Energy Converter
Convert between joules, calories, BTU, kilowatt-hours, electron volts, and more energy units.
Quick Reference — 1 unit = ? Kilojoule (kJ)
| Unit | Value |
|---|---|
| Joule (J) | 0.001 |
| Kilojoule (kJ) | 1 |
| Megajoule (MJ) | 1000 |
| Calorie (cal) | 0.004184 |
| Kilocalorie / Food Calorie (kcal) | 4.184 |
| Kilowatt-hour (kWh) | 3600 |
| Watt-hour (Wh) | 3.6 |
| BTU (British Thermal Unit) | 1.05506 |
Formula
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Everyday Uses
Food energy
Calories versus kilojoules on nutrition labels — convert so Australian and US labels tell you the same story.
Utility bills
Your bill charges per kWh; your heater is rated in joules or BTU — convert to predict what running it actually costs.
Heating and cooling
Furnaces in BTU, radiators in watts, bills in kWh — convert to compare heating options like for like.
Physics homework
Joules, calories, electron-volts, kWh — move between energy units with the precision assignments demand.
Frequently Asked Questions
How many joules are in a kilowatt-hour, and how do you convert between them?
1 kWh = 3,600,000 joules (3.6 MJ). This is because 1 kW = 1,000 W = 1,000 J/s, and 1 hour = 3,600 seconds: 1,000 × 3,600 = 3,600,000 J. To convert kWh to joules: multiply by 3,600,000. To convert joules to kWh: divide by 3,600,000. A standard electric kettle (2 kW) running for 3 minutes uses 2,000 W × 180 s = 360,000 J = 0.1 kWh. Average UK home uses ~8–10 kWh per day.
What is the difference between a calorie (cal) and a Calorie (kcal) in food?
A small calorie (cal) = the energy needed to raise 1 gram of water by 1°C = 4.184 joules. A food Calorie (Cal, written with capital C) = 1 kilocalorie (kcal) = 1,000 small calories = 4,184 joules. Nutrition labels always use kilocalories. When someone says "a banana has 100 Calories," they mean 100 kcal = 418,400 joules. A typical adult daily intake of 2,000 kcal = 8,368,000 joules = 2.32 kWh — roughly the same energy as a laptop running for 3–4 days.
What is a BTU and where is it used?
1 BTU (British Thermal Unit) = 1,055.06 joules = 0.000293 kWh. Defined as the energy needed to raise 1 pound of water by 1°F. BTUs are widely used in HVAC (heating, ventilation, air conditioning) in the US and UK: air conditioners are rated in BTU/hour (a 12,000 BTU/h unit is called a "1-ton" AC). Furnaces and boilers are rated in BTU/h or BTU/day. Gas bills in the UK use therms (1 therm = 100,000 BTU = 29.3 kWh). Natural gas energy content ≈ 1,000 BTU per cubic foot.
How do joules, watts, and watt-hours relate to each other?
Joule (J) = 1 watt × 1 second = the base SI unit of energy. Watt (W) = 1 joule per second = unit of power (rate of energy use). Watt-hour (Wh) = 1 watt used for 1 hour = 3,600 joules. Kilowatt-hour (kWh) = 1,000 Wh = 3,600,000 J. Key relationship: Energy (kWh) = Power (kW) × Time (hours). A 100W light bulb running for 10 hours uses 1 kWh. An electric car battery of 75 kWh holds 270,000,000 joules of energy.
How does the electron volt (eV) compare to everyday energy units?
1 electron volt (eV) = 1.60218 × 10⁻¹⁹ joules — an incredibly tiny unit used in atomic and particle physics to describe the kinetic energy gained by an electron accelerated through 1 volt. 1 MeV (megaelectron volt) = 10⁶ eV = 1.60 × 10⁻¹³ J. 1 GeV = 10⁹ eV. For context: a photon of visible light carries about 1.8–3.1 eV; the proton rest mass energy ≈ 938 MeV; the Higgs boson mass ≈ 125 GeV. Despite being tiny, eV is the natural unit for atomic-scale energies where joules would require awkward scientific notation.
What is an erg, and how does it compare to a joule?
1 erg = 10⁻⁷ joules — a unit from the older CGS (centimetre-gram-second) system, defined as the energy needed to exert 1 dyne of force over 1 centimetre. It is vanishingly small in everyday terms: a single erg is roughly the kinetic energy of a mosquito in flight. Ergs still appear occasionally in older physics and astronomy literature (e.g. stellar luminosity in erg/s), but the joule has replaced it in virtually all modern (SI) scientific work.
What is a therm, and how does it relate to natural gas billing?
1 therm (US) = 100,000 BTU = 105.506 megajoules = 29.3 kWh — defined as the heat content of approximately 100 cubic feet of natural gas. Many US and UK utility bills measure gas consumption in therms (or hundred cubic feet, "CCF") rather than kWh. To convert a gas bill: therms × 29.3 = kWh equivalent, useful when comparing gas costs against electric heating costs on a like-for-like energy basis.
What does "ton of TNT" mean as an energy unit, and where is it used?
1 ton of TNT ≈ 4.184 gigajoules (4,184,000,000 J) — the approximate energy released by detonating one metric ton of trinitrotoluene explosive. It is a colloquial, order-of-magnitude unit used to describe very large energy releases: nuclear weapon yields (kilotons, megatons of TNT), asteroid impacts, volcanic eruptions, and major earthquakes. For comparison, the Hiroshima bomb released about 15 kilotons of TNT-equivalent energy (~6.3 × 10¹³ J), while a large hurricane can release the thermodynamic equivalent of thousands of megatons of TNT in latent heat over its lifetime.