At a standard US household voltage of 120V, 15 amps equals exactly 1,800 watts. At a standard European or UK voltage of 230V, 15 amps equals 3,450 watts. The universal formula used to find this is Watts = Amps × Volts × Power Factor. Substituting the standard US values with a purely resistive load (Power Factor of 1.0), the math is: 1,800W = 15A × 120V × 1.0. However, treating this single number as a universal truth is a fast track to tripped breakers or melted wires, because the actual wattage shifts dramatically based on your regional voltage, phase configuration, and the power factor of the specific appliance you are running.
The Core Formula and the Assumptions That Fix It
To accurately convert amps to watts, you must lock in three variables: voltage, phase, and power factor (PF). In a pure DC circuit (like a 12V car battery), the formula is simply Watts = Amps × Volts. But in AC mains electricity, the voltage and current waveforms rarely peak at the exact same millisecond.
This phase shift creates the Power Factor, a ratio between 0 and 1.0. Resistive loads like incandescent bulbs or basic space heaters have a PF of 1.0, meaning all the current does real work. Inductive loads like AC compressors, refrigerators, and drill presses have a PF closer to 0.8. According to Fluke's technical guide on power factor, a motor drawing 15 amps at 120V with a 0.8 PF is only producing 1,440 watts of real mechanical work, even though the wiring must be sized to carry the full 15 amps of apparent power.
Amp-to-Watt Conversion Table (12A to 18A Range)
When sizing circuits or balancing loads on a subpanel, you rarely deal with exactly 15.0 amps. Below is a spec-sheet table covering the ±20% neighborhood of a 15A load. This includes the critical NEC 80% continuous load limit, which dictates the maximum wattage you can run for 3 hours or more on a standard 15-amp breaker.
| Current (Amps) | Watts @ 120V (PF=1.0) | Watts @ 230V (PF=1.0) | Max Continuous @ 120V (80% Rule) |
|---|---|---|---|
| 12A | 1,440W | 2,760W | 1,152W (at 12A limit) |
| 13A | 1,560W | 2,990W | N/A (Exceeds 12A continuous) |
| 14A | 1,680W | 3,220W | N/A (Exceeds 12A continuous) |
| 15A | 1,800W | 3,450W | N/A (Exceeds 12A continuous) |
| 16A | 1,920W | 3,680W | N/A (Breaker will trip) |
| 17A | 2,040W | 3,910W | N/A (Breaker will trip) |
| 18A | 2,160W | 4,140W | N/A (Breaker will trip) |
How the Answer Shifts: 120V, 230V, and 3-Phase Systems
The assumption that 15 amps always equals 1,800 watts only holds true for North American 120V split-phase branch circuits. If you move the exact same 15-amp draw to a different system, the wattage scales linearly with the voltage.
- 230V Single-Phase (UK/EU/AU): At the IEC standard 230V, 15A yields 3,450W. This is why European kitchens can run high-wattage kettles and ovens on standard wall plugs that would instantly trip a US 15A breaker.
- 208V 3-Phase (US Commercial): For three-phase power, the formula changes to Watts = √3 × Volts × Amps × PF. Assuming a PF of 1.0, 15A at 208V 3-phase equals roughly 5,402 watts (1.732 × 208 × 15).
- 480V 3-Phase (US Industrial): Pushing 15A through a 480V 3-phase motor circuit delivers a massive 12,470 watts. This highlights why industrial panelboards use much smaller gauge wires for high-horsepower motors compared to residential setups.
When This Conversion Becomes Meaningless
There are two specific scenarios where asking 'how many watts in 15 amps' is the wrong question entirely.
1. When the Power Factor is Unknown or Highly Reactive: If you clamp a meter around the feed to a bank of cheap, uncorrected LED drivers or an old arc welder, you might read 15 amps. But if the PF is a dismal 0.5, your real power (watts doing actual work) is only 900W at 120V. The utility company is still charging you for the apparent power (VA) in commercial settings, and your wires are still heating up from the full 15A of current. Sizing a breaker based on the 900W real power calculation would result in a fire hazard.
2. When Ignoring the NEC Continuous Load Derating: The National Electrical Code (NEC) Article 210.20(A) mandates that branch circuits supplying continuous loads (anything expected to run for 3 hours or more) must be derated to 80%. Therefore, on a 15-amp breaker, your continuous limit is 12 amps. Asking how many watts 15 amps is becomes meaningless for a space heater or grow light, because you are legally and practically limited to 1,440 continuous watts on that circuit.
Frequently Asked Questions
How many watts can a 15-amp breaker handle continuously?
A standard 15-amp breaker can handle a maximum of 1,440 watts continuously at 120V. The NEC requires continuous loads to be limited to 80% of the breaker's rating (15A × 0.80 = 12A; 12A × 120V = 1,440W). For non-continuous, intermittent loads (like a vacuum cleaner or toaster), you can safely pull the full 1,800 watts.
Does 15 amps equal 1800 watts for all household appliances?
No. That 1,800W figure assumes a Power Factor of 1.0, which only applies to purely resistive loads like toaster ovens, incandescent bulbs, and basic space heaters. Appliances with electric motors (refrigerators, AC units, washing machines) or complex switching power supplies (desktop PCs) have a power factor less than 1.0. They will draw 15 amps from the panel but produce fewer than 1,800 watts of real working power.
How many watts is 15 amps at 12 volts DC?
In a 12V DC system (common in automotive, RV, and off-grid solar setups), 15 amps equals exactly 180 watts (15A × 12V = 180W). Because the voltage is so low, DC systems require much thicker wire gauges to carry the same wattage without suffering from severe voltage drop. A 15A draw at 12V requires at least 14 AWG wire for short runs, whereas 15A at 120V can safely use standard 14 AWG NM-B Romex for much longer distances.
Can I plug a 1500W heater and a 500W TV into a 15-amp circuit?
No, this will likely trip the breaker. The combined load is 2,000 watts, which exceeds the absolute maximum capacity of a 120V, 15-amp circuit (1,800W). Furthermore, if the heater runs for more than three hours, it violates the 1,440W continuous load rule on its own. You must split these devices across two separate branch circuits.
