This paper describes an evaluation of kW value with an analysis of measured data of photovoltaic power systems for houses. The kW value indicates the capability of power stations at the large electric power demands. If the PV system has a large kW value, the capacity of thermal power station to meet with the peak demand is reduced. A new method, K90 is proposed to estimate the kW value of the PV system. With the measured data of the PV system, K90 method is evaluated. Besides, the modified duration curve is considered to investigate the actual effect of the PV system on the peak demand for the thermal power plants. Finally, an appropriate kW value of the PV system is estimated.