Guide 01 / 10
Watts and kWh: power is different from energy
Understand the difference between the number on an appliance label and the number on your electricity meter.
Watts describe a rate
Watts (W) tell you how quickly a device uses energy at a moment in time. A 100 W rating does not mean the device always uses 100 kWh. To compare devices, you also need operating time and an idea of whether the power draw stays constant. A rating plate is often a nominal value, not a measurement over a whole day.
kWh add up energy
A kilowatt-hour (kWh) is a unit of energy. A steady 1,000 W load running for one hour uses 1 kWh. A 100 W load running for ten hours also uses 1 kWh. A small load left on continuously can therefore use more energy than a powerful device used briefly.
Keep the units straight
Convert watts to kilowatts by dividing by 1,000, then multiply by hours. If a label already states annual kWh, do not multiply it by hours again: it already reports energy for the stated period. Read the test period and conditions on the label.