Kilowatt-Hours to Joules

A watt is one joule per second. A kilowatt is 1,000 joules per second, and running it for an hour of 3,600 seconds delivers 1,000 × 3,600 = 3,600,000 J. The factor is exact, so a kWh on your bill and 3.6 megajoules in a physics book are the same amount of energy.

The kilowatt-hour stuck because it matches how people use power: a 2 kW kettle running for 3 minutes uses 0.1 kWh, or 360 kJ, and a 10 W LED bulb left on for 100 hours uses 1 kWh. Joules are better for comparing very different things, from food to fuel to batteries.

Electricity bills charge per kWh, and an American household typically uses on the order of 10,000 kWh a year, which is 36 billion joules, or 36 GJ. Gas bills in the US often use therms; one therm is 100,000 BTU, about 105.5 MJ or 29.3 kWh, so you can compare a gas and an electric heater on equal terms once both are in joules.

A phone battery of around 15 Wh stores 54 kJ, while an electric car pack of 60 kWh holds 216 MJ, four thousand times more. The EPA counts 33.7 kWh of energy in a US gallon of gasoline for its MPGe ratings, which is 121 MJ. In food terms, 1 kWh is about 860 kcal, a substantial meal, which puts the 0.1 kWh of one kettle boil at around 86 kcal.

Large energy numbers are easier with prefixes: 1 MJ is a million joules, 1 GJ a billion and 1 TJ a trillion. 1 MWh is 3.6 GJ, and 1 GWh is 3.6 TJ. For food energy, convert with kcal to kJ; the unit converter also handles calories and kilojoules directly.