Amps to Watts Calculator
Convert amps to watts or watts to amps for DC, AC single-phase and AC three-phase circuits, with power factor and the apparent power (VA) a wire or breaker actually carries. Amps is what sizes a wire and breaker, not watts alone, so verify any real wiring or breaker decision against the wire size calculator or a licensed electrician -- not this page by itself.
1.0 = resistive (heaters, incandescent); 0.8-0.95 = motors and AC/compressor loads.
Watts (real power)
1,800 W
Amps
15.00 A
Apparent power
1,800 VA
what the wire and breaker see
AC single-phase: W = V × A × PF → 120.00 V × 15.00 A × 1.00 = 1,800 W.
How it's calculated
DC: W = V × A
AC single-phase: W = V × A × PF
AC three-phase: W = √3 × V(L-L) × A × PF (√3 ≈ 1.732)
V is line-to-neutral for DC and single-phase, and line-to-line for three-phase -- the convention printed on three-phase equipment nameplates. PF (power factor) is 1.0 for a purely resistive load and drops for motors, compressors and switching supplies; the reverse direction (watts → amps) just divides instead of multiplies. These are standard AC/DC power relationships from basic electrical engineering, and the same three-phase and power-factor conventions the National Electrical Code uses for motor nameplate current and Article 220 load calculations.
Common amp ratings converted to watts (PF = 1.0)
Single-phase, resistive equivalent -- the ceiling for what a breaker of that size can deliver as real power. A real AC load at PF 0.85 draws about 18% more amps than this table implies for the same wattage, which is why a motor or AC nameplate should always be sized from its amps, not backed into from watts.
| Breaker / circuit | 120 V | 240 V |
|---|---|---|
| 15 A | 1,800 W | 3,600 W |
| 20 A | 2,400 W | 4,800 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 |
Frequently Asked Questions
How do I convert amps to watts?
For DC, or an AC load at a power factor of 1.0 (a plain resistance heater or incandescent bulb), watts = volts × amps. 15 A on a 120 V circuit at PF 1.0 is 120 × 15 × 1.0 = 1,800 W. Most real AC loads have a power factor below 1.0; the same 15 A at 120 V but PF 0.85 (a motor or compressor) is only 1530 W of real power, even though the amps didn't change.
How is three-phase different, and why does this calculator use line-to-line voltage?
Three-phase power is watts = √3 × V(line-to-line) × amps × PF, where √3 ≈ 1.732 accounts for the 120° phase offset between the three legs. A 3-phase motor drawing 20 A at 208 V line-to-line, PF 0.85, is 1.732 × 208 × 20 × 0.85 ≈ 6,125 W. This calculator uses line-to-line voltage (not line-to-neutral) because that's the standard convention on three-phase equipment nameplates and in NEC load calculations -- the number printed on a 208V or 480V 3-phase motor is always the line-to-line figure.
What is power factor, and why isn't watts just volts × amps?
Power factor is the ratio of real power (watts, what actually does work and shows on your meter) to apparent power (volt-amps, what the wire and breaker actually carry). For a resistive load like a heater, PF = 1.0 and the two are equal. Motors, air conditioners and switching power supplies draw current that's partly out of phase with the voltage, so PF drops below 1.0 and the amps needed for a given wattage go up -- which is why wire and breaker sizing always starts from amps or VA, never from watts alone.
How do I convert watts back to amps?
Flip the formula: amps = watts ÷ (volts × PF) for DC or single-phase, or amps = watts ÷ (√3 × volts × PF) for three-phase. Running the 6124.5 W three-phase example above back through the reverse formula returns 20.0 A -- the same 20 A it started from, within rounding.
Is the amps number here enough to size a wire or breaker?
No -- amps is the starting input, not the answer. Wire gauge and breaker size also depend on ampacity at temperature, run length, continuous-load rules and the small-conductor breaker caps in the NEC, which the wire size calculator applies. Treat any wiring, breaker or circuit decision as safety-critical: verify it against the National Electrical Code or have a licensed electrician confirm it, not a web calculator alone.
These calculators use National Electrical Code tables and industry sizing charts and are for planning. They are not a substitute for a licensed electrician, an HVAC contractor's Manual J load calculation, or your local code, which governs. Electrical work can kill you and may require a permit; when in doubt, hire it out.