How to Calculate Electric Power from Voltage and Current

Calculates electric power as P = voltage × current. Voltage is in volts, current in amperes, and power in watts (W).

Explanation

Electric power is the energy a device uses every second. Multiply the voltage by the current. It is the wattage printed on the label of a heater or a microwave.

P = V I

$P$ is the power, in watts (W)
$V$ is the voltage, in volts (V)
$I$ is the current, in amperes (A)

One watt is one joule per second, so 1 W = 1 J/s.

Combine this with Ohm's law ( $V = I R$ ) and you also get $P = I^2 R$ and $P = \frac{V^2}{R}$ , which are handy when you know the resistance and just one of the other two quantities.

Example

The defaults are a voltage of 100 V and a current of 2 A.

P = 100 \times 2 = 200\,\mathrm{W}

The power is 200 W: for every second the device runs, it uses 200 J of energy.

Notes

Power (W) and energy (Wh or kWh) are different things. Power is the rate; energy is the total that piles up over time. Your electricity bill is based on energy, not power.

This formula gives the power for direct current, or for alternating current in which voltage and current peak together, a power factor of 1. Motors pull the two out of step, and then the real power consumed is less than voltage times current.

Household breakers set a ceiling. On a 100 V circuit rated for 20 A, the limit is 2000 W, and drawing more trips the breaker.