Energy Converter

All of these measure the same quantity, and the joule is the SI unit against which the rest are defined. The variety exists because different fields settled on different scales before anyone unified them — chemistry took the calorie, electricity the kilowatt-hour, and heating engineering the BTU.

The calorie is the one that causes genuine confusion, because there are two. The scientific calorie is 4.184 joules, the energy to warm a gram of water by one degree. The food Calorie — capitalised, and what every nutrition label means — is a kilocalorie, a thousand of those. So a 250 Calorie snack is 250 kcal, or about 1,046 kJ, which is why European labels showing kilojoules produce such alarming numbers for the same food.

The kilowatt-hour is not a rate despite containing a rate. It is a power multiplied by a time, so a 1 kW appliance running for one hour uses one kWh — and it is the unit your electricity bill counts, which makes it the most economically significant energy unit most people encounter.

For a sense of scale: a kilowatt-hour is 3.6 million joules, roughly the energy in a large chocolate bar, and about what it takes to boil ten kettles of water.