Power and energy
Energy from voltage and current
- where
- energy (J)
- voltage (V)
- current (A)
- time (s)
Explain this equation
Works out energy used from voltage, current and time, in joules. Useful when you have readings rather than a power rating.
E is energy in joules. V × I is the power in watts. t is the time in seconds the load runs.
It is two steps in one: P = V × I, then E = P × t. Volts are joules per coulomb and amps × seconds are coulombs, so the units come out in joules.
Sense check: 12 V at 2 A for a minute is 24 W × 60 s = 1,440 J. Double any one input and the energy doubles.
Watch out: on AC, V × I is only true power for resistive loads. For inductive or electronic loads you need power factor too.
Combine P = VI with E = Pt. Find the electrical power first, then multiply by elapsed seconds.
For varying loads use the appropriate average power or sum energy over separate time intervals.
Enter your values to see the result.
Common mistake
Treating apparent AC power as real energy for every load.
Limits
Steady DC, or resistive AC with RMS voltage/current; time in seconds.
Sources
The tools calculate the displayed equation. They do not automatically choose cable sizes, approve installations, supply current regulatory tables, certify equipment or replace inspection and testing.