With recent trends in utilizing renewable power to develop sustainable energy sources, WTG and PV are increasingly viable economic alternatives for sustainable power generation from conventional fossil fuels. Therefore, multi-state models are being generated to solve the intermittent power production problem of wind turbine generator (WTG) and photo voltaic (PV). However, a disadvantage of these units is the generation of highly variable electricity at several different timescales from hourly, daily, and seasonally. Related to variability is the short-term (hourly or daily) predictability of wind plant output. These problems result in serious damage to sustainable service concerns in both the design and operation of WTG and PV systems. Large-scale systems can provide a solution to overcome sustainable service problem but are costly. A sufficient design will cause outages that lead to certain cost losses on the customer side. Therefore, planning a reasonable size is a major dilemma. For this objective, the three models of load model, generation model, and service cost model should be built. For the first two models, the loss of load expected (LOEE) and the loss of load expected (LOLE) can be calculated. Then, this reliability characteristic is evaluated with the existing system of generation units to decide the range of required renewable generators. Finally, the cost model is constructed with consideration of the sustainable service cost. The fuel and operation costs obviously contribute a rather large portion to the utility cost. From the total service cost chart, the amount that the sustainable service is worth is tremendous when forced outage at a high level occurs and the customer is not fulfilled. However, the utility cost becomes the primary of the total service costs when sufficient capacity is constructed in the system, which is invested mainly to the conventional generating. The utility cost of renewable energy changes with increases in WTG and PV but remains steady. With regard to the excessive variable cost, fixed cost, and capital cost of a conventional generating system, the investment cost and maintenance cost are relatively insignificant. If renewable energy replaces conventional generators, particularly thermal sources, this component of utility costs will not occur. Also, a certain service level is ensured to supply the load.