Energy Converter
Convert energy between joules, kJ, MJ, calories, kilocalories, watt-hours, kWh, BTU, electronvolts and foot-pounds.
Updated Formula covered by automated tests
Convert energy between joules, kilojoules, megajoules, calories, kilocalories, watt-hours, kilowatt-hours, British thermal units, electronvolts and foot-pounds. The same physical quantity turns up under a different name on almost every bill and label: kilowatt-hours on an electricity bill, therms or BTU on a gas bill and a furnace rating, kilocalories or kilojoules on a food package, and joules in a physics textbook.
Putting these on one scale answers practical questions such as how a 1,500-watt heater compares with a gas furnace rated in BTU, how many kilojoules a 250-Calorie snack contains, or how much energy a phone battery stores compared with a car battery. It also helps with the confusion between energy and power: watts measure how fast energy is used, while watt-hours measure how much was used in total.
The formula
- J
- Joule, the SI unit: the energy of one watt sustained for one second, or one newton pushing through one meter
- kWh
- Kilowatt-hour, 1,000 watts for one hour, the unit utilities bill electricity in
- kcal
- Kilocalorie, written as Calorie with a capital C on US food labels
- BTU
- British thermal unit, roughly the heat that warms one pound of water by one degree Fahrenheit
Since a watt is one joule per second, a watt-hour is 3,600 joules and a kilowatt-hour is 3,600,000 joules. That single fact links electricity to every other energy unit. Heat units came from a different tradition: the calorie and BTU were first defined by how much heat raised the temperature of water, and because that varies slightly with the starting temperature, several versions existed until they were pinned to the joule.
This converter uses the thermochemical calorie (exactly 4.184 J), which is also the factor the FAO and most nutrition references use to switch between kcal and kJ, and the International Table BTU (1,055.05585262 J) used in heating and air-conditioning ratings. The electronvolt, the energy an electron gains crossing one volt, is exactly 1.602176634 × 10⁻¹⁹ J since the 2019 SI redefinition and is used for atoms, photons and particles.
Worked example
Jamal runs a 1,500-watt portable heater for 4 hours each evening in his garage workshop. He wants to compare the energy with his gas furnace’s BTU rating and, out of curiosity, with food Calories.
- 1 Energy in kilowatt-hours: 1,500 W × 4 h ÷ 1,000 = 6 kWh.
- 2 Convert to megajoules: 6 × 3.6 = 21.6 MJ, which is 21,600,000 J.
- 3 Convert to BTU: 21,600,000 ÷ 1,055.05585262 = 20,472.8 BTU.
- 4 Convert to food Calories: 21,600,000 ÷ 4,184 = 5,162.5 kcal.
Result: Each evening uses 6 kWh, equal to about 20,500 BTU or roughly 5,160 food Calories. At an electricity price of $0.15 per kWh that is $0.90 a night; plug in your own tariff from your bill.
Before you rely on the result
- Uses the thermochemical calorie and International Table BTU; other historical calorie and BTU definitions differ from these by up to about 0.15 percent.
- Converts amounts of energy only. Power ratings in watts or BTU per hour must first be multiplied by running time.
- Energy on a bill or label is not the same as energy delivered as useful heat, light or motion; appliance efficiency is not included.
Frequently asked questions
Is a food Calorie the same as a calorie?
No. The Calorie on US nutrition labels is a kilocalorie, 1,000 small calories or 4,184 joules. Labels in the EU, Australia and elsewhere show kilojoules alongside kcal; multiply kcal by 4.184 to get kJ, so a 250 kcal snack is 1,046 kJ.
My air conditioner is rated 12,000 BTU. How many kilowatts is that?
Air-conditioner and furnace ratings are actually BTU per hour, a power unit, even when the label just says BTU. 12,000 BTU per hour is 12,000 × 1,055.06 ÷ 3,600 = 3.52 kW of cooling, which is also called one "ton" of refrigeration. The electricity the unit draws is several times lower than that, because an air conditioner moves heat rather than creating it.
How do I compare a gas bill in therms with an electric bill in kWh?
A US therm is 100,000 BTU, which is about 29.3 kWh. Divide the price per therm by 29.3 to get a price per kWh of heat, then account for the efficiency of the gas appliance versus the electric one before deciding which is cheaper.
How much energy does a phone battery hold?
Battery capacity in watt-hours is printed on most batteries or can be found by multiplying the amp-hour rating by the nominal voltage. A typical smartphone battery of around 15 Wh stores 54,000 J, while a 60 kWh electric car battery stores 4,000 times as much.