Electrical Power: How to Calculate Watts from Voltage and Current
Electrical power in watts is voltage times current: a device drawing 5 A at 120 V uses 600 W. The same relation, rearranged, tells you the current a known load pulls from a circuit, which is what decides whether a breaker holds, a cable stays cool and a power supply copes. This guide covers the formula, a worked example, the sizing rules that follow from it, and the AC case where power factor matters.
What Electrical Power Is
Power is the rate at which electrical energy is converted into heat, light, motion or sound, measured in watts (one joule per second). A 60 W bulb uses energy three times as fast as a 20 W one.
Almost every part of an electrical system is rated by power or limited by current. Cables, switches and connectors overheat above a certain current; supplies and transformers are sold by wattage; breakers and fuses trip at a current which, with the voltage, sets a power ceiling. Knowing the power a load draws is the first step in choosing parts that are safe rather than guessed.
How to Calculate Electrical Power
For a DC circuit, or an AC circuit with a purely resistive load:
Power (W) = Voltage (V) × Current (A)
Voltage is the electrical pressure pushing charge through the circuit; current is the rate of charge flow. Their product is the rate of energy transfer.
Worked example. A device on a standard North American supply:
- Voltage: 120 V
- Current: 5 A
The Electrical Power Calculator multiplies voltage by current in the same way.
Rearranging. Current = Power ÷ Voltage. A 600 W device on 120 V draws 600 ÷ 120 = 5 A; the same device on a 230 V supply would draw 2.6 A. This is the check for whether a load fits a circuit's amp rating. If you know resistance rather than current, Ohm's law supplies the missing piece: V = I × R, and combining the two gives P = V² ÷ R = I² × R. The Ohm's Law Solver finds voltage, current or resistance from the other two.
Sizing Breakers, Cables and Supplies
Breakers. A 15 A breaker on 120 V can carry 120 × 15 = 1,800 W. For loads that run three hours or more, the U.S. National Electrical Code's continuous-load rule limits the circuit to 80% of the breaker rating, so plan on 12 A, or 1,440 W, for a space heater or similar. Add up the wattage of everything on the circuit; a 1,500 W heater and a 1,200 W kettle together draw 22.5 A on 120 V, which is why that combination trips a 15 A breaker.
Cables. A conductor's current rating, not its wattage, is what matters: excess current becomes heat in the copper. Size the cable for the current the load actually draws, with margin.
Power supplies. Total the power of everything the supply must drive and choose a unit with 20–30% headroom so it is not running flat out continuously.
Energy cost. Power is a rate; energy is power × time. A 600 W device running 10 hours uses 6 kWh. The Electrical Power Consumption Calculator turns a device's wattage, daily hours and your electricity rate into a monthly cost.
Common Mistakes
Confusing power with energy. Watts are a rate; watt-hours and kilowatt-hours are energy. A high-wattage device used briefly can use less energy than a low-wattage one left on all day.
Ignoring power factor on AC loads. P = V × I gives true power only for DC and resistive AC loads (heaters, incandescent lamps). For motors, transformers and many electronic supplies, voltage and current fall out of step and V × I is the apparent power in volt-amperes (VA); the real power in watts is V × I × power factor, where the power factor is the ratio of real to apparent power, between 0 and 1. The Three Phase Power Calculator includes a power-factor input for industrial loads.
Using the wrong voltage. Mains voltage is about 120 V in North America and 230 V in much of Europe. The wrong figure throws current calculations off by almost a factor of two.
Leaving no headroom. Sizing a fuse, cable or supply to exactly the calculated figure leaves nothing for surges, aging or a slightly larger load.
The Wire Size Calculator takes the current from this calculation to the next step.