How to use the Energy Converter
- Enter an amount of energy, such as 2000 for a daily food intake in kcal.
- Pick the unit you have and the unit you need, grouped into SI, electricity, food and heat, and physics units.
- Read the converted amount and the factor for one unit in each direction.
- Scan the complete list to compare every unit, and click any value to copy it.
Features
- Exact factors, e.g. 1 kWh = 3.6 MJ, 1 cal = 4.184 J
- Food Calories (kcal) and kilojoules for nutrition labels
- kWh and MWh for electricity bills and solar panels
- BTU and therms for heating and cooling
- Electronvolts, ergs and foot-pounds for physics
- Every unit at once with one-click copy
The joule and its many cousins
Energy is the capacity to do work, and its SI unit is the joule (J), named after James Prescott Joule, whose experiments in the 1840s showed that mechanical work and heat are two forms of the same thing. One joule is the work done by a force of one newton over one meter, or one watt flowing for one second. Different fields grew their own units long before the joule was agreed, which is why converting between them is so common.
Electricity: watt-hours and kilowatt-hours
A kilowatt-hour is a power of 1,000 watts sustained for one hour: 1,000 × 3,600 s = 3.6 million joules, exactly. Utilities bill in kWh, and battery packs, solar panels and electric cars are rated in Wh or kWh. Do not confuse energy (kWh) with power (kW): power is the rate, energy is the total.
Food: calories and kilocalories
A small calorie was originally the heat needed to warm one gram of water by one degree Celsius. This tool uses the thermochemical calorie of exactly 4.184 J; a slightly different “International Table” calorie of 4.1868 J also exists, and the two differ by less than 0.07%. The “Calorie” on a food label is a kilocalorie (kcal), 1,000 small calories. Labels in Europe, Australia and elsewhere also show kilojoules.
Heating: BTU and therms
The British thermal unit is the heat that raises one pound of water by one degree Fahrenheit, standardized as 1,055.06 J. Furnaces and air conditioners in the US are rated in BTU per hour, which is power, not energy. US natural-gas bills use the therm, 100,000 BTU. This converter defines the therm as exactly 100,000 BTU; the US legal therm is about 0.02% smaller, a difference that does not show on a bill.
Physics: electronvolts and ergs
An electronvolt is the energy an electron gains crossing one volt, exactly 1.602176634 × 10⁻¹⁹ J since the 2019 SI redefinition. Particle physicists use it with prefixes such as MeV and GeV. The erg (10⁻⁷ J) belongs to the older CGS system, and the foot-pound appears in US engineering and firearms data.
Worked examples
A 2,000 kcal diet: 2,000 × 4.184 = 8,368 kJ, about 2.32 kWh per day.
A 60 W bulb for 5 hours: 60 × 5 = 300 Wh = 0.3 kWh, or 1.08 MJ.
Gas bill: 50 therms × 29.3071 = 1,465.4 kWh.
Reference table
| Unit | Joules |
|---|---|
| 1 calorie (thermochemical) | 4.184 |
| 1 kilocalorie (food Calorie) | 4,184 |
| 1 BTU | 1,055.05585262 |
| 1 watt-hour | 3,600 |
| 1 kilowatt-hour | 3,600,000 |
| 1 therm (100,000 BTU) | 105,505,585.262 |
To turn food energy into a daily target, use the Calorie Calculator. Temperature and pressure questions often come up alongside energy ones; the Temperature Converter and Pressure Converter cover those.
Frequently asked questions
Is a food Calorie the same as a calorie?
No. A food Calorie (capital C) is a kilocalorie, 1,000 small calories or 4,184 joules.
How many joules are in a kilowatt-hour?
Exactly 3,600,000 joules (3.6 MJ), because a kilowatt-hour is 1,000 watts for 3,600 seconds.
What is the difference between BTU and BTU per hour?
A BTU is an amount of energy. BTU per hour (often written BTU on air conditioner labels) is a rate, which is power. 1 BTU/h is about 0.293 watts.
How many kWh are in a therm?
About 29.3071 kWh, taking a therm as 100,000 BTU.
How many kWh is 1,000 kcal?
About 1.162 kWh. Multiply kcal by 4,184 to get joules, then divide by 3,600,000.