This paper deals with estimation of total output of solar photovoltaic generators (PV's) widely distributed in future power systems. In the proposed framework of estimation, the estimated signal in each frequency band is a linear combination of observed signals. The observation gain for each observed signal in each frequency band is designed through following two steps. First, an estimation error function is defined using assumed root mean square of output of PV's, predicted distribution of future PV's in the area, and correlation coefficients, which are identified in advance based on actual synchronized measurements of PV's. Second, observation gain in each frequency band is designed so as to minimize the estimation error function. The appropriateness of the proposed framework of estimation is demonstrated in small scale and large scale examples.