Energy units

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Energy is the capacity to do work or to heat something, and different trades count it in different units: a nutrition label in kilocalories, an electricity bill in kilowatt-hours, a gas bill in therms, a boiler in BTU, a physicist in joules or electronvolts. They all measure the same thing, so any one converts to any other.

1 kWh in kJ // 3,600.00 kJ
2000 kcal in kJ // 8,368.00 kJ
1 therm in kWh // 29.31 kWh

Calories

A calorie comes in two sizes a thousand apart, and the spelling says which. The small calorie, cal, is roughly the energy that warms a gram of water by one degree. The food Calorie on a nutrition label is a thousand of those, a kilocalorie, and is written kcal, or Cal with a capital C as a label prints it.

2000 kcal in kJ // 8,368.00 kJ
250 Cal in kJ // 1,046.00 kJ
1 kcal in cal // 1,000.00 cal
1 kWh in kcal // 860.42 kcal

The words follow the same rule, because units are case-sensitive: calorie and calories in lower case are the small calorie, as a physics text writes them, and Calorie and Calories with a capital are the food Calorie. The everyday “calories” of a diet are kilocalories, so write a food figure as kcal or Cal; 2000 calories is two thousand small calories, about 8 kJ.

1 calorie in J // 4.18 J
2000 calories in kJ // 8.37 kJ
2000 Calories in kJ // 8,368.00 kJ

The calorie is the thermochemical one, exactly 4.184 joules, which is the size food energy is reckoned in. The slightly larger International Table calorie of engineering steam tables, 4.1868 joules, is not offered, so a calorie means one thing.

BTU and therms

The British thermal unit (BTU, or Btu as standards bodies write it) is about the energy that warms a pound of water by one degree Fahrenheit, and it rates boilers and furnaces. A therm (therm, therms) is 100,000 BTU and is the unit many gas bills charge in.

100000 BTU in kWh // 29.31 kWh
1 therm in BTU // 100,000.00 BTU
45 therms in kWh // 1,318.82 kWh

The BTU is the International Table one, exactly 1,055.05585262 joules, and the therm is 100,000 of those, as the UK and EU therm is. The US therm is built on an older BTU and is about 0.02% smaller, so a US gas bill converted here reads that much high.

Electronvolts

An electronvolt (eV) is the energy one electron gains crossing one volt, a tiny amount that suits atoms and particles. It is exactly 1.602176634 × 10⁻¹⁹ joules, and comes with the prefixes physics uses: keV, MeV and GeV.

1 eV in J // 1.6e-19 J
13.6 eV in J // 2.18e-18 J
1 MeV in keV // 1,000.00 keV

The boundary

An energy converts only to another energy: 2 kcal * 3 m and 2 kcal + 3 kg are refused by name rather than guessed at. Case matters throughout, so KCAL and CAL are names, not units. A BTU per hour, the unit an air conditioner is often rated in, is a power. Written BTU/h it is read as a rate of energy over time, which converts to another such rate but not to watts:

12000 BTU/h in kJ/h // 12,660.67 kJ/h

Energy that comes from multiplying other quantities, a power for a time or a charge at a voltage, is on named derived units and electricity.