An equivalent mean day for the calculation of monthly average daily utilizability is proposed in the present article. The equivalent mean day is characterized by a clearness index equal to the average of the clearness indices of the days that contribute to the useful energy. It has been shown that the monthly average daily utilizability can be calculated from the utilizability calculated for a single day, the equivalent mean day. The method has been validated for flat plate collectors of general orientation with extensive comparisons. [S0199-6231(00)00402-0].