At a standard US household voltage of 120V, 15 amps is exactly 1,800 watts. If you are measuring a 230V European or UK mains supply, 15 amps equals 3,450 watts. For a 12V DC automotive or solar system, 15 amps is just 180 watts. The exact wattage is never a fixed number; it is entirely dependent on the voltage of the circuit and whether the current is alternating (AC) or direct (DC).
Substituted for US Mains: 1,800W = 15A × 120V
Neighboring Amperage Conversions (12A to 18A Range)
When sizing wires or checking breaker limits, you rarely deal with exactly 15.0 amps on the dot. Loads fluctuate. Here is how the wattage shifts across a ±20% range (12A to 18A) for the three most common voltages you will encounter on the bench or in the panel. This assumes a purely resistive load with a Power Factor (PF) of 1.0.
| Current (Amps) | Watts at 12V DC | Watts at 120V AC (US) | Watts at 230V AC (EU/UK) |
|---|---|---|---|
| 12A (-20%) | 144 W | 1,440 W | 2,760 W |
| 13A | 156 W | 1,560 W | 2,990 W |
| 14A | 168 W | 1,680 W | 3,220 W |
| 15A (Baseline) | 180 W | 1,800 W | 3,450 W |
| 16A | 192 W | 1,920 W | 3,680 W |
| 17A | 204 W | 2,040 W | 3,910 W |
| 18A (+20%) | 216 W | 2,160 W | 4,140 W |
How Voltage, Phase, and Assumptions Shift the Wattage
The single-voltage answer of '1,800 watts' is only correct if you assume a 120V, single-phase, purely resistive circuit. The moment you change the supply voltage or introduce three-phase power, the math shifts dramatically. For three-phase systems, the formula expands to include the square root of 3 (approximately 1.732) to account for the phase angles.
Below is a data-dense breakdown of exactly what 15 amps yields across different global and industrial power standards. This table assumes a Power Factor of 1.0 (unity) for baseline comparison.
| System Type | Nominal Voltage | Formula Used | Total Watts at 15A | Common Application |
|---|---|---|---|---|
| Single-Phase AC | 120V | W = 15 × 120 | 1,800 W | US standard duplex outlets, lighting |
| Single-Phase AC | 230V | W = 15 × 230 | 3,450 W | EU/UK/AU wall sockets, heavy appliances |
| Single-Phase AC | 240V | W = 15 × 240 | 3,600 W | US dryers, HVAC, water heaters (split-phase) |
| Three-Phase AC | 208V | W = 1.732 × 15 × 208 | 5,403 W | US commercial lighting, light industrial |
| Three-Phase AC | 480V | W = 1.732 × 15 × 480 | 12,470 W | Heavy industrial motors, large HVAC chillers |
| Direct Current (DC) | 12V | W = 15 × 12 | 180 W | Automotive, RV systems, small solar setups |
| Direct Current (DC) | 48V | W = 15 × 48 | 720 W | Telecom racks, LiFePO4 battery banks |
When the Conversion is Meaningless: Power Factor and the NEC 80% Rule
There are two major scenarios where simply multiplying amps by volts will give you a dangerously incorrect answer: when dealing with inductive AC loads, and when sizing breakers for continuous duty.
The Power Factor (PF) Trap
If your 15-amp load is a resistive heater or an incandescent light bulb, the Power Factor is 1.0, and the math is simple. But if you are measuring a 15-amp AC motor, a transformer, or a switching power supply with poor correction, the current and voltage waveforms are out of phase.
In these cases, you are measuring apparent power (Volt-Amps, or VA), not real power (Watts). The true formula for single-phase AC is:
Watts = Amps × Volts × Power Factor
If a 120V motor draws 15 amps but has a lagging power factor of 0.75, it is only consuming 1,350 watts of real work, even though the wires and breaker must be sized to handle the full 1,800 VA of apparent current. As noted by Fluke's electrical testing guides, ignoring power factor in industrial settings leads to massive inefficiencies and utility penalty charges. Without knowing the PF, converting AC amps to watts is essentially a guess.
The NEC 80% Continuous Load Rule
If you are asking 'how many watts is 15 amps' because you want to know what you can safely plug into a standard US 15-amp breaker, the theoretical maximum of 1,800W is the wrong number to use.
According to NEC Article 210.20(A), if a load is expected to run continuously for three hours or more (like a space heater, a server rack, or grow lights), the circuit must be derated to 80% of the breaker's capacity.
- Breaker Rating: 15 Amps
- Continuous Limit (80%): 12 Amps
- Max Continuous Watts at 120V: 1,440 Watts
If you pull a continuous 15A (1,800W) on a 15A breaker, the thermal-magnetic trip mechanism will eventually heat up and open the circuit, usually within 45 to 90 minutes. Always size your continuous loads to 1,440W or less on a 15A branch circuit, or upgrade to a 20A breaker with 12 AWG wire to get a 1,920W continuous limit.
Frequently Asked Questions
Can I run a 1,500-watt space heater on a 15-amp breaker?
Yes, but only if it is the primary load on the circuit. A 1,500W heater at 120V draws 12.5 amps. Because it is under the 15A absolute maximum, it will not trip the breaker immediately. However, because space heaters often run for more than three hours, they technically violate the NEC 80% continuous load rule (1,440W max). In practice, it is safer to run a 1,500W heater on a dedicated 20-amp circuit.
Why does my 12V inverter draw more than 15 amps to output 180 watts?
Inverters are not 100% efficient. If you are pulling 180 watts of AC power from a 12V DC inverter that operates at 85% efficiency, the inverter must pull roughly 212 watts from the battery. At 12V, that means the DC wiring will actually carry about 17.6 amps, not 15 amps. Always size your DC-side wiring and fuses for the inverter's maximum input current, not just the AC output wattage.
How many watts can a 15-amp generator produce?
Generators are typically rated by their running watts and surge watts. A standard portable generator with a 15-amp output limit at 120V will produce a maximum of 1,800 running watts. However, you should never run a generator at 100% capacity for extended periods. For reliable operation and to handle motor startup surges, keep your continuous load around 1,400 to 1,500 watts.
