Energy Converter: J, kWh, Btu, calorie, eV & More
Convert energy between joule (J), kilowatt-hour (kWh), Btu, calorie, electron-volt (eV), and 50+ units. Whether you need kWh to J, Btu to kWh, calories to joules, or any SI, imperial, or specialty unit (therm, ton TNT, Hartree), this converter uses the SI base (joule) for accurate results. Free, runs in your browser—no sign-up required.
What is energy?
Energy is the capacity to do work or produce heat. The SI unit is the joule (J): 1 J = 1 N·m (one newton-meter). In everyday and engineering use you’ll see kilowatt-hour (kWh) for electricity (1 kWh = 3.6×10⁶ J), Btu (British thermal unit) for heating and cooling (1 Btu ≈ 1055 J), calorie or kilocalorie for food and chemistry (1 kcal ≈ 4186 J), and electron-volt (eV) in physics (1 eV ≈ 1.602×10⁻¹⁹ J). This converter supports 50+ units including SI prefixes (kJ, MJ, GJ), mechanical (foot-pound, kgf·m), thermal (Btu, therm, ton-hour refrigeration), TNT-equivalent (ton of explosives, kiloton, megaton), and atomic (eV, keV, MeV, Hartree, Rydberg).
Every value is normalized to joules under the hood, so you can convert from electric bills (kWh), heating specs (Btu), or physics (eV) to J or any other unit in one step.
How energy conversion works
To convert between any two energy units, we express both in joules (J). Multiply your value by the source unit’s factor (how many J one unit equals), then divide by the target unit’s factor:
Example: convert 1 kWh to kJ. 1 kWh = 3.6×10⁶ J = 3600 kJ. Another: convert 1000 Btu to kWh. 1000 Btu ≈ 1.055×10⁶ J ≈ 0.293 kWh. The tool above does this for all 50+ units.
Key energy units explained
Each unit’s factor to J. 1 kWh = 3.6×10⁶ J; 1 Btu (IT) ≈ 1055 J; 1 kcal (IT) ≈ 4186.8 J; 1 eV ≈ 1.602×10⁻¹⁹ J.
Joule (J)
Factor: 1 (SI base unit)
1 J = 1 N·m = 1 W·s. Used in physics, mechanics, and electrical definitions. 1 kWh = 3.6×10⁶ J.
Kilowatt-hour (kWh)
Factor: 1 kWh = 3.6×10⁶ J
Standard for electricity billing and appliance energy. 1 kW for 1 hour = 1 kWh.
Btu (IT)
Factor: 1 Btu ≈ 1055.06 J
British thermal unit. Used for HVAC, heating, and cooling capacity (e.g. BTU/h).
Kilocalorie (IT) [kcal]
Factor: 1 kcal ≈ 4186.8 J
Food “calorie” (Cal). 1 kcal = 1000 cal (IT). Used in nutrition and chemistry.
Electron-volt (eV)
Factor: 1 eV ≈ 1.602×10⁻¹⁹ J
Energy gained by one electron across 1 V. Common in particle and atomic physics (keV, MeV).
Foot-pound (ft·lbf)
Factor: 1 ft·lbf ≈ 1.35582 J
Mechanical work in US/UK. 1 lbf through 1 ft. Often used for torque in lb·ft—context distinguishes.
When to use J vs kWh vs Btu vs calorie
Joule (J) and its multiples (kJ, MJ) are the SI choice for physics and engineering calculations. kWh is the standard for electrical energy (bills, battery capacity, appliance labels). Btu is common in US HVAC, boilers, and cooling (Btu/h for power). Calorie/kcal is used in nutrition (food energy) and some chemistry. eV and MeV are used in particle and nuclear physics. Foot-pound and inch-pound appear in mechanical work in imperial contexts. Ton of explosives (and kiloton, megaton) are used for explosive energy. This converter includes all of these so you can switch between disciplines and regions.
Who uses an energy converter?
Electrical engineers and consumers convert kWh to J or Btu for comparison with thermal or mechanical energy. HVAC and building engineers work with Btu, ton-hour (refrigeration), and sometimes kWh. Physicists and chemists use eV, keV, MeV, Hartree, or Rydberg and need J or vice versa. Nutrition and food science use kcal (Calories) and may convert to kJ. Students and educators use it to check homework (e.g. “convert 1 kWh to joules”) and to relate formulas (in J) to real-world units. All conversions run in your browser—no data is sent to a server.
Why joule is the SI unit
Energy has the same dimension as work: force × distance. In SI, force is in newtons (N) and distance in meters (m), so the coherent unit is the joule (J): 1 J = 1 N·m. It is also 1 W·s (watt-second), so electrical energy in J is power in W times time in s. The joule is named after James Joule. The kWh, Btu, and calorie are accepted for use with SI in specific contexts but are not part of the coherent set. Using J keeps formulas (e.g. kinetic energy ½mv², or E = P×t) dimensionally consistent. This converter uses J as the internal base so every result is traceable to SI.
Common energy conversions at a glance
Handy reference for kWh to J, Btu to kWh, calories to joules, and other everyday conversions.
| From | To | Formula | Example |
|---|---|---|---|
| kWh | J | × 3.6×10⁶ | 1 kWh = 3.6×10⁶ J |
| Btu | kJ | × 1.055 | 1000 Btu ≈ 1055 kJ |
| kcal | kJ | × 4.1868 | 1 kcal ≈ 4.187 kJ |
| ft·lbf | J | × 1.35582 | 1 ft·lbf ≈ 1.356 J |
| eV | J | × 1.602×10⁻¹⁹ | 1 eV ≈ 1.602×10⁻¹⁹ J |
Avoiding common mistakes
Don’t confuse energy (joule, kWh, Btu) with power (watt, kW, Btu/h): energy is power × time. A 1 kW heater running 1 hour uses 1 kWh of energy. For rates, use the Power Converter. Don’t mix calorie (IT) with calorie (th)—they differ slightly (1 cal (th) ≈ 4.184 J, 1 cal (IT) ≈ 4.1868 J); this converter supports both. Ton of explosives (and kiloton, megaton) are units of energy (≈ 4.184 GJ per ton), not mass. When in doubt, convert everything to J with this tool, then apply your formulas.
Energy Conversion FAQ
? How do I convert kWh to joules?
Multiply kWh by 3.6×10⁶ (or 3 600 000). So 1 kWh = 3.6×10⁶ J. To convert J to kWh, divide by 3.6×10⁶. 1 kWh is the energy delivered by 1 kW in 1 hour.
? What is the SI unit for energy?
The joule (J). 1 J = 1 N·m = 1 W·s. Energy has the same dimension as work (force × distance). Common multiples are kJ (1000 J), MJ (10⁶ J), and GJ (10⁹ J). The kilowatt-hour (kWh) is accepted for use with SI for electrical energy.
? How do you convert Btu to kWh?
Multiply Btu (IT) by 0.000293071. So 3412 Btu ≈ 1 kWh. To convert kWh to Btu, multiply by 3412. 1 Btu ≈ 1055 J and 1 kWh = 3.6×10⁶ J, so 1 kWh ≈ 3412 Btu.
? How many joules are in a calorie?
1 calorie (IT) ≈ 4.1868 J. 1 kilocalorie (kcal, food “Calorie”) = 1000 cal ≈ 4186.8 J. Calorie (th) ≈ 4.184 J. This converter supports both IT and th so you can match nutrition or chemistry conventions.
? What is the difference between Btu (IT) and Btu (th)?
Btu (IT) uses the international steam table calorie; Btu (th) uses the thermochemical calorie. 1 Btu (IT) ≈ 1055.06 J; 1 Btu (th) ≈ 1054.35 J. The difference is small; HVAC and specs usually mean Btu (IT). This converter includes both.
? How do I convert electron-volt to joule?
Multiply eV by 1.602176634×10⁻¹⁹. So 1 eV ≈ 1.602×10⁻¹⁹ J. For keV multiply by 1.602×10⁻¹⁶; for MeV by 1.602×10⁻¹³. To convert J to eV, divide by 1.602×10⁻¹⁹ (or multiply by 6.242×10¹⁸).