Forecasts of insolation have a wide range of applications. With the dissemination of photovoltaic, PV, systems such forecasts also becoming important in the renewable energy field. In previous studies we developed a method to forecast hourly insolation, one-day ahead of time. Currently, we are modifying the proposed method to apply it in the problem of hour-ahead forecast of insolation. In this short-communication we present a method to forecast insolation one hour ahead of time, and we evaluate its performance with 1 year of data of Tsukuba city. Finally, we discuss the accuracy of the forecasts and the error reduction that occurs when changing the forecast horizon from 1 day ahead to 1 hour ahead of time.