Energy Converter

How it works

Every energy unit is defined as some number of joules. The calculator multiplies your value by the size of the unit it is in to get joules, then divides by the size of the unit you want.

A watt-hour is 3,600 J and a kilowatt-hour 3,600,000 J, exactly. The calorie here is the thermochemical calorie, exactly 4.184 J, and a kilocalorie (the food 'Calorie' with a capital C) is 4,184 J. The British thermal unit is the International Table BTU, 1,055.05585262 J. The therm is the US therm used on gas bills, exactly 105,480,400 J (about 99,976 BTU). The foot-pound is the exact pound-force times the exact foot, and the electronvolt is exactly 1.602176634 × 10⁻¹⁹ J under the 2019 SI.

The conversion factor is what one unit of the 'from' unit is worth in the 'to' unit, so you can apply it to other values by hand. The table lists the same energy in every unit the converter knows.

Formula

joules    = value × (size of the from unit in J)
converted = joules ÷ (size of the to unit in J)
factor    = size of from unit ÷ size of to unit

1 Wh = 3,600         1 kWh = 3,600,000     1 cal = 4.184     1 kcal = 4,184
1 BTU = 1,055.05585262   1 therm (US) = 105,480,400   1 ft·lbf = 4.4482216152605 × 0.3048
1 eV = 1.602176634 × 10⁻¹⁹

Example

1 kWh, the default, is 3,600,000 J, which is 3,600,000 ÷ 1,055.05585262 = 3,412.141633 BTU. The factor from kWh to BTU is 3,412.1416331279.

The table shows the same kilowatt-hour as 3,600 kJ, 860.42065 kcal, 0.03413 therms and 2,655,223.737398 ft·lbf. A 2,000 kcal daily diet is 2,000 × 4,184 = 8,368 kJ, or 8,368,000 ÷ 3,600,000 = 2.324444 kWh.

Assumptions and limitations

• The calorie is the thermochemical calorie (4.184 J exactly). The International Table calorie is 4.1868 J, 0.07% larger. Food Calories are kilocalories.
• The BTU is the International Table BTU (1,055.05585262 J). The thermochemical BTU (1,054.350 J) and the 59 °F BTU (1,054.804 J) differ by up to 0.07%.
• The therm is the US therm, 105,480,400 J exactly, the unit on US gas bills (1 therm = 29.300111 kWh). The EC therm, 100,000 International Table BTU = 105,505,585.262 J, is 0.024% larger and is not offered.
• The foot-pound uses the exact pound-force (0.45359237 kg × 9.80665 m/s²) and the exact foot (0.3048 m): 1.3558179483 J.
• A negative value is converted as entered, which is useful for net work or heat flow out of a system.
• Results are shown to six decimal places; very small values (for example a few joules in kWh) may display as 0 even though the conversion is computed in full precision.

Frequently asked questions