Joules and watt-hours
1 J = 1 watt for 1 second. 1 Wh = 3,600 J. 1 kWh = 3.6 MJ — the standard unit on electricity bills.
Energy conversion
Convert any energy,
instantly.
Joules, kilowatt-hours, calories, BTU and electronvolts — every common energy unit, side by side.
kWh
3.6
MJ1 kWh = 3.6 MJ
Quick to:
1 kWh in all units
| Joules | 3600000 | J |
| Kilojoules | 3600 | kJ |
| Megajoules | 3.6 | MJ |
| Watt-hours | 1000 | Wh |
| Megawatt-hours | 0.001 | MWh |
| Calories (cal) | 860420.65 | cal |
| Kilocalories (food cal) | 860.42065 | kcal |
| British Thermal Units | 3412.1416 | BTU |
| Therms (US) | 0.034129563 | thm |
| Foot-pounds | 2655223.7 | ft·lb |
| Electronvolts | 2.246943e+25 | eV |
About energy conversion
Energy is the capacity to do work. The SI unit is the joule (J), but utilities bill in kilowatt-hours, food labels use kilocalories, and HVAC systems are rated in BTU. They all measure the same physical quantity.
Calories
A small calorie is 4.184 J. A food Calorie (capital C) is actually a kilocalorie = 4,184 J. A 2,000 kcal/day diet equals about 8.4 MJ.
British Thermal Units
1 BTU ≈ 1,055.06 J. Air conditioners and gas appliances are commonly rated in BTU/h. 1 therm = 100,000 BTU ≈ 105.5 MJ — the unit on US natural-gas bills.
Electronvolts
1 eV ≈ 1.602 × 10⁻¹⁹ J. Visible light photons are around 2–3 eV; chemical bonds a few eV; nuclear reactions millions of eV (MeV).
Frequently asked questions
- How many joules are in a kilowatt-hour?
- 1 kWh = 3,600,000 J (3.6 MJ). It's the energy used by a 1 kW appliance running for one hour.
- How do I convert calories to joules?
- Multiply by 4.184. A food calorie (kcal) is 1,000 small calories, so 1 kcal = 4,184 J.
- What is a BTU?
- A British Thermal Unit is the energy needed to raise one pound of water by 1 °F — about 1,055.06 J. Common in HVAC and US heating ratings.
- How big is an electronvolt?
- 1 eV ≈ 1.602 × 10⁻¹⁹ J — the energy gained by an electron crossing a 1-volt potential. Used throughout atomic and particle physics.