About Energy Converter
Convert between common energy-converter units instantly. Free online converter. Free, no sign-up required.
How to use
- Type the energy amount into the Value field. The default is set up for nutrition use (kilocalories to kilojoules), but any positive number works whether you are converting battery capacity in Wh or chemical bond energy in eV.
- Pick the source unit from the From dropdown. Available units: joules (J), kilojoules (kJ), calories, kilocalories (food Calories), watt-hours, kilowatt-hours, BTU, and electron volts.
- Choose the target unit in the To dropdown. The most useful pairings are kcal-to-kJ for food labels, kWh-to-BTU for HVAC sizing, and joules-to-eV for physics work — but any combination is valid.
- Use the swap button between the dropdowns to instantly reverse direction. This is the fastest way to verify a result by round-tripping through both units.
- Read the converted value in the highlighted box. Extremely small numbers (like joules expressed in eV) automatically switch to scientific notation so you keep all your significant figures.
- When working with food labels, watch for the calorie/Calorie trap: the “Calories” on a US nutrition label are actually kilocalories. A 2,000 Calorie daily diet means 2,000 kcal, or 8,368 kJ — the converter handles both naming conventions.
Examples
Daily diet to electricity
A 2,000 kcal daily diet equals 8,368 kJ or 2.32 kWh of metabolic energy. That is the same energy as running a 100 W bulb for 23 hours — a useful reminder that humans are roughly 100 W heat sources.
Natural gas billing
1 therm = 100,000 BTU ≈ 105.5 MJ ≈ 29.3 kWh. So a winter gas bill showing 80 therms used represents 8,440,000 BTU or 2,344 kWh — handy when comparing gas heating costs to an electric heat pump quoted in kWh.
Photon energy in eV
A green visible-light photon at 550 nm carries 3.61 × 10⁻¹⁹ J, which the converter expresses as 2.25 eV. Electron volts are the natural unit for atomic and chemical processes; joules are clumsy at that scale.
Frequently asked questions
How do I convert calories to joules?
Multiply small calories (cal) by 4.184 to get joules. Multiply food Calories or kilocalories (kcal) by 4,184 to get joules, or by 4.184 to get kilojoules. The factor 4.184 is the thermochemical calorie definition: the energy required to raise 1 gram of water by 1 °C at standard conditions. Other historical calorie definitions (15 °C calorie, 4 °C calorie) differ by tenths of a percent and are rarely used today.
What is the difference between a calorie and a kilocalorie?
1 kcal = 1,000 cal. The lowercase calorie (cal) is the small physics-textbook unit; the kilocalorie (kcal) is 1,000 of those. Confusingly, US food labels write “Calories” with a capital C to mean kilocalories — so a 200 “Calorie” snack actually contains 200,000 small calories or about 837 kJ. The rest of the world generally labels in kJ to avoid the confusion.
How do I convert kWh to BTU?
Multiply kWh by 3,412.142 to get BTU. So 10 kWh ≈ 34,121 BTU. One BTU is the energy needed to raise 1 pound of water by 1 °F at standard conditions. HVAC equipment is sized in BTU/h (often called just “BTU” colloquially), while electricity meters report in kWh. Converting between them is essential when comparing electric heat pumps (rated in kW or BTU/h) against gas furnaces (rated in BTU/h).
What does eV (electron volts) mean and when is it used?
1 eV = 1.602 × 10⁻¹⁹ J — the energy gained by one electron crossing a 1-volt potential. It is the standard unit for atomic, molecular, and particle physics because typical processes happen at 1-10 eV scales. Visible light photons are a few eV; chemical bond energies are 1-10 eV; X-ray photons are keV (thousand eV); particle accelerator beams reach TeV (trillion eV).
How much energy is in a kilowatt-hour?
1 kWh = 3,600,000 J = 3.6 MJ = 3,412 BTU = 860.4 kcal. It is the energy a 1,000-watt device uses in one hour, or a 100 W bulb in 10 hours. Residential electricity in the US averages around 11-30 cents per kWh depending on region, making it the natural unit for household energy economics. Tesla Model 3 batteries store 50-82 kWh; a single charge equals 180-295 MJ.
Can I convert energy to power or vice versa?
Not directly — energy and power are different physical quantities. Energy (J, kWh, BTU) is total work; power (W, kW, hp) is energy per unit time. They are related by time: energy = power × time. A 1,000 W heater running for 1 hour uses 1 kWh; a 100 W bulb running for 10 hours also uses 1 kWh. Use the Time Converter alongside this tool when converting between rated power and total energy use.
Why does this converter show electron volts when I am working with everyday energy?
The eV unit is included because visitors often arrive looking up battery cell chemistries, photovoltaic photon energies, or atomic spectra — all natively in eV. If you are only working with household-scale energy (kWh, BTU, kcal), you can safely ignore eV. Conversely, if you are deep in physics or chemistry, joules and kilojoules will be far more useful than the everyday units.
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