The values are generally expressed in kWh/m²/day. This is the amount of solar energy that strikes a square metre of the earth''s surface in a single day. Of course this value is averaged to account for differences in the days'' length. There are several units that are used throughout the world.

The conversions based on surface area as follows:
1 kWh/m²/day = 317.1 btu/ft²/day = 3.6MJ/m²/day

The raw energy conversions are:
1kWh = 3412 Btu = 3.6MJ = 859.8kcal