Energy & Power Converter

Joules, watts, horsepower — one tool.

Convert between joules, calories, kilocalories, watt-hours, kilowatt-hours and horsepower.

How it works

Energy units are converted through joules: 1 calorie = 4.184 J, 1 kilocalorie = 4,184 J, and 1 kilowatt-hour = 3.6 million J. Horsepower is a unit of power (energy per second), equal to about 745.7 watts.

When it helps

Helpful for comparing food energy with electricity use, reading appliance labels, or physics homework.

Energy versus power

Energy is an amount of work or heat, measured in joules, calories or kilowatt-hours. Power is how fast that energy is used, measured in watts or horsepower. A 1,000-watt heater running for one hour uses 1 kilowatt-hour of energy.

Food labels use kilocalories (often written 'Calories' with a capital C), electricity bills use kilowatt-hours, and physics uses joules. They all measure the same thing, so you can compare a sandwich with a phone charge.

Worked examples

A 500 kcal meal contains 500 × 4,184 ≈ 2.09 million joules, which is about 0.58 kWh — roughly the energy a 60 W bulb uses in ten hours. A car with a 150 hp engine can deliver about 112 kW at full power.

A typical phone battery stores around 15 watt-hours, so a full charge is about 54,000 joules or roughly 13 kilocalories.

Frequently asked questions

Are food Calories the same as kilocalories?

Yes. The 'Calorie' on food labels is a kilocalorie.

Which horsepower is used?

Mechanical (imperial) horsepower, about 745.7 W. Metric horsepower is slightly less, about 735.5 W.

How do I estimate an appliance's electricity cost?

Multiply its power in kilowatts by the hours used to get kWh, then multiply by your price per kWh.

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