Power and energy
Energy in joules
- where
- energy (J)
- power (W)
- time (s)
Explain this equation
Works out energy used from power and time, in joules. Handy for heating and battery sums in SI units.
E is energy in joules. P is the power in watts and t is the running time in seconds.
A watt is one joule per second, so watts multiplied by seconds gives joules. Power is the rate; energy is the total built up over time.
Sense check: 1 kW for one minute is 60,000 J. 1 kW for an hour is 3.6 million J, which is 1 kWh.
Watch out: the time must be in seconds for joules. If the power varies, use the average over the period.
A watt is one joule per second. Multiply constant power by time in seconds to obtain joules.
Power describes the rate; energy describes the accumulated amount. A low-power device running for a long time can use substantial energy.
Enter your values to see the result.
Common mistake
Using hours and labelling the result joules.
Limits
Constant power, or average power for the period.
Sources
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