If you are looking for the exact conversion amps to watts for a standard 15-amp circuit, the direct answer is 1,800 watts on a 120V single-phase system, 3,600 watts on a 240V single-phase system, and 4,866 watts on a 208V 3-phase system (assuming a 0.9 Power Factor). The foundational formula substituting these values is: Watts = 15A × 120V × 1.0 (PF) = 1,800W. However, treating this single calculation as a universal rule will lead to undersized wire and tripped breakers on the jobsite. The true conversion depends entirely on three fixed assumptions: your system voltage, the number of phases, and the load's Power Factor (PF).
The Core Formulas: DC, Single-Phase AC, and 3-Phase AC
To convert amperage to wattage accurately, you must match the formula to the circuit topology. Using the DC formula on an industrial 3-phase motor will result in a calculation that is off by nearly 70%.
- DC Circuits:
Watts = Amps × Volts - Single-Phase AC (120V/240V):
Watts = Amps × Volts × Power Factor - 3-Phase AC (208V/480V):
Watts = Amps × Volts × Power Factor × 1.732
Quick Reference: Amps to Watts Neighboring Values (±20%)
When sizing components, you rarely land on exactly 15.0 amps. Below is a quick-reference table spanning a ±20% range around our 15A baseline (12A to 18A). This table assumes a Power Factor of 1.0 for single-phase (purely resistive loads like space heaters) and 0.9 for 3-phase (typical inductive motor loads).
| Amps (A) | Watts @ 120V (1Φ) | Watts @ 240V (1Φ) | Watts @ 208V (3Φ, 0.9 PF) |
|---|---|---|---|
| 12A | 1,440W | 2,880W | 3,893W |
| 13A | 1,560W | 3,120W | 4,217W |
| 14A | 1,680W | 3,360W | 4,542W |
| 15A | 1,800W | 3,600W | 4,866W |
| 16A | 1,920W | 3,840W | 5,190W |
| 17A | 2,040W | 4,080W | 5,515W |
| 18A | 2,160W | 4,320W | 5,839W |
How Voltage and Phase Shift the Conversion
The amperage drawn by a load is inversely proportional to the voltage applied. This is why a 3,600W electric dryer pulls 30A on a 120V theoretical circuit, but only 15A on a standard 240V residential split-phase circuit.
When you move from a US standard 120V receptacle to a 230V European standard or a 240V US dryer outlet, the wattage doubles for the exact same amperage. In 3-phase industrial environments (commonly 208V or 480V in the US), the power delivery is continuous across the phases. A 15A draw on a 480V 3-phase system yields 11,188W (15 × 480 × 0.9 × 1.732). This massive increase in power transfer is exactly why industrial facilities use 3-phase: they can deliver more watts using thinner, less expensive copper wire because the current per leg is kept low.
When the Conversion is Meaningless: The Power Factor Trap
The conversion from amps to watts becomes entirely meaningless if you do not know the load's Power Factor (PF) and you are dealing with inductive AC loads.
Amperage measured with a clamp meter gives you Apparent Power (Volt-Amps, or VA). Wattage is Real Power (W). For resistive loads (incandescent bulbs, toasters, resistive heaters), PF is 1.0, meaning VA equals Watts. But for motors, transformers, and LED drivers with cheap ballasts, the current waveform lags the voltage waveform.
If your clamp meter reads 15A on a 120V compressor circuit, you might assume 1,800W. But if that motor has a poor PF of 0.75, the real wattage consuming energy and generating heat is only 1,350W. The remaining 450W is "reactive power" bouncing back and forth in the magnetic field. Sizing a solar inverter or a UPS based on the 1,800W assumption will cause you to overspend, but sizing your wire based only on the 1,350W real power will cause the wire to overheat, because the wire must carry the full 15A of apparent current. As noted in All About Circuits' AC theory guide, conductors must always be sized for the apparent current (Amps), not just the real power (Watts).
Decision Path: Sizing Breaker and Wire for Calculated Watts
Use this decision tree to translate your calculated wattage back into physical hardware. Per NFPA 70 (NEC) guidelines, continuous loads (on for 3+ hours) must be derated to 80% of the breaker rating.
| Calculated Watts (120V 1Φ) | Load Type | Required Ampacity | Concrete Hardware Pick |
|---|---|---|---|
| Under 1,440W | Continuous or Non-Continuous | 15A | 14 AWG Copper, 15A Breaker |
| 1,441W to 1,920W | Non-Continuous (e.g., power tools) | 15A - 20A | 12 AWG Copper, 20A Breaker |
| 1,441W to 1,920W | Continuous (e.g., baseboard heat) | 20A (derated) | 12 AWG Copper, 20A Breaker |
| 1,921W to 2,400W | Any | 20A+ | 10 AWG Copper, 30A Breaker |
The Default Pick: If your conversion amps to watts calculation lands exactly on 1,800W at 120V and the load is continuous, do not use a 15A breaker (1800W / 120V = 15A; 15A × 80% = 12A max continuous). Pull 12 AWG THHN in conduit and terminate on a 20A Square D QO or Homeline single-pole breaker.
Frequently Asked Questions
Does the conversion change for DC battery systems?
Yes, it simplifies. In a 12V DC solar or automotive system, the formula is strictly Watts = Amps × Volts with no Power Factor multiplier. A 15A draw on a 12V LiFePO4 battery bank is exactly 180W. However, because the voltage is so low, the current required for high wattage is massive; a 1,800W inverter pulling from a 12V battery will draw 150A, requiring 1/0 AWG battery cables.
Why did my clamp meter read 15A but my Kill-A-Watt meter read 1,400W?
Your clamp meter measures apparent current (Amps), while the Kill-A-Watt measures real power (Watts). The discrepancy is caused by a Power Factor of roughly 0.77 (1400W / (15A × 120V) = 0.77). This is highly typical for older refrigerator compressors or uncorrected fluorescent lighting ballasts. Always size your wire for the 15A clamp meter reading, not the 1,400W reading.
