Luminous efficacy refers to the amount of light produced by a source compared to the amount of heat generated. It is measured as the number of lighting Lumens for each Watt of heat energy.

If measured in terms of luminous efficacy (the number of lighting lumens for each Watt of associated heat energy)

The luminous efficacy of daylight is considerably higher than that of most electric lighting alternatives. Thus, using natural daylight within a space results in less heat per lumen than lighting it by artifical means.

As virtually all of the energy from any light source is eventually converted to heat within a building, the considered use of daylight can also reduce air-conditioning cooling loads.