Converters
Convert energy between watt-hours, kilowatt-hours, joules, megajoules, and BTU.
Enter your numbers to see the answer.
Energy converter guide
The energy converter links watt-hours, kilowatt-hours, joules, megajoules, and British thermal units, the units that appear on electricity bills, gas bills, battery labels, and heating equipment. The joule is the SI unit, one watt for one second, so a watt-hour is exactly 3,600 J and a kilowatt-hour is exactly 3.6 MJ. The BTU is the heat that raises one pound of water by one degree Fahrenheit; the converter uses the International Table BTU of 1,055.05585262 J, the definition NIST lists and the one used for fuel and HVAC ratings. All conversions run through joules. Use it to compare an electric heater’s kWh with a gas appliance’s BTU, read a battery pack’s Wh against a spec in MJ, or convert utility figures that mix units. This page converts energy, not power: kW, hp, and BTU per hour belong to the power converter, and dividing energy by time gets you there.
Energy (to) = energy (from) × joules per from-unit ÷ joules per to-unit. Factors (J): watt-hour 3,600, kilowatt-hour 3,600,000, joule 1, megajoule 1,000,000, BTU (IT) 1,055.05585262.
Worked example with the defaults: 1 kWh to megajoules. Convert to joules: 1 × 3,600,000 = 3,600,000 J. Divide by the joules in a megajoule: 3,600,000 ÷ 1,000,000 = 3.6 MJ, the displayed figure. The detail rows show 1,000 Wh and 3,600,000 ÷ 1,055.05585262 = 3,412.14 BTU. A household using 10 kWh a day uses 36 MJ, and a gas therm of 100,000 BTU is 100,000 × 1,055.05585262 = 105,505,585 J, or 29.31 kWh.
Enter the amount as a decimal in Wh, kWh, J, MJ, or BTU. Kilojoules divide by 1,000 to MJ; gigajoules, the unit on many Canadian gas bills, multiply by 1,000 to MJ. A therm is 100,000 BTU and a cubic foot of natural gas is roughly 1,037 BTU, so enter those as BTU. Food calories (kcal) are 4,184 J each, so 2,000 kcal is 8.368 MJ. Results round to two decimals; convert to a smaller unit if a figure shows as 0.
Unit conversion is exact when the conversion factor is exact, so the only real uncertainty here is rounding and the precision of your input. Where a unit has more than one definition — US versus imperial measures, or short versus metric tons — the wrong variant produces a plausible-looking but wrong answer. The units note above states which definition this page uses. On this page the figure rests entirely on value, from and to, so start there if the energy converter returns something you did not expect.
Good to know: kWh is energy and kW is power. A 3 kW oven running for half an hour uses 1.5 kWh, and this converter takes the 1.5, not the 3. “BTU” printed on a furnace or air conditioner is really BTU per hour, a power rating, and does not belong here. Several BTU definitions differ by up to 0.1%; the tool uses the International Table value, the one in NIST SP 811 and in fuel-pricing tables. Battery labels in mAh are charge, not energy; multiply by voltage to get Wh.
Conversion results use stated mathematical factors and display precision. For regulated, medical, engineering, or contractual work, confirm the required standard and precision.
Sources
How it works
Convert energy between watt-hours, kilowatt-hours, joules, megajoules, and BTU. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Value = 1 · From = kWh · To = MJ → 3.6 MJ (1 kWh converted to MJ). Change an input above and this example updates with your numbers.
Common questions
3,412.14 BTU, from 3,600,000 J ÷ 1,055.056 J. So 10 kWh is 34,121.42 BTU and 1 BTU is 0.000293 kWh. To compare heating fuels, convert both to kWh or MJ before dividing by price.
Exactly 3.6 MJ, because a kilowatt-hour is 1,000 W × 3,600 s = 3,600,000 J. Going back, 1 MJ is 0.2778 kWh. A 100 MJ gas bill is 27.78 kWh, and 1 GJ is 277.78 kWh.
1,055.056 J for the International Table BTU (1,055.05585262 J exactly). The thermochemical BTU is 1,054.35 J, about 0.07% smaller; the difference is negligible for utility figures but the converter uses the IT value throughout.
29.31 kWh, since a therm is 100,000 BTU and 100,000 × 1,055.056 J = 105.5 MJ. Enter 100,000 BTU in the converter to see all five units. A cubic foot of gas at about 1,037 BTU is 0.304 kWh.
No. A joule is one watt for one second; a watt-hour is one watt for one hour, so 1 Wh = 3,600 J. A phone battery rated 20 Wh stores 72,000 J or 72 kJ, and a 60 kWh EV battery holds 216 MJ.