Energy Converter – Joules, kWh, Calories and BTU

Energy units and their definitions

The joule (J) is the SI unit of energy: the work done by a force of one newton over one metre, or one watt for one second. The converter uses these exact or standard values:

• 1 watt-hour (Wh) = 3,600 J; 1 kilowatt-hour (kWh) = 3,600,000 J = 3.6 MJ
• 1 calorie (cal) = 4.184 J (thermochemical calorie); 1 kilocalorie (kcal) = 4,184 J
• 1 British thermal unit (BTU) = 1,055.056 J (International Table BTU)

Electricity: kilowatt-hours

Electricity bills are measured in kilowatt-hours. One kWh is the energy a 1,000 W appliance uses in one hour, or a 100 W device in ten hours. To estimate what an appliance costs to run, use the electricity cost calculator, which multiplies power, hours of use and your price per kWh.

Food energy: calories and kilojoules

Food labels show energy in kilocalories, often written as “Calories” with a capital C, and in kilojoules. One kcal is 4.184 kJ, so a 250 kcal snack contains about 1,046 kJ. The calorie on a nutrition label is therefore 1,000 times larger than the scientific calorie used in chemistry.

Heating: BTU and kWh

Boilers, furnaces and air conditioners in the United States are rated in BTU, and gas is sometimes billed in therms (100,000 BTU, about 29.3 kWh). One kWh equals about 3,412 BTU. When comparing a heat pump rated in kW with a furnace rated in BTU per hour, use the power converter, because those ratings are power rather than energy.

Energy versus power

Energy is an amount; power is the rate at which energy is used. A kilowatt (kW) is power, a kilowatt-hour (kWh) is energy. A 2 kW heater running for three hours uses 6 kWh.

Sources

Conversion factors follow the SI definitions in the BIPM SI Brochure and the exact factors for US customary and imperial units in NIST Special Publication 811.