Energy converter

Convert to joules, then to the target unit. A daily diet of 2,000 kcal: 2,000 × 4,184 = 8,368,000 J, which is 8,368 kJ — and divided by 3,600,000, about 2.3 kWh. A full day of food holds roughly as much energy as running a 1,000-watt microwave for two hours and twenty minutes.

Because nutrition labels in many countries write “Calorie” or “Cal” for what is really a kilocalorie. The small calorie, once defined by heating one gram of water by one degree Celsius, is 4.184 J; the food Calorie is a thousand of them. European Union labels avoid the confusion by law: Regulation 1169/2011 requires energy in both kilojoules and kilocalories.

There are also two small calories. The thermochemical calorie used here is exactly 4.184 J; the International Table calorie used in some engineering is 4.1868 J. The 0.07% difference only shows in the fourth digit.

A kilowatt is power, the rate at which energy is used; a kilowatt-hour is energy, the amount used. A 2 kW heater running for three hours uses 6 kWh. Electricity is billed in kilowatt-hours, which is why a kWh — one kilowatt sustained for one hour — equals 1,000 W × 3,600 s = 3.6 million joules exactly.

Out of habit from the American heating and cooling trade. A British thermal unit was the heat needed to warm one pound of water by one degree Fahrenheit; this site uses the International Table value, 1,055.056 J. Air conditioners are rated in BTU per hour, which is a power, and a 12,000 BTU/h unit — one “ton of refrigeration” — removes about 3.5 kW of heat. At the other end of the scale, the electronvolt measures the energy of single particles: exactly 1.602176634 × 10⁻¹⁹ J since the 2019 SI revision.