The efficiency of a lighting system is calculated by dividing input power (WATTS) vs. output (LUMENS or PPF).

Watts is an international measurement representing 1 unit of input power. It is the most important measurement for determining how expensive a light is to run because it’s the one used by electrical utilities for calculating the cost of electricity.

Electrical utilities calculate costs based on kilowatt hours (kWh). A kilowatt hour is how much energy gets used by a 1000w appliance in 1 hour. For example, a 2000 watt appliance would use up 1 kWh in 1/2 an hour. A 500 watt appliance would take 2 hours to use up 1 kWh.

The best light is the one that outputs the most power for the least amount of money.

Divide WATTS with an output measurement, either LUMENS or PPF. This results in a number you can use to compare the efficiency of different light systems. The lower the number, the cheaper a light costs to run.

Sometimes efficiency measurements are already calculated by light manufacturers and published with their product specifications. Efficiency measurements are expressed as PPF/W (PPF per watt), or lm/W (LUMENS per watt). PPF/W is sometimes alternatively expressed as µmoles/joule.