In this article, first the various components of the cash flow of the project of “power supply through the grid and heat through a boiler” ("Ι" ), the project of “power and heat supply through DG generator” ("ΙΙ" ), and the difference project, are expanded, for the option installation of DG generators in CNG station and carwash complexes. Next, for different electrical capacities, the cash flow components of the difference project, including the investment cost, annual benefits and costs and salvage value are calculated and the optimum electrical capacity and six corresponding economical parameters of the difference project are determined, according to maximize the net present worth of the difference project, using MATLAB software. In addition, the sensitivity of the optimum electrical capacity and the net present worth of the difference project relative to various parameters, including the power purchase and sales tariffs and gas purchase tariff, is investigated. The type of technology considered is gas engine and the system type is CHP, additionally it is assumed that the generator operates in grid support (base-load) mode. The present study takes into consideration not only the constraint of maximum possible value of investment, but also those constraints relative to proper power flow and accepted fault levels in the power grid. A new approach which simultaneously takes into account the electrical load profile and the power bills has been adopted in the mathematical modeling. Consumed electrical load profile of CNG station under study located in Tehran, during one day period, has been extracted through a Power Harmonics Analyser device. Also consumed thermal load profile for the nearby carwash, has been determined by estimating cars traffic at different working hours and calculating consumed thermal load per car.