Weather data Monthly calculations

This simplified computation performs quick weather data evaluations using only geographic site database data (monthly irradiation values) and evaluates horizon, tilt, rows, sun-shields, and IAM effects.

This method exploits "average month" properties. Using real weather data from a given month, an "average day" is constructed by averaging irradiances for each hour separately (all 8:xx values, all 9:xx values, etc.). The resulting average profile closely matches a "clear day" model profile with reduced amplitude such that the daily integral equals the monthly global value. This also applies to the diffuse component.

Conversely, for the middle of each month, a "clear day" is constructed with amplitude matched to the given monthly irradiation. This "average day" is assumed representative of the month, allowing all corrections (transposition, shading, etc.) to be applied using the solar geometry of the month's middle 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 of the monthly diffuse estimation correlation model (about 5%) may induce an additional 1 to 4% error.