⚡ Energy
Energy Converter
Convert food calories, electricity bills in kWh, gas therms and scientific joules. Enter a value and press Convert.
About the energy converter
Energy is the capacity to do work, measured in joules. Nutrition labels use kilocalories (what most people call "calories"), electricity is billed in kilowatt-hours, and gas in therms or BTU — this tool translates between all of them.
Factors used
- 1 calorie (thermochemical) = 4.184 J
- 1 kilocalorie (food Calorie) = 4,184 J
- 1 watt-hour = 3,600 J · 1 kWh = 3.6 MJ
- 1 BTU (International Table) = 1,055.05585262 J
- 1 electronvolt = 1.602176634 × 10⁻¹⁹ J
How to convert energy
Energy is the capacity to do work, measured in joules (J). Food energy is quoted in kilocalories (kcal), often written simply as Calories, where 1 kcal equals 4.184 kJ exactly. Household electricity is billed in kilowatt-hours, and 1 kWh is 3,600,000 J because one kilowatt for one hour is 1,000 watts for 3,600 seconds. The British thermal unit and the electronvolt cover heating and atomic scales respectively.
Common energy conversions
| Unit | 1 kilocalorie (food) equals |
|---|---|
| Joule (J) | 4,184 |
| Calorie (cal) | 1,000 |
| Kilojoule (kJ) | 4.184 |
| British thermal unit (BTU) | 3.9656668 |
| Watt-hour (Wh) | 1.1622222 |
| Kilowatt-hour (kWh) | 0.0011622222 |
Everyday uses
- Understanding electricity bills
- Comparing food energy with exercise
- Heating, cooling and boiler sizing
- Physics and chemistry homework
- Battery and fuel energy content
Frequently asked questions
More worked examples
- A 2,000 W kettle switched on for 3 minutes uses 0.1 kWh, because 3 minutes is 0.05 of an hour.
- One kilowatt-hour is 3,600,000 joules, or 3.6 megajoules.
- One food Calorie, the kind on a nutrition label, is 4,184 joules.
- A 2,000 kcal daily intake is about 8.37 megajoules.
Quick reference
| Unit | Equivalent |
|---|---|
| 1 joule | 1 watt-second |
| 1 kilojoule | 1,000 J |
| 1 calorie (small) | 4.184 J |
| 1 food Calorie (kcal) | 4,184 J |
| 1 kilowatt-hour | 3.6 MJ |
| 1 BTU | 1,055.06 J |
| 1 therm | 105.5 MJ |
Common mistakes to avoid
- Mixing the small calorie with the food Calorie. The nutrition figure is a kilocalorie, so it is 1,000 times larger.
- Treating kilowatts and kilowatt-hours as interchangeable. One is a rate of power and the other is an amount of energy.
- Forgetting that appliances differ enormously in rate. A phone charger and an oven both use electricity, but at wildly different speeds.
- Reading a label wattage as average consumption. Many appliances cycle on and off, so the real figure is lower.
Further questions
Sources and standards
The joule is the SI unit of energy, and the calorie has more than one definition in use. This page uses the thermochemical calorie of 4.184 J exactly; NIST SP 811 lists the factors for the international table calorie and the several BTU variants.
- BIPM — The International System of Units (SI Brochure)
- NIST Special Publication 811 — Guide for the Use of the SI
Joules to calories, kWh to joules and kW to hp
One calorie is 4.184 joules, so 1 calorie to joules is a fixed factor; 1 food calorie to kJ is 4.184 kJ, because a food calorie is a kilocalorie. 1 kWh to joules is 3.6 million, since an hour is 3,600 seconds and a kilowatt is 1,000 watts.
| From | To | Value |
|---|---|---|
| 1 calorie | joules | 4.184 J |
| 1 food calorie | kilojoules | 4.184 kJ |
| 1 kWh | joules | 3,600,000 J |
| 1 hp | kW | 0.7457 kW |
| 1 kW | hp | 1.341 hp |
kVA to kW
kVA and kW are not interchangeable without a power factor: kW = kVA × power factor. A generator rated 10 kVA at a power factor of 0.8 delivers 8 kW, while the same 10 kVA at unity power factor delivers 10 kW. That is why UPS and generator labels quote both numbers.
The dedicated pages are joules to calories, kW to hp and hp to kW.