The solar irradiance and its prediction interval are forecasted with the aid of a meteorological model for the prediction of the photovoltaic systems generation. The solar irradiances of the same target day are computed utilizing a model with different initial conditions, and the accuracy of the forecasting is discussed in this paper. Forecasting reliability is also estimated from the variance of the forecasted irradiances. The relationship between the forecasted reliability and the forecasting error is derived, and the prediction interval of the solar irradiance forecasting is evaluated from both its reliability and relationship. The performance of the probabilistic forecasting of the solar irradiance which consists of the prediction interval is discussed in the observation. The size of the prediction interval changes as the forecasting reliability varies. The observed data in the interval is almost the same rate as the coverage rate determined in advance. (C) 2016 Elsevier Ltd. All rights reserved.