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This simplified computation performs quick meteo evaluations, using geographical site database only (i.e. monthly irradiation values), and evaluates horizon, tilt, sheds and sun-shields, as well IAM effects.

This method takes advantage of  so-called "average months" properties. With real meteo data of a given month, when constructing an "average day" by averaging separately the irradiances at each hour (i.e. all irradiances at 8:00, 9:00, 10:00, etc), we obtain an average profile which is very close to the "Clear day" model profile, with amplitude reduced in such a way that the day integral matches the monthly global value. This also holds for the diffuse component.

Therefore, inversely, for the middle of each month, we construct a "clear day" with amplitude suited to the given monthly irradiation. We then assume that this "average day" is representative of the month, so that we can apply all mentioned corrections (transposition, shadings, etc) using the solar geometry of this middle-month day.

Accuracy

This procedure avoids constructing synthetic hourly values, and gives instantaneous evaluations with very acceptable accuracy.

With respect to an hourly computation, monthly calculations for Geneva show that the transposition on tilted plane induces a yearly MBE of the order of:

If only the global monthly values are known, the uncertainty on the monthly diffuse estimation correlation model (about 5%) may induce 1 to 4 % error more.