Convert energy units instantly — Joules, kilowatt-hours, BTU, calories, electronvolts and more. Click any row in the conversion table to set it as the target unit.
All Energy Units — Click a row to select as target
Unit
Name
Value
About Energy Units
Energy is the capacity to do work. The SI unit is the Joule (J), but many practical units exist across different fields: kWh for electricity billing, BTU for heating/cooling in the US, calories for food, and eV for atomic physics. All these units measure the same physical quantity — they're just scaled differently for convenience in their respective domains.
⚡ SI & Metric Units
The Joule (J) is the SI base unit. Multiples: 1 kJ = 1,000 J, 1 MJ = 1,000,000 J. The kilowatt-hour (kWh) = 3.6 MJ is standard for electricity. The erg (CGS unit) = 10⁻⁷ J was the older scientific unit. The electronvolt (eV) = 1.602×10⁻¹⁹ J is used in atomic and particle physics.
🇺🇸 Imperial & Dietary Units
The BTU (British Thermal Unit) = 1,055.06 J is used in US heating and cooling systems. The calorie (cal) = 4.184 J is the heat to raise 1 g of water by 1°C. The kilocalorie (kcal) = 4,184 J is the food Calorie on nutrition labels. The therm = 105,480,400 J is used for natural gas billing. 1 ft·lb = 1.356 J in mechanical engineering.
= 8,368,000 J = 8.37 MJ = 2.324 kWh = 7,930 BTU Average adult daily energy requirement
Natural Gas (1 Therm)
1 therm = 105.48 MJ
= 29.3 kWh = 100,000 BTU = 25,200 kcal Approx. 100 cubic feet of natural gas
Car Crash (60 mph)
~500 kJ = 500,000 J
= 0.139 kWh = 119,570 cal = 474 BTU KE = ½mv² for 1,500 kg at 27 m/s
Who Uses Energy Conversion?
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Electrical Engineers
Convert between kWh (billing), joules (circuit analysis) and MJ (large-scale systems). Essential for battery capacity, solar panel output and grid calculations.
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HVAC Engineers
US engineers use BTU for heating/cooling loads. Converting BTU to kW or kJ is essential when using international equipment datasheets and efficiency calculations.
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Nutritionists & Dietitians
Convert food energy between kcal (nutrition labels), kJ (used in Europe/Australia) and Joules for scientific calculations in metabolism research.
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Physicists & Chemists
Convert between eV (atomic scale), erg (CGS) and joules (SI). Reaction enthalpies in kJ/mol need conversion to BTU/lb or kcal/mol for chemical engineering.
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Renewable Energy Engineers
Convert solar panel output (kWh/day), battery storage (kWh or MJ) and grid energy (MWh) for system sizing, ROI calculations and energy independence planning.
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Physics Students
Convert between SI (J, kJ) and practical units (kcal, kWh) for thermodynamics, mechanics and electromagnetism problems. Energy = force × distance requires Joules.
Mental Math Tips
kWh → MJ: multiply by 3.6
1 kWh = 3.6 MJ exactly (1,000 W × 3,600 s). Quick: 10 kWh × 3.6 = 36 MJ. Reverse: MJ ÷ 3.6 = kWh. A phone battery (~15 Wh = 0.015 kWh) holds about 54 kJ = 0.054 MJ.
kcal → kJ: multiply by 4.18
1 kcal ≈ 4.18 kJ (exact: 4.184 kJ). A 500 kcal meal ≈ 2,090 kJ ≈ 2.09 MJ. To go from kJ to kcal: divide by 4.18. European food labels show kJ; US labels show kcal (Calories).
BTU ≈ 1 kJ (rough guide)
1 BTU = 1.055 kJ ≈ 1 kJ for quick estimates. A 10,000 BTU AC unit ≈ 10 MJ/hour cooling capacity. More precisely: 1 BTU = 0.293 Wh, so 3,412 BTU = 1 kWh.
The Joule (J) is the SI unit of energy. It equals the work done when a force of 1 Newton moves an object 1 metre, or the energy dissipated as heat when 1 amp flows through 1 ohm for 1 second. 1 J = 0.000278 Wh = 0.239 cal = 0.000948 BTU. A typical LED bulb uses about 10 J per second (10 W).
1 kilocalorie (kcal) = 1,000 calories (cal). The 'Calorie' (capital C) on food nutrition labels is actually a kilocalorie. So a 200 kcal snack = 200,000 small calories = 836,800 J. In Europe, food labels show both kcal and kJ (1 kcal = 4.184 kJ).
Multiply by 3,600,000: 1 kWh = 3,600,000 J = 3.6 MJ. This is because 1 kW = 1,000 J/s and 1 hour = 3,600 seconds. Example: a 50 kWh EV battery holds 50 × 3.6 = 180 MJ of energy. To convert Joules to kWh: divide by 3,600,000.
A BTU (British Thermal Unit) is the energy needed to raise 1 pound of water by 1°F. 1 BTU = 1,055.06 J ≈ 0.293 Wh ≈ 252 calories. BTUs are used in the US for air conditioners and furnaces. A 12,000 BTU (1-ton) AC can remove 12,000 BTU/hour of heat ≈ 3.5 kW.
1 thermochemical calorie = 4.184 J. 1 kcal (food Calorie) = 4,184 J. To convert calories to joules: multiply by 4.184. Example: 100 cal × 4.184 = 418.4 J. To convert joules to calories: divide by 4.184.
Energy is the total work done, measured in Joules or kWh. Power is the rate of energy transfer, measured in Watts (W = J/s). If a 100W bulb runs for 1 hour: energy = 100W × 3,600s = 360,000 J = 0.1 kWh. Energy = Power × Time. Your electricity bill charges for energy (kWh), not power.
An electronvolt (eV) is the kinetic energy gained by an electron accelerating through 1 volt of potential difference. 1 eV = 1.602 × 10⁻¹⁹ J. Used in atomic and particle physics where Joules are impractically large. Visible light photons have energies of about 1.8–3.1 eV; X-rays are 100 eV to 100 keV.