Power Formula

Electrical power is voltage times current. For AC loads, multiply by the power factor to get real power in watts.

P = V × I × PF

What the terms mean

Symbol Meaning
P Real power, in watts
V Voltage, in volts
I Current, in amperes
PF Power factor (0 to 1)

Worked example

A driver at 120 V drawing 0.5 A with a 0.9 power factor uses 120 × 0.5 × 0.9 = 54 W.

Power factor counts. LED drivers with a low power factor draw more current than their watts suggest, which matters for circuit and breaker sizing.

See the Power Factor Calculator and Energy Consumption Formula.

The electrical power formula

Electrical power is voltage times current: P = V × I (watts = volts × amps). It tells you how much energy a device draws per second and is the starting point for sizing circuits, drivers, and wiring. Rearranged, it also gives current (I = P ÷ V) or voltage (V = P ÷ I) when you know the other two. For example, a 60W lamp on a 120V circuit draws 60 ÷ 120 = 0.5 amps.

For lighting, this is how you check a circuit isn’t overloaded: add the wattage of all fixtures, divide by the circuit voltage to get amps, and keep it safely below the breaker rating (a common rule is no more than 80% of the rating continuously). For DC and LED systems, the same P = V × I applies at the driver’s output.

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