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The albedo coefficient is the fraction of global incident irradiation reflected by the ground in front of a tilted plane.

This effect takes place during the transposition computation of the horizontal irradiation onto a tilted plane. The albedo "seen" by the plane is of course null for an horizontal plane, and increases with tilt.

Its contribution is proportional to   (1 - cos i) / 2,  where i = plane tilt  (see for example the Hay model). This means 0 for horizontal, 0.5 for vertical, and 0.067 for a 30° plane.

In the project definition, the albedo values can be adjusted each month in order to take any possible snow-cover into consideration. The value usually admitted in the urban localities is of the order of 0.14 to 0.22, and can go up till 0.8 for a snow-cover. The usual value is 0.2 (default in PVsyst).

Ideally, the best value for a given situation may be obtained by a direct measurement on the site  (it is the ratio of the irradiances measured by a pyranometer turned toward ground, and another one measuring the global horizontal).

In practice, except for vertical planes, this value does not take on any great importance as the albedo component is relatively weak in the incident global irradiation (this  contribution can be visualized in the monthly results of your simulation).

Moreover for shed systems (for example ground power plants), only the first row is "seeing" the albedo, the shading factor on the albedo component is  (n-1)/n  (where n = number of rows).

The following table gives some usual values for the albedo factor: