To convert 15 amps to wattage on a standard US 120V AC circuit, the answer is 1,800 watts (assuming a power factor of 1.0). The core formula used is Watts = Amps × Volts × Power Factor. Substituting our baseline values: 15A × 120V × 1.0 = 1,800W. This direct conversion assumes a purely resistive load—like a space heater or incandescent bulb—where voltage and current are perfectly in phase. If you are working with inductive loads like motors, the actual wattage will be lower due to power factor losses.
The Core Assumptions: Voltage, Phase, and Power Factor
Amps measure the flow of electrical current, while watts measure the actual work (power) being done. You cannot convert amps to wattage without fixing three critical variables: voltage, phase count, and power factor (PF).
If you only have an amperage reading and do not know the system voltage, the conversion is physically impossible. Furthermore, if you are measuring an AC inductive load (like an HVAC compressor) and the power factor is unknown, calculating wattage using only
Amps × Volts will give you Apparent Power (VA), not True Power (Watts). For industrial motors, assuming a PF of 1.0 will dangerously overestimate the actual wattage.For DC circuits or purely resistive AC loads (heating elements, toasters, incandescent lights), the power factor is exactly 1.0. For inductive AC loads, the power factor typically ranges from 0.75 to 0.95. According to Electronics Tutorials, the phase angle difference between voltage and current dictates this ratio, meaning a 15A motor with a 0.80 PF only performs 1,440W of real work despite drawing 1,800VA from the grid.
Amps to Watts Conversion Table (15A Baseline ±20%)
The table below maps the wattage for currents surrounding our 15A baseline (±20%, from 12A to 18A) across standard single-phase voltages. This assumes a resistive load (PF = 1.0).
| Current (Amps) | Wattage @ 120V (US) | Wattage @ 230V (EU/UK) | Wattage @ 240V (US Split-Phase) |
|---|---|---|---|
| 12.0 A (-20%) | 1,440 W | 2,760 W | 2,880 W |
| 13.5 A (-10%) | 1,620 W | 3,105 W | 3,240 W |
| 15.0 A (Baseline) | 1,800 W | 3,450 W | 3,600 W |
| 16.5 A (+10%) | 1,980 W | 3,795 W | 3,960 W |
| 18.0 A (+20%) | 2,160 W | 4,140 W | 4,320 W |
How the Math Shifts: 120V vs 230V vs 3-Phase
The formula changes structurally depending on the power delivery architecture. Here is how the math scales across the three most common systems you will encounter on the bench or jobsite:
1. Single-Phase 120V / 230V
The standard formula applies directly. W = A × V × PF. A 15A draw on a European 230V outlet yields 3,450W, which is nearly double the 1,800W drawn by the same 15A on a US 120V outlet. This is why high-wattage appliances (dryers, ovens) use 240V in the US—to keep the amperage (and required wire thickness) manageable.
2. Three-Phase AC (Industrial / Commercial)
For balanced 3-phase systems, you must multiply by the square root of 3 (approximately 1.732) to account for the phase overlap. The formula becomes: W = A × V × PF × 1.732.
Worked Example: A 15A 3-phase motor running on a 400V European supply with a 0.85 power factor.
15A × 400V × 0.85 × 1.732 = 8,833 Watts.
As All About Circuits notes, failing to include the 1.732 multiplier in 3-phase calculations is the most common error leading to undersized breakers in commercial panels.
Decision Path: Sizing Wire and Breakers for Your Wattage
Once you have converted your amps to wattage, you must size your overcurrent protection and conductors. The National Electrical Code (NEC) requires continuous loads (running 3 hours or more) to be derated to 80% of the breaker's capacity. Use this decision tree to select your hardware for a 120V single-phase circuit:
| Calculated Wattage (120V) | Continuous Amp Draw | Required Breaker Size | Minimum Copper Wire (THHN/NM-B) |
|---|---|---|---|
| < 1,440 W | < 12.0 A | 15 Amp | 14 AWG |
| 1,440 W – 1,920 W | 12.0 A – 16.0 A | 20 Amp | 12 AWG |
| 1,920 W – 2,400 W | 16.0 A – 20.0 A | 25 Amp or 30 Amp | 10 AWG |
| > 2,400 W | > 20.0 A | STOP: Switch to a 240V circuit design. | |
If you are wiring a 1,800W (15A) continuous-duty baseboard heater on a 120V circuit, the 80% continuous load rule dictates you cannot use a 15A breaker (15A × 0.8 = 12A max continuous). You must step up. Install a 20A breaker and pull 12 AWG copper wire.
FAQ: Edge Cases and Meaningless Conversions
Why does my 15A motor trip a 15A breaker if it only draws 1,800W?
Wattage calculations assume steady-state running current. Motors have a Locked Rotor Amperage (LRA) or inrush current that can be 5 to 7 times higher than the running amps for the first few milliseconds. A 15A motor might momentarily pull 90A on startup. You must size the breaker for the inrush (using a slow-blow fuse or a motor-rated breaker with magnetic trip settings), not just the running wattage.
Can I convert DC amps to watts using the same formula?
Yes. In DC circuits, there is no phase angle and no power factor to worry about. The formula is strictly Watts = Amps × Volts. A 15A draw on a 12V DC solar battery bank is exactly 180W. For deeper insights into DC vs AC power calculations, reference the Fluke guide on power factor and electrical measurements.
What if my multimeter reads amps but the nameplate lists watts?
Nameplate wattage is often the maximum thermal output or mechanical output, not the electrical input. If a power supply nameplate says "Output: 500W" and you measure 5A on the 120V AC input side, the input wattage is 600W (assuming PF 1.0). The 100W difference is lost as heat due to the power supply's efficiency rating (in this case, ~83% efficient). Always use the input-side voltage and measured AC amps to size your branch circuit wiring.
