How to convert amps to watts (the formula)
Power is voltage times current, so watts come from amps as soon as you know the voltage — and, on AC, the power factor. Three cases:
| System | Formula | Example |
|---|---|---|
| DC | W = A × V | 20 A × 120 V = 2400 W |
| AC single-phase | W = A × V × PF | 20 A, 120 V, PF 1 = 2400 W |
| AC three-phase | W = √3 × A × V × PF | 20 A, 480 V, PF 1 = 16,628 W |
The √3 (about 1.732) in the three-phase formula comes from three-phase geometry. To get kilowatts, divide the watts by 1000.
On DC — solar arrays, RVs, and battery banks — there's no power factor, so watts are simply amps × volts. A 12 V load pulling 10 A is 120 W; on a 24 V system the same 10 A is 240 W. It's the same reason higher-voltage battery banks move more power on smaller wire.
How many watts is X amps?
The common breaker and appliance currents converted to watts at power factor 1.0 (single-phase):
| Current | 120 V | 240 V |
|---|---|---|
| 10 A | 1200 W | 2400 W |
| 15 A | 1800 W | 3600 W |
| 20 A | 2400 W | 4800 W |
| 30 A | 3600 W | 7200 W |
| 50 A | 6000 W | 12,000 W |
These are the maximum watts at each current — but a breaker isn't meant to run at its full rating continuously, which is where the next rule comes in.
Amps to watts and the 80% rule
A breaker should only carry 80% of its rating on a continuous load (running three hours or more). So the usable continuous watts are lower than the table maximums:
15 A circuit → 12 A continuous → about 1440 W at 120 V.
20 A circuit → 16 A continuous → about 1920 W at 120 V (3840 W at 240 V).
30 A circuit → 24 A continuous → about 5760 W at 240 V.
Use these when you're checking whether a heater, EV charger, or other long-running load fits a circuit.
To go the other way — from a wattage to the amps and the circuit it needs — use the watts to amps calculator and the Breaker Size Calculator.
Power factor and three-phase
On AC, the real power in watts is amps × volts × power factor. Resistive loads (heaters, incandescent bulbs) are power factor 1.0; motors run around 0.8, which gives fewer real watts for the same amps. Three-phase multiplies by an extra √3:
15 A at 120 V:
• at PF 1.0 (resistive) → 1800 W
• at PF 0.8 (motor) → 1440 W of real power
And three-phase: 20 A at 480 V → 16,628 W (16.6 kW).
Amps, watts, kilowatts, and volts
These all connect. Divide watts by 1000 for kilowatts. For the reverse — a wattage back to amps — use watts to amps. To find watts from volts and amps framed the other way, see volts to watts. And for a kilowatt rating, the kW to amps calculator does the current.
Frequently asked questions
ConversionsWhat is the formula to convert amps to watts?
ConversionsHow many watts is 15 amps?
ConversionsHow many watts is 20 amps?
ConversionsHow many watts is 30 amps?
ConversionsHow do I convert amps to watts at 240V?
ConversionsDoes power factor change amps to watts?
ConversionsHow do I convert amps to watts for three-phase?
ConversionsHow many watts can a 20 amp circuit handle?
This calculator gives estimates from the standard power formula. Real AC loads vary with power factor, and circuit sizing follows the NEC and your local authority having jurisdiction (AHJ). When in doubt, hire a licensed electrician.