In a conventional economic analysis, the availability of system is considered as a constant value. However, some factors such as increasing components' failure rate due to degradation, reducing failure rates by replacement or repairing, and stops in operation because of overhauls change the availability during the lifetime of a system. Furthermore, due to overhauls and degradation, maintenance costs are not identical in different years. This paper presents a new approach for economic analysis of thermal systems in which change in the availability of system during its lifetime is considered. A combined gas turbine cycle and desalination is studied. The instantaneous availability of system is calculated using state space method with time-varying failure rates and considering overhauls. Then, the average availability of producing electricity and fresh water in each year of lifetime is applied to the economic analysis. In addition, maintenance costs are calculated according to the overhauls and the number of components repairs in each year. Finally, some economic indicators are compared in two cases of variable and constant availability, using the life cycle cost analysis method. Considering time-varying availability, increase in payback period is observed by 9 months and reduction in net present value by about $18 million. (C) 2017 Elsevier B.V. All rights reserved.