energy · electrical conversion
Amps to Watts Calculator
Convert amps to watts — or watts to amps — for DC, single-phase AC and three-phase AC. Pick a circuit, enter two numbers, get the answer with the working shown.
the formula
Watts = Amps × Volts for DC and resistive AC. For AC with motors or electronics, multiply by the power factor: W = A × V × PF. Three-phase adds √3 (≈ 1.732): W = A × V × PF × √3. So 15 amps at 120 volts is 1,800 W — and a 20-amp, 120-volt circuit tops out at 2,400 W (1,920 W continuous).
your circuit
The breaker rating or measured amperage of the load.
The power draw in watts — from the appliance nameplate or a kill-a-watt meter.
Line voltage. North America: 120 V or 240 V. Europe/Asia: 230 V. Batteries: 12 or 24 V.
Three-phase needs √3 in the formula; single-phase AC can carry a power factor below 1.
1.0 for heaters and bulbs, 0.8–0.95 for motors and electronics. Leave at 1 if unknown.
Or start from a standard circuit
result
Power draw
1,800 W
15 A × 120 V = 1,800 W
Continuous safe load (80% rule)
1,440 W
NEC 210.19/210.20: loads running 3+ hours stay under 80% of the breaker rating.
Current draw
15.0 A
15 A at 80% = 12.0 A continuous
W = 15 A × 120 V = 1,800 W
how the math works
The amps-to-watts formula, three ways
One multiplication for DC, one extra factor for AC power factor, one more for three-phase. Here is each variant with the numbers the calculator uses.
DC and resistive AC: W = A × V
Batteries, solar panels, heaters, incandescent bulbs and kettles convert current to power without a phase gap, so watts are simply amps times volts. A 12 V circuit pulling 15 A delivers 180 W; a 120 V space heater on a 12.5 A draw is 1,500 W — the rating on its nameplate.
AC with motors and electronics: W = A × V × PF
Motors, compressors, chargers and anything with a transformer draw current slightly out of phase with the voltage. The power factor (PF) expresses how much of the apparent power (VA) is real power (W). Nameplates list PF; if yours does not, 0.9 is a safe household estimate and 1.0 is never far wrong for a rough check. A 120 V air conditioner rated 10 A with PF 0.9 draws 1,080 W real power, not the 1,200 VA its plug would suggest.
Three-phase: W = A × V × PF × √3
Three live conductors deliver power in overlapping waves, so a three-phase circuit carries √3 (≈ 1.732) times the power of a single-phase circuit at the same amps and volts. This is why a 50 A, 240 V three-phase welder outlet supports far more than a 50 A, 240 V single-phase one: 50 × 240 × 0.9 × 1.732 ≈ 18,700 W versus 10,800 W.
| Current | Voltage | Type | PF | Formula | Result |
|---|---|---|---|---|---|
| 15 A | 120 V | AC 1-phase | 1.0 | 15 × 120 | 1,800 W |
| 20 A | 120 V | AC 1-phase | 1.0 | 20 × 120 | 2,400 W |
| 15 A | 240 V | AC 1-phase | 1.0 | 15 × 240 | 3,600 W |
| 10 A | 230 V | AC 1-phase | 0.9 | 10 × 230 × 0.9 | 2,070 W |
| 20 A | 240 V | AC 3-phase | 0.9 | 20 × 240 × 0.9 × 1.732 | 7,482 W |
| 100 A | 240 V | AC 1-phase | 1.0 | 100 × 240 | 24,000 W |
How many watts can a standard circuit handle?
The breaker rating multiplied by the voltage is the hard ceiling; the continuous-load column is the number that matters for anything that runs for hours — space heaters, EV chargers, water features. The National Electrical Code (NEC 210.19 and 210.20) requires continuous loads to stay at or under 80% of the breaker rating.
| Breaker | Voltage | Typical use | Max watts | Continuous (80%) |
|---|---|---|---|---|
| 15 A | 120 V | General outlets | 1,800 W | 1,440 W |
| 20 A | 120 V | Kitchen, bathroom, garage | 2,400 W | 1,920 W |
| 30 A | 240 V | Dryer, water heater | 7,200 W | 5,760 W |
| 40 A | 240 V | Range, heat pump | 9,600 W | 7,680 W |
| 50 A | 240 V | Range, RV pedestal, EV charger | 12,000 W | 9,600 W |
| 100 A | 240 V | Whole-home service | 24,000 W | 19,200 W |
Why the 80% rule exists
A breaker protects the wire, not the appliance. Wire heats with the square of the current, and heat builds over time: a load at 95% of rating might never trip the breaker on a cold day and still cook the insulation inside a wall over a winter of continuous use. The 80% continuous-load rule keeps wiring temperature inside its rated envelope. Short bursts — a kettle, a toaster, a power tool — may use the full breaker rating; anything running 3 hours or more may not.
Related tools and guides
- Portable Generator Wattage Sizing Calculator — size a generator from an appliance checklist, in watts and amps.
- Appliance Electricity Bill Calculator — turn the watts you just calculated into a monthly cost.
- Amps vs Watts vs Volts — what each unit actually measures, with the water analogy done properly.
- Amps to Watts Chart — every common circuit from 1 to 200 amps, 120 V and 240 V, print-ready.
- How Many Watts Can a Circuit Handle? — the breaker-by-breaker answer with the 80% rule worked through.
- Energy tools category — every calculator on Daily Utility Lab.
faq
Amps to watts, answered
How many watts is 15 amps?
1,800 watts at 120 volts: watts = amps × volts, so 15 × 120 = 1,800 W. On a 240-volt circuit the same 15 amps is 3,600 W. For continuous loads longer than 3 hours, the NEC 80% rule caps a 15-amp circuit at 1,440 W.
How do I convert amps to watts?
Multiply amps by volts. For DC and resistive AC loads the result is watts directly: W = A × V. For AC motors and most household electronics, multiply by the power factor too: W = A × V × PF. For three-phase, add √3 (≈ 1.732): W = A × V × PF × √3.
What is the difference between VA and watts?
VA (volt-amperes) is apparent power — volts × amps as the circuit actually delivers them. Watts is real power — the part that does work. The ratio watts ÷ VA is the power factor (0 to 1). Resistive loads like heaters have PF ≈ 1, so VA ≈ W; motors and electronics often sit at 0.8–0.9.
How many watts can a 20 amp breaker handle?
2,400 watts at 120 volts (20 × 120). For continuous loads running 3 hours or more, the NEC 80% rule limits it to 1,920 watts. The breaker will eventually trip above 2,400 W, and sustained loads near the limit will heat the wiring.
Does three-phase power change the amps-to-watts formula?
Yes. Three-phase adds a factor of √3 (about 1.732): W = A × V × PF × √3, where V is the line-to-line voltage. So 20 amps at 240 volts three-phase with PF 0.9 is 20 × 240 × 0.9 × 1.732 ≈ 7,482 W — versus 4,320 W single-phase at the same amps and volts.