Convert between joules, calories, BTU, kilowatt-hours, and other energy units
Energy is the capacity to do work, and it comes in many forms—heat, electricity, motion, and chemical bonds. Different fields use different units: physicists use joules, nutritionists use kilocalories (food Calories), HVAC engineers use BTU, and electric utilities use kilowatt-hours. Understanding these conversions is essential for comparing energy sources, calculating efficiency, and managing consumption.
The joule (J) is the SI unit of energy, defined as the work done when a force of one newton moves an object one meter. Kilowatt-hours (kWh) measure electrical energy consumption—what appears on your electric bill. Calories measure heat energy, with the 'food Calorie' actually being a kilocalorie (1000 small calories).
This converter handles all common energy units from tiny electron-volts used in atomic physics to massive BTUs used in heating systems, making it useful for scientists, engineers, nutritionists, and homeowners alike.
SI unit of energy. 1 J = 1 kg·m²/s². Basis for all other energy conversions.
Food energy. 1 food Calorie = 1 kcal = 4,184 J. Daily intake: 2000-2500 kcal.
Utility billing unit. 1 kWh = 3.6 MJ. Average US home uses 900 kWh/month.
HVAC and heating. 1 BTU heats 1 lb water by 1°F. = 1,055 J.
| From | To | Multiply By | Application |
|---|---|---|---|
| Kilocalories | Kilojoules | 4.184 | Nutrition labels |
| kWh | BTU | 3,412 | HVAC efficiency |
| BTU | Joules | 1,055.06 | Heating calcs |
| Calories (small) | Joules | 4.184 | Chemistry |
| Foot-pounds | Joules | 1.356 | Mechanical work |
| Watt-hours | Joules | 3,600 | Batteries |
US uses Calories (kcal), EU uses kilojoules. 1 Calorie = 4.184 kJ. A 500 Cal meal = 2,092 kJ.
Electric bill shows kWh. A 60W bulb for 10 hours = 0.6 kWh. At $0.12/kWh = $0.07.
AC units rated in BTU/hour. 12,000 BTU/hr = 1 ton of cooling. Bigger isn't always better.
Batteries rated in Wh or mAh. Convert mAh to Wh: (mAh × voltage) ÷ 1000.
A small calorie (cal) heats 1 gram of water by 1°C. A food Calorie (Cal, capital C) equals 1 kilocalorie (1000 small calories). Nutrition labels use Calories (kcal), though they often just write 'calories' lowercase.
Your bill shows kWh. Multiply by 3,412 for BTU, by 3.6 for megajoules, or by 860 for kilocalories. A 1,000 kWh bill = 3.41 million BTU = 3,600 MJ.
The US uses Calories (kcal), while the EU requires kilojoules (kJ) and may also show kcal. This is part of metric vs. imperial differences. International products often show both: '200 kcal (838 kJ)'.
Average adult needs 2,000-2,500 kcal/day (8,400-10,500 kJ). This equals about 2.4-3 kWh of energy—enough to power a 100W light bulb for 24-30 hours.
A homeowner burned 80 therms of natural gas last month and wants to compare that to a heat pump's kWh consumption.
Result2,344 kWh
1 therm equals 100,000 BTU, which is 29.3 kWh. Multiply 80 therms by 29.3 to get 2,344 kWh of equivalent electrical energy at 100% efficiency.
Multiply kWh by 3,412 to get BTU. A 1.5 kW space heater running for one hour uses 1.5 kWh, equal to 5,118 BTU of heat output. For AC, manufacturers list BTU/hour ratings, so divide BTU/hr by 3,412 to estimate the electrical draw at 100% efficiency.
Nutritionists adopted the large calorie (kcal) because the small calorie (4.184 joules) is too tiny to be useful for meal sizes. One food Calorie equals 1,000 small calories, or 4,184 joules. A 500 Calorie meal therefore contains 2,092 kJ of chemical energy.
Joules, kWh, and watt-hours describe energy regardless of form, but kWh is conventional for electricity and BTU or therm for combustion heat. The units convert directly (1 kWh = 3,412 BTU), but real systems lose energy to heat, friction, or radiation, so input rarely equals useful output.
A refrigerator uses around 1-2 kWh per day, a microwave uses 1.2 kWh per hour of cook time, a clothes dryer uses 3-4 kWh per load, and a central AC uses 3-5 kWh per cooling hour. Check the yellow EnergyGuide label for annual kWh estimates.
A therm equals 100,000 BTU or about 29.3 kWh. US natural gas utilities bill in therms because it standardizes payment despite seasonal variations in gas heating value. One therm of gas at 95% furnace efficiency delivers roughly 27.8 kWh of usable heat.
One watt is one joule per second. A kilowatt sustained for one hour (3,600 seconds) equals 1,000 J/s times 3,600 s = 3,600,000 joules, or 3.6 MJ. This is an exact definition, not a measurement, so the conversion has no rounding error.
No. BTU measures energy, while BTU/hour measures power (rate of energy transfer). A 12,000 BTU/hour AC delivers 12,000 BTU of cooling each hour it runs. Confusing the two leads to oversizing equipment, which wastes electricity and shortens compressor life.
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