Energy use is power multiplied by run time. Convert watt-hours to kilowatt-hours to match how utilities bill.
What the terms mean
| Symbol | Meaning |
|---|---|
| kWh | Energy, in kilowatt-hours |
| P | Power per fixture, in watts |
| h | Hours of operation |
| n | Number of fixtures |
Worked example
Ten 40 W fixtures running 10 hours a day use (40 × 10 × 10) ÷ 1000 = 4 kWh per day.
See the Power Formula and the Cost Savings Calculator.
Calculating lighting energy use
Energy use is simply power multiplied by time: energy (Wh) = power (W) × hours. Utilities bill in kilowatt-hours, so divide watt-hours by 1,000 to get kWh, then multiply by your electricity rate for cost. For example, a 100W fixture running 5 hours a day uses 500 Wh = 0.5 kWh daily; over a 30-day month that’s 15 kWh, or about $2.25 at $0.15/kWh.
For a whole installation, add up the wattage of all fixtures, multiply by their daily run hours, and scale to the period you care about. This is where LED efficiency pays off: swapping that 100W fixture for a 15W LED of equal brightness cuts the same usage to about 2.25 kWh a month — and occupancy sensors or daylight dimming reduce it further by cutting run time.