretion involved, spending decisions are affected to a significant degree by uncertainty, particularly concerns about economic and financial conditions and trends. We have experienced periods of time when economic weakness caused a significant slowdown in decisions to proceed with installation and replacement project work.
Operating Environment and Management Emphasis
In 2020, the advent of a global pandemic led to some delays in service and construction, including delayed project starts and air pockets or pauses during 2020 and 2021. We experienced increasing demand in 2022 and 2023, and we expect that the demand environment, especially for industrial and technology customers, will remain at high levels in 2024. Although we have largely recovered from negative impacts caused by the COVID-19 pandemic, we continue to experience increased labor costs, supply constraints and cost increases, and delays in delivery of various materials and equipment. We expect that constraints and delays will continue to abate in 2024; however, we anticipate that pressure on cost and availability, especially for skilled labor, will continue throughout 2024.
We have a credit facility in place with terms we believe are favorable that does not expire until July 2027. As of March 31, 2024, we had $768.8 million of credit available to borrow under our credit facility. We have strong surety relationships to support our bonding needs, and we believe our relationships with the surety markets are strong and benefit from our operating history and financial position. We have generated positive free cash flow in each of the last twenty-five calendar years and will continue our emphasis in this area. We believe that the relative size and strength of