Energy Converter

The calorie problem

There are two calories. The small calorie is the energy to heat one gram of water by one degree Celsius, about 4.184 joules. The large Calorie, properly capitalised and also called the kilocalorie, is a thousand of those. Food labelling uses the large one, so a 500 Calorie meal is 500 kilocalories or about 2.1 million joules. European labels avoid the ambiguity by printing kcal, and this single unit collision causes more confusion than any other in everyday measurement.

Energy is not power

Power is the rate of energy transfer. A watt is one joule per second, so a kilowatt-hour is a rate multiplied by a time: 1,000 watts for one hour, which is 3.6 million joules. Electricity is billed in kWh because it measures the energy actually consumed, whereas the wattage on an appliance tells you only how fast it draws while running. A 2 kW heater run for 30 minutes uses 1 kWh.

BTU and the confusing shorthand

One British thermal unit is the energy to raise a pound of water by one degree Fahrenheit, about 1,055 joules. Air conditioning is rated in BTU per hour, but the per hour is almost always dropped in conversation and marketing, so a 12,000 BTU unit means 12,000 BTU/h, which is a rate, not a quantity. Confusing the two makes any sizing calculation meaningless.

The electronvolt

At the other extreme, the electronvolt is the energy an electron gains crossing one volt: about 1.602 times ten to the minus nineteen joules. It is used in particle physics because joules are absurdly large at that scale, in the same way pascals are inconveniently small for tyre pressure.

Practical reference points

A joule is roughly the energy to lift an apple one metre. A kilowatt-hour runs a modern fridge for about a day. A therm, used for gas billing, is 100,000 BTU. Keeping one anchor in each system makes conversions easier to sanity-check.