Watts to Amps Calculator: AC, Three-Phase and DC
Watts to amps in short: I = P / V. A 1500 W space heater on 120 V draws 12.5 A. On three-phase, I = P / (√3 · V · PF): an 11 kW EV charger on 400 V needs 15.9 A per line. For motors, enter the power factor from the nameplate.
Source: OpenStax, OpenStax University Physics Vol. 2, 9.5 Electrical Energy and Power (P = IV). Updated: .
How it is calculated
Watts to amps formulas
- DC: I = P / V
- Single-phase AC: I = P / (V · PF)
- Three-phase AC: I = P / (√3 · VL · PF), VL = line-to-line voltage
The other way round: P = V · I (DC), P = V · I · PF (single-phase), P = √3 · V · I · PF (three-phase). √3 ≈ 1.732 comes from the 120° phase shift between the three lines.
Watts to amps at 120 V and 240 V (PF = 1)
| Watts | at 120 V | at 240 V |
|---|---|---|
| 100 W | 0.83 A | 0.42 A |
| 500 W | 4.17 A | 2.08 A |
| 1000 W | 8.33 A | 4.17 A |
| 1500 W | 12.5 A | 6.25 A |
| 2000 W | 16.67 A | 8.33 A |
| 3000 W | 25 A | 12.5 A |
Power factor, VA and reactive power
In motors, transformers and power supplies current and voltage are out of step. The apparent power S = V · I (VA) is then larger than the real power P (W); their ratio is the power factor PF = P / S. Reactive power Q = √(S² − P²) oscillates between supply and device. Wires and breakers must carry the current, i.e. the apparent power.
Watts to amp-hours
Amp-hours (Ah) measure charge: Ah = A · h = W · h / V. A 60 W load on 12 V draws 5 A and uses 20 Ah in 4 hours. For power banks, see the mAh to Wh converter.
Limitations
The calculation uses RMS values and a balanced three-phase load. Motor starting currents are several times the rated current. The breaker size shown is a rough guide; wire size, installation method and the local electrical code decide.
Frequently asked questions
How do you convert watts to amps?
Divide watts by volts: I = P / V. 1500 W at 120 V is 12.5 A. For AC motors also divide by the power factor.
How many amps is 1000 watts?
8.33 A at 120 V, 4.17 A at 240 V and 83.3 A at 12 V DC.
How do you convert watts to amps on three-phase?
I = P / (√3 · V · PF). Example: 11 kW at 400 V, PF 1 → 11000 / (1.732 · 400) = 15.9 A per line; at 208 V it would be 30.5 A.
How many watts can a 15 amp circuit handle?
15 A × 120 V = 1800 W in theory; for continuous loads the NEC 80 % rule limits it to 1440 W.
How do you convert watts to amp-hours?
Watts divided by volts gives amps; times hours gives amp-hours. 60 W / 12 V = 5 A, so 4 h = 20 Ah.
Sources and legal basis
- OpenStax: OpenStax University Physics Vol. 2, 9.5 Electrical Energy and Power (P = IV)
- Lehrmaterial: University of Louisville ECE 252, Lesson 13B: Three-Phase Power (P = √3 V_L I_L cos θ)
As of:
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