Amps to watts formula
DC or AC single-phase: watts = voltage × amps (AC with a power factor other than 1: watts = voltage × amps × power factor). AC three-phase: watts = √3 × line-to-line voltage × amps × power factor.
- 15 amps at 120V, single-phase: 120 × 15 = 1,800 W
- 20 amps at 120V: 120 × 20 = 2,400 W
- 10 amps at 240V: 240 × 10 = 2,400 W
- 10 amps at 12V DC: 12 × 10 = 120 W
- 30 amps at 208V three-phase, PF 0.9: √3 × 208 × 30 × 0.9 = 9,727 W
How many watts is 15 amps? How many watts is 20 amps?
At 120V, 15 amps is 1,800 watts and 20 amps is 2,400 watts. Those are the full-load (100%) numbers; for a load running 3 or more hours, the NEC's general continuous-load guidance caps a 15A circuit at 12A (1,440W) and a 20A circuit at 16A (1,920W).
Volts to watts (with amps)
"Volts to watts" always needs the current too -- power = voltage × current, so you can't get watts from voltage alone. Once you know the amps a device draws (from its nameplate or a meter), multiply by the voltage (and power factor, for an AC motor or other inductive load) to get watts, exactly as the calculator above does.
15A and 20A circuit wattage, with the continuous-load guidance
| Circuit | Full-load watts (120V) | 80%-continuous watts (120V) |
|---|---|---|
| 15A | 1,800 W | 1,440 W |
| 20A | 2,400 W | 1,920 W |
The 80% figure is the NEC's general continuous-load guidance (210.19(A)(1)(a) / 210.20(A)) for a load running 3 hours or more, not a hard cap on every load. Check your local code and a licensed electrician for a real installation.
Common amp draws converted to watts
| Amps | Watts @ 120V | Watts @ 240V |
|---|---|---|
| 5 A | 600 W | 1,200 W |
| 10 A | 1,200 W | 2,400 W |
| 15 A | 1,800 W | 3,600 W |
| 20 A | 2,400 W | 4,800 W |
| 25 A | 3,000 W | 6,000 W |
| 30 A | 3,600 W | 7,200 W |
| 40 A | 4,800 W | 9,600 W |
| 50 A | 6,000 W | 12,000 W |
Single-phase AC, power factor 1 (purely resistive). Use the calculator above for DC, three-phase or a different power factor.
More electrical & energy tools
- Watts to amps calculator: the reverse conversion, plus "how many amps is 1500 watts".
- Voltage drop calculator: check how much voltage a wire run loses at that current.
- Electricity cost calculator: turn watts and hours of use into a dollar cost.
Frequently asked questions
How many watts is 15 amps?
At 120V (a standard US household circuit), 15 amps is 1,800 watts (15 × 120). But a 15A circuit shouldn't be loaded past 80% continuously (12A, or 1,440 watts) for a load running 3+ hours -- see the breaker note below.
How many watts is 20 amps?
At 120V, 20 amps is 2,400 watts (20 × 120). The 80%-continuous-load guidance caps a 20A circuit at 16A (1,920 watts) for a continuous load.
How do I convert amps to watts?
For DC or AC single-phase: watts = volts × amps (× power factor for AC with a power factor other than 1). For AC three-phase: watts = √3 × line-to-line volts × amps × power factor. Example: 10 amps at 240V, single-phase, power factor 1: 240 × 10 = 2,400 watts.
Is "amps and voltage to watts" the same formula as "volts to watts"?
Yes -- watts always needs both volts and amps (power = voltage x current); "volts to watts" just means you already have the amps and are solving watts = volts x amps, exactly what this calculator does. There's no way to get watts from voltage alone without knowing the current.
Does DC use the same formula as AC?
DC uses the simplest form: watts = volts × amps, with no power factor. For example, 10 amps at 12V DC (a common solar/RV/automotive voltage) is 12 × 10 = 120 watts.
How is three-phase different?
Three-phase adds a √3 (about 1.732) factor because power is split across three conductors: watts = √3 × line-to-line volts × amps × power factor. Example: 30 amps at 208V line-to-line, power factor 0.9: √3 × 208 × 30 × 0.9 = 9,727 watts.
What's a safe continuous wattage for a 15A or 20A circuit?
The NEC's general continuous-load guidance (210.19(A)(1)(a) / 210.20(A)) says not to load a breaker past 80% for a load running 3+ hours. That's 12A (1,440W at 120V) on a 15A circuit, and 16A (1,920W at 120V) on a 20A circuit. Short-duration loads can use the full rating; check your local code and an electrician for anything borderline.
What is power factor and does it change the answer?
Power factor (PF) is 1 for purely resistive AC loads (heaters, incandescent bulbs) and typically 0.8-0.9 for motors and other inductive loads. A lower PF means fewer watts for the same amps and voltage, since not all the current does useful work. Leave PF at 1 for a resistive-load estimate, or enter the value from the load's nameplate for a motor or other inductive device.
Is amps the same as watts?
No -- amps measure current (the rate of charge flow) and watts measure power (the rate of energy use). They're related by voltage: watts = volts × amps, which is what this calculator solves.
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