Converting energy between joules, calories and kWh
A snack wrapper says 250 Calories, a physics problem wants the answer in joules, and your power company bills you in kilowatt-hours. All three numbers can describe the exact same amount of energy — they're just different-sized buckets for measuring it.
The method
Pick joules as the common unit and convert everything through it. The fixed relationships: 1 kilocalorie (dietary "Calorie") = 4,184 joules, 1 calorie (small c, the chemistry one) = 4.184 joules, and 1 kilowatt-hour = 3,600,000 joules (because a kWh is 1,000 watts running for 3,600 seconds, and a watt is one joule per second: 1,000 × 3,600 = 3,600,000). To convert A to B: convert A to joules, then divide by B's joules-per-unit.
A real example: a 250-Calorie snack in kilojoules
A food label's "Calorie" is a dietary kilocalorie. Convert to joules: 250 × 4.184 = 1,046 joules... but nutrition labels in kJ use kilojoules, so divide by 1,000 the other way: 250 × 4.184 = 1,046 kJ directly, since 1 kcal = 4.184 kJ exactly. A 250-Calorie snack is 1,046 kJ — which is why EU labels often show "1046 kJ / 250 kcal" side by side.
A real example: a 1,000-watt microwave run for 3 minutes, in kcal
First get the energy in kWh: 1,000 watts is 1 kW, and 3 minutes is 3÷60 = 0.05 hours, so energy used is 1 × 0.05 = 0.05 kWh. Convert to joules: 0.05 × 3,600,000 = 180,000 joules. Convert to kcal: 180,000 ÷ 4,184 = 43.02 kcal. Running that microwave for 3 minutes uses about 43 kcal worth of electrical energy — for comparison, that's roughly the energy in half a cracker, which is a useful reminder that a kilowatt-hour is a lot of energy and a kilocalorie is a little.
A real example: a 30 kWh monthly appliance reading in joules
Multiply straight through: 30 × 3,600,000 = 108,000,000 joules, or 108 megajoules. Utility bills use kWh because 108 million of anything is an awkward number to put on an invoice — the unit was chosen for convenience, not because the underlying physics changed.
Why BTU still shows up on furnaces and boilers
The British Thermal Unit (1 BTU ≈ 1,055.06 joules) predates the metric system and stuck around in heating, cooling and US appliance ratings for the same reason Fahrenheit stuck around in US weather reports: industries standardize once and rarely switch. A furnace rated "80,000 BTU/hour" is describing a rate of energy delivery, not a one-time amount — that distinction matters, and it's covered in the power conversion below.
Run your own numbers
The free Energy Converter converts between joules, kilojoules, calories, kilocalories, watt-hours, kilowatt-hours and BTU instantly, entirely on your device.