Energy Converter
Convert energy between joules and kilojoules, watt-hours and kilowatt-hours, calories and food Calories, BTU, foot-pounds, and electronvolts. The same quantity runs through an electricity bill, a nutrition label, and a physics problem under three different names, and this puts them on one scale.
Converters
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How it works
Energy, and the units that hide it
1 J = 1 W·s · 1 kWh = 3.6 MJ · 1 kcal = 4,184 J · 1 BTU = 1,055.06 J
Every unit here is a fixed multiple of the joule. Watt-hours come from power multiplied by time, calories from heating water, and BTU from heating a pound of it — different origins, one quantity.
- Kilowatt-hours are what an electricity meter counts, which is why the bill is in energy and not power.
- A nutrition label's Calorie (capital C) is a kilocalorie. Labels outside the US usually print kJ alongside it.
- The electronvolt is here for physics and chemistry work: 1 eV is 1.602176634 × 10⁻¹⁹ J, fixed exactly by the 2019 SI redefinition.
- Energy is not power. A 2 kW heater running for 30 minutes uses 1 kWh — the converter handles the units, not the multiplication by time.
For reference
Everyday amounts of energy
| What | kJ | kcal | kWh |
|---|---|---|---|
| 1 kilowatt-hour of electricity | 3,600 | 860.4 | 1.000 |
| 100 Calorie snack | 418.4 | 100 | 0.116 |
| Daily 2,000 Calorie diet | 8,368 | 2,000 | 2.324 |
| AA alkaline battery | 13.5 | 3.2 | 0.0038 |
| 1 BTU | 1.055 | 0.252 | 0.00029 |
The AA figure assumes roughly 2,500 mAh at 1.5 V; real capacity depends heavily on the load.
How to use
4 clear steps.
Enter the energy amount.
Select its unit — food Calories are listed as kilocalories.
Select the target unit and read the result.
Compare every unit at once in the full table.
Good to know
Frequently asked questions
- Is a food Calorie the same as a calorie?
- No. The Calorie on a nutrition label is a kilocalorie — 1,000 small calories, or 4,184 joules. A 500 Calorie meal is 500 kcal, which is 2,092 kJ.
- How many joules are in a kilowatt-hour?
- Exactly 3,600,000 J, because one watt is one joule per second and an hour is 3,600 seconds. That makes a kWh 3.6 MJ, or about 860 kcal.
- Which calorie definition does this use?
- The thermochemical calorie, 4.184 J, which is the one food labelling is based on. The International Steam Table calorie is 4.1868 J — a 0.07% difference that only matters in engineering tables.