At a standard US residential voltage of 120V (DC or purely resistive AC), 15 amps equals exactly 1,800 watts. The fundamental formula used to derive this is Power (W) = Voltage (V) × Current (A). Substituting the standard values: 120V × 15A = 1,800W. However, treating this single number as a universal truth is a fast track to tripped breakers or melted wire insulation. The true wattage shifts dramatically based on your voltage, phase configuration, and the power factor of the load you are measuring.
The Core Formula and the Assumptions That Fix Your Answer
The conversion from amps to watts is not a fixed constant; it is entirely dependent on three assumptions: voltage, phase, and power factor (PF). If you change any of these variables, the 1,800W answer changes with it.
1. Voltage Shifts (120V vs. 230V/240V)
In North America, standard branch circuits operate at 120V, but heavy appliances (dryers, ovens, EV chargers) use split-phase 240V. In Europe and much of the world, standard single-phase mains are 230V.
- At 120V: 15A × 120V = 1,800 Watts
- At 230V (EU/UK): 15A × 230V = 3,450 Watts
- At 240V (US Split-Phase): 15A × 240V = 3,600 Watts
2. Phase Shifts (Single-Phase vs. 3-Phase)
For 3-phase industrial or commercial power, the formula introduces the square root of 3 (approximately 1.732) to account for the phase angles. The formula becomes: Power = √3 × Voltage × Current × PF.
- At 208V 3-Phase (US Commercial): 1.732 × 208V × 15A × 1.0 = 5,404 Watts
- At 400V 3-Phase (EU Commercial): 1.732 × 400V × 15A × 1.0 = 10,392 Watts
15 Amp Wattage Reference Table (12A to 18A Range)
When sizing wires or checking loads on a bench, you rarely sit at exactly 15.0 amps. The table below provides a ±20% reference range (12A to 18A) calculated at a standard 120V single-phase supply with a power factor of 1.0. This is the baseline for resistive loads like incandescent lighting, space heaters, and toasters.
| Current (Amps) | Voltage | Power Factor | Calculated Watts (W) | NEC 80% Continuous Limit |
|---|---|---|---|---|
| 12A | 120V | 1.0 | 1,440 W | 1,152 W |
| 13A | 120V | 1.0 | 1,560 W | 1,248 W |
| 14A | 120V | 1.0 | 1,680 W | 1,344 W |
| 15A | 120V | 1.0 | 1,800 W | 1,440 W |
| 16A | 120V | 1.0 | 1,920 W | 1,536 W |
| 17A | 120V | 1.0 | 2,040 W | 1,632 W |
| 18A | 120V | 1.0 | 2,160 W | 1,728 W |
Note: The "NEC 80% Continuous Limit" column reflects the National Electrical Code (NEC) requirement that a breaker should only be loaded to 80% of its rating if the load will run continuously for 3 hours or more. Therefore, a 15A breaker can only safely handle 1,440W of continuous heating or lighting load.
When the Amp-to-Watt Conversion Becomes Meaningless
The straightforward W = V × A math completely falls apart when you introduce inductive or capacitive loads—like AC compressors, refrigerator motors, or fluorescent ballasts. These devices introduce a phase shift between voltage and current, creating a Power Factor (PF) of less than 1.0.
When the power factor is unknown or unmeasured, converting amps directly to watts is just guessing. Here is why:
- Apparent Power (VA) vs. Real Power (W): A 15A induction motor running on a 120V circuit with a 0.80 power factor draws 1,800 Volt-Amps (VA) of apparent power. This is what the wire must carry and what the breaker monitors for heat. However, the actual mechanical work output (Real Power) is only 1,440 Watts (120V × 15A × 0.80).
- The Utility Penalty: Your residential meter only bills you for Real Power (Watts). But if you are sizing a generator, an inverter, or an uninterruptible power supply (UPS), you must size it for the Apparent Power (VA). A 1,500W UPS will instantly overload and shut down if you connect a 15A motor with a poor power factor, even though the "watts" seem to match.
To get a true wattage reading on inductive loads, you cannot just multiply the clamp-meter amperage by the wall voltage. You must use a true-power wattmeter (like a Kill A Watt or a Fluke power quality analyzer) that samples both voltage and current waveforms simultaneously to calculate the real power factor.
Frequently Asked Questions
How many watts can a 15 amp breaker handle continuously?
According to NEC Article 210.20(A), a 15 amp breaker can handle a maximum continuous load (defined as running for 3 hours or more) of 12 amps. At 120V, this equates to 1,440 watts. If you plug in a 1,500W space heater and run it on high for an afternoon, the thermal element inside the breaker will eventually heat up and trip the circuit, even though 1,500W is technically below the 1,800W absolute peak limit.
How many watts is 15 amps at 240 volts?
At 240V single-phase (like a US baseboard heater or EV charger circuit), 15 amps equals 3,600 watts (15A × 240V = 3,600W). This is why 240V circuits are used for high-wattage appliances; they deliver twice the power while keeping the current—and therefore the required wire thickness and heat generation—the same as a 120V circuit drawing half the wattage.
How many watts can a 15 amp 120V generator supply?
A portable generator rated for 15 amps at 120V can supply a peak of 1,800 watts. However, generators have two ratings: running (continuous) watts and starting (surge) watts. A 1,800W peak generator usually has a continuous running rating closer to 1,400W to 1,500W. Always check the manufacturer's data plate for the continuous rating, and never exceed it for more than a few minutes, or you risk damaging the generator's alternator windings.
Why does my 15 amp space heater trip a 15 amp breaker?
Most high-setting space heaters draw exactly 1,500 watts, which pulls 12.5 amps at 120V. While this is under the 15A absolute limit, breakers are thermal-magnetic devices. If the breaker is in a warm panel, if the wire connections are slightly loose (adding resistance and heat), or if you have other loads on the same circuit (like a TV and some LED lights adding another 1-2 amps), the cumulative heat will push the thermal trip mechanism past its threshold. For heating appliances, always use a dedicated 20A circuit if possible, or ensure the 15A circuit has absolutely zero shared loads.
