Energy Converter

Convert between joules, kilojoules, calories, food calories, kilowatt-hours, BTU, electronvolts, therms, tonnes of TNT and more. This converter is useful for physics, food energy, electricity bills, heating values, engineering checks and quick comparisons between everyday and scientific energy units. Enter a value in any unit and all other units update in real time.

Convert energy

Choose a category or use all units together. The calculator converts your input into joules first, then converts that joule value into every other energy unit on the page. This keeps scientific, electrical, thermal and food-energy conversions consistent.

Quick references

Full conversion table

Enter an energy value above to see all conversions.

Food energy quick-reference

Common foods and their energy content in both kcal and kJ.

Real-world energy reference

Familiar events and objects — from a photon of light to a nuclear bomb.

kWh cost estimator

Estimate the electricity cost of any energy amount at your tariff.

Energy (kWh)

Rate (pence per kWh)

Estimated cost

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UK Ofgem price cap rate: ~24.5p/kWh (2025). Check your bill for your exact tariff.

Conversion formulas

How it works

This converter uses joules as the base unit. When you enter a value, the calculator first converts the input into joules, then divides by the joule value of each target unit.

Energy units are used in different contexts. Joules and kilojoules are common in science and engineering, kilowatt-hours are used on electricity bills, calories and kilocalories are used for food energy, BTU and therms are used for heating, and electronvolts are used in atomic and particle physics.

Formula used

energy in joules = input value × joule factor of input unit
converted value = energy in joules ÷ joule factor of target unit

Common reference values include: 1 kWh = 3,600,000 J, 1 kcal = 4,184 J, 1 Wh = 3,600 J, 1 BTU ≈ 1,055.06 J, 1 therm ≈ 105,480,400 J and 1 tonne of TNT = 4.184 × 10⁹ J.

Worked example

Example: convert 2 kWh to joules

A kilowatt-hour is 3,600,000 joules.

2 kWh × 3,600,000 = 7,200,000 J

So, 2 kWh is 7.2 MJ.

Example: convert 500 kcal to kJ

A food calorie, or kilocalorie, is 4.184 kJ.

500 kcal × 4.184 = 2,092 kJ

So, 500 kcal is about 2,092 kJ.

Common mistakes

Confusing calories and kilocalories

Food labels usually use kilocalories, often written as kcal or Calories with a capital C. One food Calorie equals 1,000 small calories.

Mixing power and energy

Kilowatts measure power. Kilowatt-hours measure energy. A 2 kW heater running for 3 hours uses 6 kWh.

Using kWh for instant power

kWh is not how fast energy is used. It is the total energy used over time. Use watts or kilowatts for power.

Rounding scientific values too early

Very small or very large units, such as electronvolts and TNT equivalent, may need scientific notation for sensible precision.

Common questions

How many joules are in 1 kWh?⌄

1 kilowatt-hour equals 3,600,000 joules, or 3.6 megajoules.

Is kcal the same as Calories on food labels?⌄

In everyday food labelling, Calories usually means kilocalories. So 1 food Calorie is 1 kcal, which equals 4,184 joules.

What is the difference between watts and watt-hours?⌄

Watts measure power, which is the rate of energy use. Watt-hours measure total energy used over time.

Why are electronvolts included?⌄

Electronvolts are useful in atomic, nuclear and particle physics because joules are often too large for energies at that scale.

Sources, assumptions and disclaimer

This converter is for general education, study, engineering checks and everyday comparisons. It uses standard fixed conversion factors where possible. Results are rounded for readability, so use appropriate significant figures for professional engineering, billing, scientific or safety-critical work.

Base unit: all conversions are handled through joules.
Food energy: kcal values are treated as food Calories, where 1 kcal = 4,184 J.
Electricity: 1 kWh is treated as 3.6 MJ, matching electricity-billing convention.
Heating units: BTU, MMBTU and therm values are approximate standard conversion references.
TNT equivalent: TNT is a comparison unit and should not be used for safety, explosives or engineering-risk calculations.
Disclaimer: this page does not replace calibrated instruments, official billing data, safety calculations, engineering standards or professional advice.