Preliminary validation of models predicting insolation on tilted surfaces

Using a data base consisting of hourly values of direct normal insolation, diffuse and global insolation on the horizontal, and insolation on seven tilted surfaces, three models used to estimate insolation on tilted surfaces are analyzed. The data base is shown to be sufficiently accurate to test the insolation models. All of the models show significant errors in their estimates of insolation on vertical surfaces, but accurately predict insolation on surfaces tilted at small angles.