Wattage Calculator
Watts = volts × amps. Solve for wattage, amps, or volts — and see watt-hours and kWh for however long the device runs each day.
Example: with Solve for Watts (W = V × A) · Voltage (volts) 120 · Current (amps) 10 · Power in watts (amps & volts modes) 1500 · Hours of use per day (for watt-hours) 3 → Result: 1,200 W.
Computed by the calculator below using its default values. Change any input to see your own numbers.
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Check it outHow to calculate watts, amps, and watt-hours
The watt formula is W = V × A: a 120 V outlet delivering 10 A supplies 1,200 W. Electricians usually flip it to find current — A = W ÷ V — so a 1,500 W space heater on a 120 V circuit draws 12.5 A, which is why it trips a 15 A breaker the moment anything else shares the circuit (code limits continuous loads to 80%, or 12 A).
To calculate watt-hours, multiply power by runtime: a 1,200 W load running 3 hours a day uses 3,600 Wh = 3.6 kWh, about $0.61 a day at $0.17 per kWh. One caveat for AC devices: W = V × A is exact for resistive loads like heaters, kettles, and incandescent bulbs; motors and electronics need a power factor, W = V × A × PF.
How it’s calculated
Power P = V × I (watts = volts × amps); current I = P ÷ V; voltage V = P ÷ I. Daily energy = P × hours, shown in watt-hours and kilowatt-hours (1 kWh = 1,000 Wh). These relations are exact for DC and resistive AC loads; for reactive AC loads (motors, electronics) multiply by the power factor: W = V × A × PF.
Results update as you type and are estimates, not professional advice — verify important decisions with a qualified professional.
Common mistakes
- Loading a circuit to 100% of the breaker — the NEC limits continuous loads to 80%, so a 15 A, 120 V circuit should carry at most 1,440 W continuously.
- Confusing watts with watt-hours — watts measure power right now; watt-hours measure energy used over time.
- Using W = V × A for motors and electronics without the power factor — apparent VA overstates real watts.
Frequently asked questions
How do you calculate watts?
Multiply volts by amps: W = V × A. A 120 V circuit carrying 15 A supplies 1,800 W.
How many amps is 1,500 watts?
Amps = watts ÷ volts. On 120 V, 1,500 ÷ 120 = 12.5 A; on a 240 V circuit it is only 6.25 A.
How do I calculate watt hours?
Multiply watts by hours of use: a 100 W TV running 5 hours uses 500 Wh, or 0.5 kWh. Utilities bill you per kWh.
Does watts = volts × amps work for AC power?
For resistive loads like heaters and kettles, yes. Motors, LEDs, and electronics draw reactive power, so real watts = volts × amps × power factor (often 0.5–0.95).