The amount of radiation received by a surface can be expressed as the amount of energy received by a unit of surface area per unit time. Note that energy per unit of time is called power.

The most common units for expressing the amount of solar radiation received by an object are Watts per square meter (W/m²) or kiloWatt-hours per square meter per day (kWh/m²/day).

The Watt is a unit of power, which is equal to one Joule per second. Somewhat confusingly, a kiloWatt-hour (kWh) is a unit of energy instead of power and is equal to exerting 1 kiloWatt of power (1000 Watts) during a one-hour period. Consequently, 1 kWh is equal to 3.6 million Joules since in a one-hour period there are 3,600 seconds multiplied by 1000 Watts. This is also the same amount of energy needed to power one 100 Watt light bulb for ten hours since in a 10-hour period there are 36,000 seconds multiplied by 100 Watts.

1 kWh/day is equal to 41.67 Watts of power. This conversion is done by converting 1 day into 24 hours and dividing 1kWh by 24 hours. The result is 0.04167 kiloWatt since units of days and hours cancel each other. For the final conversion 0.04167 is multiplied by 1000, since there are 1000 Watts in 1 kiloWatt and the result is 41.67 Watts. Another way to look at the conversion equation is shown below:

(1 kiloWatt hour / 1 day) * (1 day / 24 hours) = 0.04167 kiloWatt * (1000 Watts / 1 kiloWatt) = 41.67 Watts