What the numbers mean
Ten minutes and a whole day can use the same energy.
The hairdryer uses 250 watt-hours in 10 minutes. At 10 watts, the LED takes 25 hours to use that amount. Energy use depends on power multiplied by time, so minutes alone cannot tell the story.
Read the sourcesWatts tell you the pace
A watt measures how quickly energy is being used. A watt-hour measures the amount used over time. The US Energy Information Administration makes this distinction with a bulb example. In our model, the hairdryer draws 150 times the LED's power, so it needs only one-hundred-and-fiftieth of the time to use equal energy. [1]
Turn the minutes into hours
Ten minutes is one-sixth of an hour. Multiply 1,500 watts by one-sixth to get 250 watt-hours, or 0.25 kilowatt-hours. Then divide by the LED's 10 watts to get 25 hours. The same power-times-time calculation works for another device once its actual power use and running time are known. [1]
Read the right number on the bulb
Watts measure a bulb's electrical input; lumens measure its light output. The US Department of Energy lists these separately when comparing bulbs. For an energy calculation, use input watts. For a brightness comparison, use lumens. Our two devices perform different jobs, so equal energy use is not a comparison of how efficiently they do the same job. [2]