Key facts
- 1 kWh = 3.6 million joules = 860.4 kcal — the unit on your electricity bill.
- 1 kcal = 4,184 J; food “Calories” (capital C) are kilocalories.
- 1 BTU = 1,055.06 J — the unit behind air-conditioner ratings.
- A 100 W bulb left on for 10 hours uses exactly 1 kWh.
Electricity bills, food labels, heaters and air conditioners all measure energy — in different units. This free energy converter translates between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours, BTU and electronvolts instantly.
Enter a value and pick your units. It is the fastest way to connect the physics (joules) with the bill (kWh) and the diet (kcal).
How to use
- Enter a value.
- Choose the from and to units.
- Read the answer and the full table of equivalents below.
How energy conversion works
| Term | Meaning |
|---|---|
| from-unit in joules | Each unit’s exact definition in joules. |
| to-unit in joules | The target unit’s definition in joules. |
A watt-hour is just power × time: 1 Wh = 1 watt for 1 hour = 3,600 joules.
Worked examples
The electricity bill
100 W bulb × 10 hours: 100 × 10 = 1,000 Wh = 1 kWh.
1 kWh → joules: 3,600,000 J (3.6 MJ); → kcal: 3,600,000 ÷ 4,184 = 860.4 kcal.
Food energy
500 kcal meal → kJ: 500 × 4.184 = 2,092 kJ.
That is about the energy a 70 kg person burns running for 35–40 minutes.
Energy units in joules
| Unit | Equals (J) | You meet it in |
|---|---|---|
| Joule (J) | 1 | Physics, SI base |
| Kilojoule (kJ) | 1,000 | Food labels (metric) |
| Calorie (cal) | 4.184 | Chemistry |
| Kilocalorie (kcal) | 4,184 | Food “Calories” |
| Watt-hour (Wh) | 3,600 | Batteries, small devices |
| Kilowatt-hour (kWh) | 3,600,000 | Electricity bills |
| BTU | 1,055.06 | Heating, air conditioning |
Frequently asked questions
What is the difference between kW and kWh?
kW (kilowatt) is power — the rate of energy use. kWh is energy — power × time. A 2 kW heater running 3 hours uses 6 kWh.
Are food Calories the same as kcal?
Yes. The “Calorie” on food labels (capital C) is the kilocalorie = 1,000 chemistry calories = 4,184 J.
What is a BTU?
British Thermal Unit — the energy to heat one pound of water by 1 °F (1,055.06 J). Air conditioners and heaters are rated in BTU per hour.
How much is 1 kWh in plain terms?
Running a 100 W bulb for 10 hours, or a 2 kW kettle for 30 minutes. At typical tariffs it costs roughly $0.10–$0.35 depending on country.
Why do food labels show kJ and kcal?
Metric countries quote kilojoules (the SI unit); kcal persists from older nutrition science. This converter switches between them instantly.
Key takeaways
- 1 kWh = 3.6 MJ = 860.4 kcal; 1 kcal = 4,184 J; 1 BTU = 1,055.06 J.
- kW is power (rate); kWh is energy (amount) — don’t mix them.
- Food “Calories” are kilocalories: divide kJ by 4.184 to get kcal.
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