Energy Converter

Calorie with a capital C

The calorie of chemistry is the energy needed to warm one gram of water by one degree Celsius — about 4.184 joules. The Calorie on a nutrition label is a kilocalorie: a thousand of those. A 2000 Calorie daily intake is 2,000,000 small calories, or 8.368 megajoules. European labels sidestep the whole mess by printing kilojoules first, which is why a chocolate bar shows “1100 kJ / 263 kcal”.

Your electricity bill is an energy bill, not a power bill

A kilowatt is a rate; a kilowatt-hour is an amount. Running a 2 kW heater for 30 minutes uses 1 kWh. Utilities bill for kWh because that is what they actually deliver, and one kWh is exactly 3.6 megajoules. A single kWh is roughly the energy in 860 food Calories — about a large restaurant meal, which is a useful sanity check on how much energy a human body actually handles.

BTU and therms: the heating units

A BTU is the energy to raise one pound of water by one degree Fahrenheit. It survives in American heating, air conditioning and gas billing. A therm is exactly 100,000 BTU, and US gas bills are usually in therms or CCF. Air-conditioner capacity quoted as “12,000 BTU” is really 12,000 BTU per hour — a power rating with the “per hour” dropped, equal to about 3.5 kW. The missing time unit causes constant confusion.

Why there is more than one BTU

Because water's heat capacity varies slightly with temperature, several BTU definitions exist that differ by a fraction of a percent. This converter uses the International Table BTU (1055.05585262 J), the one used in engineering and energy billing.

Frequently Asked Questions