Energy Calculator
Calculate energy from power and time in joules, kilowatt-hours, and kilocalories.
How it works
- 1Enter the power in watts and the duration. Choose seconds, minutes, or hours — the tool converts to seconds automatically.
- 2The formula E = P × t multiplies power by time to give energy in joules (the SI unit of energy).
- 3Results are shown in joules and converted to kilowatt-hours and kilocalories for everyday comparison.
Use cases
- Estimate how much electricity an appliance uses (e.g. a 1500 W heater for 2 hours = 3 kWh).
- Calculate the energy a battery or power supply delivers for sizing and selection.
- Convert between energy units for coursework, engineering reports, or nutrition comparisons.
Frequently asked questions
What formula does the energy calculator use?
It uses E = P × t, where E is energy in joules (J), P is power in watts (W), and t is time in seconds (s). One joule equals one watt sustained for one second (1 J = 1 W·s).
How does a joule relate to kWh and kcal?
One kilowatt-hour (kWh) equals 3,600,000 J — the energy a 1000 W device uses in one hour. One kilocalorie (kcal, the food Calorie) equals 4184 J.
Can you walk through a worked example?
A 100 W bulb running for 1 hour: t = 3600 s, so E = 100 × 3600 = 360,000 J. That is 360,000 ÷ 3,600,000 = 0.1 kWh, or about 86 kcal.
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