Energy Converter
Convert energy between joules, kilowatt-hours, calories, BTU, therms and electronvolts, with a full results table for every unit.
Energy Converter — input your values
What does this conversion calculator do?
The energy converter spans the units used in physics, nutrition, electricity billing and heating: joules and their multiples, watt-hours and kilowatt-hours, calories and kilocalories, BTU, therms and electronvolts.
All conversions pass through the joule as the SI reference base. This makes it easy to compare an electricity bill (kWh) with food energy (kcal) or a gas bill (therms) on a common scale.
Formula and variables
1 cal (thermochemical) = 4.184 J; 1 kWh = 3.6 MJ; 1 BTU ≈ 1,055 J.
| Symbol | Meaning |
|---|---|
| factor(from) | How many joules one 'from' unit equals |
| factor(to) | How many joules one 'to' unit equals |
Worked example
Convert 1 kilowatt-hour into joules.
- 1 kWh = 3,600,000 J because 1 kW = 1,000 J/s and 1 hour = 3,600 s.
Answer: 1 kWh = 3,600,000 J (3.6 MJ).
Tips and common mistakes
Tips
- Food labels list Calories with a capital C, which are kilocalories: 1 Cal = 1 kcal = 4,184 J.
- An electricity bill in kWh converts to joules by multiplying by 3,600,000 — useful for comparing with appliance ratings.
- One therm is a common gas billing unit equal to 100,000 BTU, or about 105.5 MJ.
- The electronvolt is tiny (1.602 × 10⁻¹⁹ J) — it is the natural energy unit for particle physics and quantum mechanics.
Common mistakes to avoid
- Confusing food Calories (kcal) with small calories (cal) — a factor of 1,000.
- Mixing up power (watts) with energy (watt-hours): power × time = energy.
- Using the old calorie definition — the modern thermochemical calorie is 4.184 J, while the international table calorie is 4.1868 J.
Energy Converter — frequently asked questions
How many joules are in a kilowatt-hour?
Exactly 3,600,000 joules (3.6 MJ), because one kilowatt is 1,000 joules per second over 3,600 seconds.
What is the difference between a calorie and a kilocalorie?
A kilocalorie is 1,000 calories. Food energy labeled in Calories actually refers to kilocalories: 1 dietary Calorie = 4,184 J.
What is a BTU?
A British Thermal Unit is the energy to heat one pound of water by one degree Fahrenheit, equal to about 1,055 J. It is used for heating and air-conditioning ratings in the US.
How do I convert kWh to calories?
Multiply by 860.421: 1 kWh = 3.6 MJ ÷ 4.184 J/cal ≈ 860,421 cal ≈ 860 kcal.
What is a therm?
A therm is 100,000 BTU, about 105.5 MJ. Natural gas is billed in therms in the US and in kilowatt-hours or megajoules elsewhere.
What is an electronvolt?
The energy gained by an electron moved through a potential difference of one volt — 1.602176634 × 10⁻¹⁹ J. It is the standard unit in particle physics.