Circuit breakers, fuses and battery packs are rated in amps, while appliances and generators are rated in watts. To check whether a circuit can run a load, or how much power a measured current represents, multiply current by voltage — and, for AC loads that are not purely resistive, by the power factor. This calculator does that for DC, single-phase and three-phase systems and shows the result in watts, kilowatts, kVA and horsepower, along with the 80% continuous-use figure.
How to use the amps to watts calculator
- Choose DC, AC single-phase or AC three-phase.
- Enter the current in amps — a measured load current or a circuit rating.
- Enter the voltage. For three-phase, choose line-to-line or line-to-neutral.
- For AC loads, enter the power factor: 1 for heaters and incandescent lights, about 0.8–0.9 for motors.
- Read the real power, apparent power, horsepower equivalent and the energy used in one hour.
Amps to watts formulas
Apparent power uses the same formulas without the power factor and is expressed in VA or kVA. For a three-phase supply measured line-to-neutral, replace √3 with 3.
Worked examples
A 15 A circuit at 120 V (the default)
P = 120 V × 15 A × 1 = 1,800 W (1.8 kW, about 2.4 hp of electrical power).
For loads that run three hours or more, use 80% of that: 1,440 W.
A 30 A, 240 V circuit such as a water heater or dryer: 240 × 30 = 7,200 W, or 5,760 W for a continuous load.
A three-phase load. 50 A at 480 V with a power factor of 0.85 gives √3 × 480 × 50 × 0.85 = 35.33 kW of real power from 41.57 kVA of apparent power.
Circuit capacity at a glance
| Circuit | Maximum watts | Continuous (80%) |
|---|---|---|
| 15 A, 120 V | 1,800 W | 1,440 W |
| 20 A, 120 V | 2,400 W | 1,920 W |
| 30 A, 240 V | 7,200 W | 5,760 W |
| 40 A, 240 V | 9,600 W | 7,680 W |
| 50 A, 240 V | 12,000 W | 9,600 W |
The 80% figure comes from the National Electrical Code’s rule that circuits serving continuous loads be rated at 125% of the load — the same requirement viewed from the other side. A Level 2 EV charger drawing 40 A, for instance, needs a 50 A circuit.
Measuring current correctly
If you measure current with a clamp meter, clamp around one conductor only; clamping a whole cable cancels the outgoing and returning currents and reads near zero. On AC, meters show RMS values, which is what these formulas expect. For loads with electronics, such as computers and LED drivers, the power factor may be 0.6–0.95, so multiplying volts by amps overstates the real watts — a plug-in power meter that reports watts directly is more accurate.
To go the other way, use the watts to amps calculator; for equipment rated in kVA, the kVA calculator. The electricity cost calculator turns watts and hours of use into a cost, and the power converter translates watts into BTU per hour or horsepower.
Calculations are for estimation only. Do not exceed breaker or receptacle ratings, follow your local electrical code, and have a licensed electrician add or modify circuits.
Frequently asked questions
How many watts is 15 amps?
At 120 V, 15 A × 120 V = 1,800 W. Because loads that run three hours or longer should stay at 80% of a circuit's rating, a 15 A household circuit is good for about 1,440 W continuously.
How many watts can a 20 amp circuit handle?
A 20 A, 120 V circuit carries up to 2,400 W, or 1,920 W for continuous loads. A 20 A circuit at 240 V doubles both figures: 4,800 W maximum and 3,840 W continuous.
What is the formula to convert amps to watts?
DC: P = V × I. Single-phase AC: P = V × I × PF. Three-phase AC: P = √3 × V × I × PF with the line-to-line voltage. With a resistive load the power factor is 1, so watts simply equal volts times amps.
What is the difference between watts and VA?
Watts measure real power, the energy actually converted to heat, light or motion. Volt-amperes (VA) are simply volts times amps, the apparent power the wiring must carry. They are equal only when the power factor is 1; otherwise VA is larger.