Energy Unit Converter
Convert between Joules, Calories, kWh, BTU, and more energy units. Perfect for physics, nutrition, and engineering calculations.
Why There Are So Many Energy Units
The units above all measure the same underlying thing, the capacity to do work or produce heat, but they were standardized by completely different fields at completely different scales. An electronvolt (eV) describes the energy of a single electron crossing a one-volt gap, a number so small it needs scientific notation to write down. A kilowatt-hour describes a full hour of a running appliance, a number large enough to show up on a monthly bill. Both are "energy." Neither unit is remotely convenient for the other's job.
That mismatch in scale is exactly why so many separate units survived instead of the world settling on just Joules: nutrition labels in kilocalories, electricity bills in kilowatt-hours, US heating and cooling systems in BTU, and particle physics in electronvolts, each picked the unit that keeps its everyday numbers in a readable range.
Every Unit in Joules, and Where It's Used
The exact factor this converter uses for each unit. From an electronvolt to a kilowatt-hour is about 25 orders of magnitude.
| Unit | 1 unit in joules | Used for |
|---|---|---|
| Therms (100,000 BTU) | 105,505,600 | US and older UK natural gas bills |
| Kilowatt-hours (kWh) | 3,600,000 | Electricity and gas bills, EV batteries |
| Megajoules (MJ) | 1,000,000 | Fuel energy content, gas calorific values |
| Kilocalories (kcal) | 4,184 | Food "Calories" in the US and elsewhere |
| Watt-hours (Wh) | 3,600 | Batteries, power banks, laptops |
| BTU | 1,055.056 | US heating, cooling and appliances |
| Kilojoules (kJ) | 1,000 | Food labels in the EU, Australia and New Zealand |
| Calories (cal) | 4.184 | Chemistry (small calorie) |
| Foot-pounds (ft·lbf) | 1.355818 | US mechanical work, torque-related energy |
| Joules (J) | 1 | SI unit; physics and engineering |
| Ergs | 1.000000e-7 | Older CGS physics texts |
| Electronvolts (eV) | 1.602177e-19 | Atomic and particle physics |
The electronvolt's factor is exact, because the elementary charge has been a defined constant since the 2019 SI redefinition. The BTU and therm use the International Table BTU (1,055.056 J).
How much energy is a 200-Calorie snack, really?
A food label's "200 Calories" is 200 kilocalories. Here's what that is in other units.
Worked Example: A Gas Bill in kWh
A UK gas meter shows 120 cubic metres used this month, but the bill charges per kWh. Suppliers convert using the gas's calorific value, printed on the bill, and a standard volume correction factor of 1.02264.
Step 1: Volume × correction × calorific value gives megajoules
Step 2: Divide by 3.6, because 1 kWh = 3.6 MJ
About 1,346 kWh. US gas bills in therms convert more simply: 1 therm is 29.31 kWh, so 50 therms is about 1,465 kWh. Use the converter above with Therms → Kilowatt-hours to check your own bill.
Frequently Asked Questions
What is the difference between calories and Calories?
Lowercase "calorie" (cal) is the scientific unit. Uppercase "Calorie" (Cal) in food labels actually means kilocalorie (kcal), which is 1000 calories. So 1 food Calorie = 1 kcal = 1000 cal.
Is a calorie always 4.184 joules?
Almost. Food labels and this converter use the thermochemical calorie, exactly 4.184 J, which is also the factor EU labelling law uses to turn kcal into kJ. Older engineering tables sometimes use the International Table calorie, 4.1868 J. The difference is under 0.1%.
How much is a kWh worth?
One kWh is the energy used by a 1000-watt device running for 1 hour. It's equivalent to 3.6 million Joules or about 860 kilocalories. The average US home uses roughly 30 kWh a day, and many European homes use much less.
What is an electronvolt, and why use such a tiny unit?
An electronvolt (eV) is the energy gained by an electron moving through a 1-volt potential, about 1.602×10⁻¹⁹ J. It stays useful precisely because it's tiny: particle and atomic physics deal with energies at that scale, and expressing them in Joules would mean writing out negative exponents on every single number instead of just one.
Is energy the same thing as power?
No, and this is the mix-up behind most unit confusion. Energy is a total amount, like the fuel in a tank. Power is a rate, energy per unit time, like how fast that fuel is being burned. A kilowatt-hour is energy; a kilowatt on its own is power. This converter only handles energy units, not power units like watts or horsepower.
Why is the BTU based on pounds and Fahrenheit instead of the metric system?
Because it was standardized in a period and region using Imperial units, the same reason the ton of refrigeration and Rankine scale exist. It has stuck around in US HVAC, heating, and construction industries specifically because so much existing equipment, code, and documentation is already written in BTU.
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