1.3 The 'Efficiency' Of Daylight
Efficacy is a measure of how well a light source converts 'watts in' to 'lumens (or light) out'. For electric lighting, 'watts in' refers to the electric power. For daylighting, 'watts in' refers to natural light. In either case virtually all of that energy is converted to heat at a rate of about 3.4 BTUs per watt.
The table below shows that daylighting, in all forms, compares well against electric lighting. It introduces less heat per lumen than most electric sources and can REDUCE the cooling loads in a building due to lighting. However (and it's a big however), this assumes the daylighting can be effectively distributed and fully utilized for illumination.
Efficacy of Light Sources