What are Input Watts?
The total power consumed to produce light is measured as input watts. For HID and fluorescent lights, this includes both the power used by the ballast and the power used by the bulb.
How Input Watts Determine Efficacy
Growers who consider energy efficiency a top priority can reference input watts in determining the efficacy of a light. Efficacy can be calculated by dividing the lumen output of a light by its rated input wattage. The resulting number is measured as lumens per watt (coded as “LPW” or “lm/W”).