Energy Converter

Convert Joules, calories, kWh, BTU, eV, and more energy units.

Click any card to use that unit as input.

Joule
J
SI base unit of energy
Kilojoule
kJ
1 kJ = 1,000 J
Calorie
cal
Small calorie (thermochemical)
Kilocalorie
kcal
= 1 food Calorie (kcal)
Watt-hour
Wh
1 Wh = 3,600 J
Kilowatt-hour
kWh
Electricity billing unit
BTU
BTU
British Thermal Unit
Foot-pound
ft·lb
Mechanical energy unit
Electronvolt
eV
Atomic/particle physics
Real-World Energy Contexts
1 food Calorie (kcal)
4.2 kJ = 1.0 kcal = 1.162×10^-3 kWh
Energy in a dietary Calorie — calories on nutrition labels
1 kWh (kilowatt-hour)
3.60 MJ = 860.4 kcal = 1.000×10^0 kWh
Running a 1,000 W heater for 1 hour — standard electricity unit
Avg daily human metabolism
8.37 MJ = 2000.0 kcal = 2.324×10^0 kWh
About 2,000 kcal/day consumed by an average adult
1 litre of petrol (gasoline)
34.20 MJ = 8174.0 kcal = 9.500×10^0 kWh
Energy content of approximately 1 litre of petrol
Lightning bolt (typical)
1.0 GJ = 239005.7 kcal = 2.778×10^2 kWh
Around 1 GJ, though most is lost as heat and light
Hiroshima bomb (approx)
63000.0 GJ = 15057361376.7 kcal = 1.750×10^7 kWh
Equivalent to ~15 kilotons of TNT
Conversion Formulas
1 kJ = 1,000 J
1 kcal = 4,184 J
1 cal = 4.184 J
1 Wh = 3,600 J
1 kWh = 3,600,000 J
1 BTU = 1,055.056 J
1 ft·lb = 1.355818 J
1 eV = 1.602 × 10⁻¹⁹ J
Note on scales: Electronvolts (eV) are used in atomic physics — 1 eV is extraordinarily tiny (1.6 × 10⁻¹⁹ J). Food Calories (written with capital C) = kilocalories (kcal), not small calories (cal). kWh is the practical unit for electrical energy — check your electricity bill.

Frequently Asked Questions

What is the difference between a calorie and a kilocalorie?

A small calorie (cal) is the energy needed to raise 1 gram of water by 1°C. A kilocalorie (kcal) = 1,000 cal. Food Calories (capital C on nutrition labels) are actually kilocalories — a "200 Calorie" snack contains 200 kcal = 200,000 small calories = 836.8 kJ. Always clarify which "calorie" is meant.

How many joules are in a kilowatt-hour?

1 kilowatt-hour (kWh) = 3,600,000 joules (3.6 MJ). A kWh is the energy used by a 1,000 W appliance running for 1 hour. It's the standard billing unit for electricity. A typical home uses 10,000–30,000 kWh per year depending on climate and size.

What is a BTU and when is it used?

A BTU (British Thermal Unit) is the heat needed to raise 1 pound of water by 1°F. 1 BTU ≈ 1,055 joules. BTUs are used in the US to rate HVAC systems (air conditioner and furnace capacity), water heaters, and gas appliances. A typical 2,000 sq ft home might need a 36,000 BTU/hr (3-ton) air conditioner.

What is an electronvolt and where is it used?

An electronvolt (eV) is the energy gained by an electron accelerating through 1 volt of electric potential. 1 eV = 1.602 × 10⁻¹⁹ joules — an extremely small unit. It's used in particle physics, chemistry, and semiconductor engineering where atomic-scale energies are involved. The energy of a photon of visible light is roughly 1.8–3.1 eV.