Joules, kWh, and calories — the key energy units
Joules (J), kilojoules (kJ), and megajoules (MJ) are the SI units of energy. The kilowatt-hour (kWh) is the unit on your electricity bill and equals 3.6 MJ. With calories there are two sizes that must not be confused: the kilocalorie (kcal) on nutrition labels equals exactly 4184 J (4.184 kJ), while the small calorie (cal) is only 4.184 J — they differ by a factor of 1000. The BTU (British thermal unit) equals 1055.06 J and is used in heating and air-conditioning.
Energy conversions are exact and rest on fixed factors — for more unit pairs, use our energy conversion hub.