Energy Converter | Joules, kWh, BTU & Calories
Convert between 15 energy units: joules, kJ, MJ, GJ, Wh, kWh, MWh, cal, kcal, BTU, therm, ft·lb, eV, keV, MeV. Live results organized by category with smart formatting and NIST-sourced conversion factors.
Enter a value above or click a preset to see all energy unit conversions.
What Is the Energy Converter | Joules, kWh, BTU & Calories?
Energy is the capacity to do work or transfer heat. The SI unit is the joule (J), defined as the work done by a force of one newton through a distance of one metre: 1 J = 1 N·m = 1 kg·m²·s⁻². All other energy units are fixed multiples of the joule, so converting between them is a simple matter of multiplying by the right factor.
Different fields settled on different units before SI standardisation, and all remain in common use. Electricians and utilities use kilowatt-hours (kWh) because power meters count watt-hours directly. Nutritionists use kilocalories (kcal) because early 19th-century calorimetry measured heat in terms of water temperature rise. HVAC engineers in the United States use BTU because British imperial units dominated American industry. Particle physicists use electronvolts (eV) because accelerators measure particle energies by the voltage through which they were accelerated.
The conversion factors used in this tool are sourced from NIST SP 811 (Guide for the Use of the International System of Units) and NIST CODATA 2018 for the electronvolt. The electronvolt is now exact under the 2019 SI redefinition: 1 eV = 1.602176634 × 10⁻¹⁹ J (exact). The thermochemical calorie (4.184 J, exact) differs from the International Table calorie (4.1868 J), this converter uses the thermochemical definition, which is standard in chemistry and biology.
Formula
| Unit | Symbol | Joules (exact / NIST) | Primary use |
|---|---|---|---|
| Joule | J | 1 (definition) | SI base unit of energy |
| Kilojoule | kJ | 1 × 10³ | Food labels (many countries), chemistry |
| Megajoule | MJ | 1 × 10⁶ | Industrial energy, gas billing (AU/UK) |
| Gigajoule | GJ | 1 × 10⁹ | Large-scale energy, oil equivalent |
| Watt-hour | Wh | 3,600 | Small batteries, portable devices |
| Kilowatt-hour | kWh | 3,600,000 | Electricity billing worldwide |
| Megawatt-hour | MWh | 3.6 × 10⁹ | Grid-scale storage, large generation |
| Calorie (thermo.) | cal | 4.184 (exact) | Chemistry, physics (water heat) |
| Kilocalorie | kcal | 4,184 | Food energy (nutrition labels) |
| BTU (Int'l Table) | BTU | 1,055.05585262 | HVAC, natural gas, US engineering |
| Therm (US) | therm | 105,480,400 | US natural gas billing (100,000 BTU) |
| Foot-pound | ft·lb | 1.3558179483 | Torque / mechanical work (US) |
| Electronvolt | eV | 1.602176634 × 10⁻¹⁹ (exact) | Atomic physics, semiconductor band gaps |
| Megaelectronvolt | MeV | 1.602176634 × 10⁻¹³ | Nuclear & particle physics |
How to Use
- 1Enter your value: Type any number into the Value field. Decimals, negatives, and scientific notation (e.g. 1.5e6) are all supported.
- 2Select the from unit: Choose your input unit from the grouped dropdown: SI Energy, Electrical, Thermal/Chemical, Mechanical, or Atomic/Quantum.
- 3Try a preset: Click a preset (1 food Cal, 2000 kcal/day, 1 kWh, 1 BTU, 1 therm, 1 MeV) to load a common energy value instantly.
- 4Read all conversions: All 15 equivalent values appear instantly, organized by category. Very large or very small values are shown in scientific notation automatically.
- 5Switch direction: Click any result row to make that unit the new "from" unit. The converter immediately recalculates from that value, no extra steps.
- 6Copy individual values: Click the "copy" button on any row to copy that specific value and unit abbreviation to your clipboard.
- 7Reset: Click Reset or press Esc to clear the input and return to defaults. Your last inputs are saved to browser storage and restored on your next visit.
Example Calculation
Example 1: Food energy, 2000 kcal daily diet
Example 2: Electricity billing, 1 kWh
Example 3: Particle physics, 1 MeV proton kinetic energy
Understanding Energy Converter | Joules, kWh, BTU & Calories
Why So Many Energy Units?
Every major energy unit was developed in a specific historical and scientific context, long before international standardisation. The calorie was defined in 1824 to describe heat absorbed by water, it predates the concept of energy conservation. The BTU was defined in the 1870s to characterise the performance of steam boilers in the American industrial revolution. The kilowatt-hour emerged with the commercial electricity industry in the 1880s. The electronvolt was coined in the 1920s as particle accelerators came online.
Today, the joule is the official SI unit and should be preferred in scientific writing. But every legacy unit remains entrenched in its home field, both because practitioners have physical intuition for those scales and because equipment, textbooks, and regulations are written in those units. Fluency in converting between them is a practical skill for anyone working across engineering, physics, nutrition, or energy policy.
Energy in Context: A Scale Reference
- ›1 eV: energy of a visible-light photon (~2 eV for green light). Bond energies in chemistry are a few eV.
- ›1 cal (4.2 J): energy to raise 1 gram of water by 1 °C. A single peanut contains about 5 kcal.
- ›1 Wh (3.6 kJ): small battery energy. A AA battery holds roughly 3 Wh.
- ›1 kcal (4.18 kJ): 1 food Calorie. A banana ≈ 90 kcal; a slice of pizza ≈ 300 kcal.
- ›1 kWh (3.6 MJ): electricity billing unit. Running a 1000 W kettle for one hour uses 1 kWh ≈ $0.10–0.20.
- ›1 therm (≈ 105.5 MJ): US natural gas billing. Heating a home for a cold night uses roughly 1–3 therms.
- ›1 GJ: annual energy budget of an energy-efficient home in a mild climate. A barrel of oil contains about 6.1 GJ.
The Calorie Confusion, cal vs kcal vs Cal
The word "calorie" is used in three different ways, which causes persistent confusion:
- ›Small calorie (cal): the thermochemical calorie; 4.184 J exactly. Used in chemistry to measure heat of reaction. Rarely appears in everyday contexts.
- ›Large calorie / kilocalorie (kcal): 1000 small calories = 4184 J. This is the unit used on food labels in most of the world.
- ›Food Calorie (Cal): written with a capital C to distinguish it from the small calorie; numerically identical to 1 kcal. The USA uses "Calories" on nutrition facts labels, always meaning kcal.
The practical rule: when reading a food label anywhere, the number shown is always in kilocalories, whether it says "kcal", "Cal", "kJ", or just "calories." This converter uses the thermochemical calorie (4.184 J) for both cal and kcal.
BTU and the HVAC Industry
The British Thermal Unit (BTU) is defined as the heat required to raise one pound of water by one degree Fahrenheit. The International Table BTU (used in this converter) equals exactly 1055.05585262 J per NIST SP 811. HVAC systems in the United States are still rated in BTU/hour (sometimes written BTU/h or BTUH), a 12,000 BTU/h unit is commonly called a "one ton" air conditioner (derived from the cooling power of melting one ton of ice over 24 hours). The US also uses therms for natural gas billing: 1 therm = 100,000 BTU = 1.054804 × 10⁸ J.
Electronvolts in Physics and Engineering
The electronvolt (eV) is the energy gained by one elementary charge moving through a potential difference of one volt. Since the 2019 SI redefinition, the elementary charge is exact (e = 1.602176634 × 10⁻¹⁹ C), so 1 eV = 1.602176634 × 10⁻¹⁹ J exactly. This value is sourced from NIST CODATA 2018 (physics.nist.gov).
- ›Photons: a green photon has energy ≈ 2.3 eV; X-ray photons have 1–100 keV.
- ›Semiconductor band gaps: silicon 1.12 eV, GaAs 1.42 eV, determines what wavelengths a material absorbs or emits.
- ›Particle physics: proton rest mass ≈ 938 MeV; LHC collisions reach 13 × 10¹² eV (13 TeV) per proton.
- ›Nuclear reactions: uranium-235 fission releases ≈ 202 MeV per event, about 80 TJ per kilogram.
Energy Conversion Efficiency
Converting between energy units is always lossless, you are just changing the scale. Converting between energy forms (chemical → electrical → mechanical) always involves real losses, governed by the second law of thermodynamics. Common real-world efficiencies:
| Conversion | Typical efficiency | Note |
|---|---|---|
| Coal / gas → electricity (power plant) | 35–45% | Remaining energy is waste heat |
| Natural gas → heat (furnace) | 80–98% | Modern condensing boilers: ~98% |
| Electricity → heat (resistance heater) | 100% | Electrically perfect; expensive at grid rates |
| Electricity → heat (heat pump) | 200–400% | COP × input = output (moves heat, not creates it) |
| Gasoline → motion (car engine) | 20–40% | EVs achieve 85–95% electrical to mechanical |
| Food energy → mechanical work (human) | ~25% | Rest lost as body heat |
| Solar cell → electricity | 15–25% | Crystalline silicon at standard test conditions |
Frequently Asked Questions
What is the difference between a calorie (cal) and a Calorie (kcal)?
This is one of the most common sources of confusion in nutrition and science:
- • Small calorie (cal): 4.184 J. The amount of heat needed to raise 1 gram of water by 1 °C. Used in chemistry.
- • Kilocalorie (kcal): 4,184 J = 1,000 small calories. The energy to heat 1 kg of water by 1 °C.
- • Food Calorie (Cal with capital C): exactly 1 kcal. This is what nutrition labels always mean, in the US, the word "calories" on food packaging always means kilocalories.
So when a banana is labelled "90 calories," it means 90 kcal = 376,560 J. This converter treats kcal as the food Calorie standard.
How is a kilowatt-hour (kWh) related to joules?
A kilowatt-hour is power × time: 1 kW used for 1 hour.
1 kWh = 1000 W × 3600 s = 3,600,000 J = 3.6 MJ
- • Electricity bills charge per kWh because meters measure watt-hours directly.
- • 1 kWh ≈ 860 kcal, roughly the energy in a substantial meal.
- • 1 kWh ≈ 3412 BTU, used when comparing electricity to gas heating costs.
- • An average US home uses about 900 kWh per month (≈ 3.24 GJ).
What is a BTU and when is it used?
BTU stands for British Thermal Unit, the heat needed to raise one pound of water by one degree Fahrenheit.
1 BTU (Int'l Table) = 1,055.056 J
- • HVAC: air conditioners and heaters are rated in BTU/h. A "12,000 BTU" unit is called one "ton" of cooling.
- • Natural gas: gas usage in the US is billed in therms (100,000 BTU) or MCF (thousand cubic feet ≈ 1,020,000 BTU).
- • Boilers and furnaces: still rated in BTU/h in American engineering standards.
The BTU used here is the International Table BTU (1055.05585262 J) per NIST SP 811, the standard definition used in engineering since 1956.
What is an electronvolt (eV) and why is it used in physics?
An electronvolt is the energy gained by a single electron when it accelerates through a potential difference of one volt:
1 eV = 1.602176634 × 10⁻¹⁹ J (exact since 2019 SI redefinition)
At the atomic and subatomic scale, joules are inconveniently large. The eV gives naturally sized numbers:
- • Visible light photons: 1.8–3.1 eV
- • Silicon band gap: 1.12 eV (determines solar cell efficiency)
- • X-ray photons: 0.1–100 keV
- • Proton rest-mass energy: 938 MeV
- • LHC collision energy: 13 TeV (13 × 10¹² eV)
What is a therm and how is it used for natural gas billing?
A therm is a unit of heat energy equal to 100,000 BTU ≈ 105.48 MJ. It is the standard billing unit for natural gas in the United States.
- • Natural gas meters measure volume (cubic feet or cubic metres), which utilities convert to therms using the gas's heat content.
- • 1 therm ≈ 96 cubic feet of natural gas at average energy content.
- • US average price: roughly $1.00–1.50 per therm (varies by region and season).
- • A typical US household uses 50–75 therms per month in winter for heating.
In the UK, gas is billed in kWh (1 therm ≈ 29.3 kWh). In Australia and many other countries, natural gas is billed in MJ.
What are the conversion factors exact and where do they come from?
This converter uses NIST SP 811 (Guide for the Use of the International System of Units) and NIST CODATA 2018 as primary sources:
- • Thermochemical calorie: 4.184 J exactly, defined by the U.S. National Bureau of Standards in 1956.
- • BTU (International Table): 1055.05585262 J, the internationally agreed definition since 1956.
- • Electronvolt: 1.602176634 × 10⁻¹⁹ J exactly, fixed by the 2019 SI redefinition of the ampere.
- • Foot-pound: 1.3558179483314004 J, derived from the exact definition of the yard (1 yard = 0.9144 m).
- • Therm (US): 1.054804 × 10⁸ J exactly, defined by US law (15 USC § 206).
All SI unit multiples (kJ, MJ, GJ, kWh, MWh) are exact by definition.
How do I convert energy units manually without a calculator?
The method is always the same: convert to joules first, then convert to the target unit.
- Multiply the input value by the factor for the "from" unit to get joules.
- Divide the joule value by the factor for the "to" unit.
Example: convert 500 kcal to kWh
- • 500 kcal × 4184 J/kcal = 2,092,000 J
- • 2,092,000 J ÷ 3,600,000 J/kWh = 0.5811 kWh
Quick mental shortcuts: 1 kWh ≈ 860 kcal; 1 kWh ≈ 3412 BTU; 1 kcal ≈ 4.18 kJ.
Does this converter save my inputs between sessions?
Yes, your last value and selected unit are saved to your browser's localStorage:
- • The value and from-unit are persisted automatically as you type.
- • All data stays in your browser, nothing is sent to any server.
- • When you return, your last inputs are restored so you can continue where you left off.
- • Click Reset or press Esc to clear the form and delete the saved data.