Energy and the joule
The joule is the SI unit of energy. One watt-hour equals exactly 3,600 joules because a watt is one joule per second. A kilowatt-hour therefore equals 3.6 million joules.
Calories and food labels
This tool uses the thermochemical calorie: one calorie equals exactly 4.184 joules. A food Calorie, written with a capital C in common usage, is a kilocalorie and equals 4,184 joules.
British thermal units
Several BTU definitions exist. This converter uses the International Table value of approximately 1,055.05585262 joules. Check the convention required by a specific heating or engineering document.
Follow one quantity through the units
A 2 kWh amount is 2,000 Wh. Multiplying by 3,600 J per Wh gives 7,200,000 J, or 7,200 kJ. Converting the result back to kWh should return 2. This check confirms both the factor and the direction. A surprising result often comes from confusing kW, a power rate, with kWh, accumulated energy, or from reading “calorie” when the source actually means a food kilocalorie.
Match precision to the source
A utility meter, nutrition label, heating specification, and laboratory measurement can follow different rounding and reporting rules. Preserve the original unit and significant digits in your notes, then state the converted unit explicitly. Do not infer device efficiency or usable heat from an energy-unit conversion; those require additional measured inputs. The underlying SI relationships and non-SI factors can be checked against the NIST conversion factors by quantity.
Energy and power are different: energy is an amount, while power is the rate of using or transferring energy. Appliance cost also depends on time, efficiency, and the applicable tariff.