In more detail
One kilowatt-hour is 3.6 megajoules, or about 3,412 BTU. Energy in kWh is watts × hours / 1,000. Bills charge per kWh, usually plus a fixed daily or monthly charge, and time-of-use tariffs price each kWh differently by hour. The power on an appliance label is its maximum; fridges, heaters and air conditioners switch on and off under a thermostat, so their real use is often well below rated watts × hours.
Why it matters
kWh is the common currency for comparing appliance running costs, sizing solar panels and working out what it costs to charge an electric car. Measuring a device with a plug-in energy meter for a day gives a much better figure than multiplying the label rating.
Example
1,500 W heater, 4 h/day, 30 days: 1,500 × 4 × 30 / 1,000 = 180 kWh → $27.00 at $0.15/kWh
10 W LED bulb, 5 h/day, 30 days: 10 × 5 × 30 / 1,000 = 1.5 kWh → $0.225
60 kWh EV battery from empty, 90 % charging efficiency: 60 / 0.9 = 66.7 kWh → $10.00
Common confusion
A kilowatt is a rate (power); a kilowatt-hour is an amount (energy). A 2 kW kettle running for 3 minutes uses only 0.1 kWh. Battery capacities in Wh follow the same logic; see Battery capacity.