The direct conversion from amps to watts for a standard 15-amp circuit at 120V AC (assuming a purely resistive load with a power factor of 1.0) is exactly 1,800 watts. The formula used is Watts = Amps × Volts × Power Factor, which substitutes as W = 15A × 120V × 1.0 = 1800W. However, if you are calculating for inductive loads like an air compressor where the power factor drops to 0.8, that same 15 amps yields only 1,440 watts of real working power. Because wattage is entirely dependent on your system's voltage and phase configuration, a single universal conversion chart does not exist.
The Core Formula and Neighboring Values (15A Anchor)
To convert amps to watts in any DC circuit or purely resistive AC circuit (like baseboard heaters or incandescent lighting), you use the basic power equation: P = I × V. For alternating current (AC) circuits with motors, transformers, or switching power supplies, you must introduce the Power Factor (PF) to account for the phase shift between voltage and current: P = I × V × PF.
Below is a quick-reference table showing the ±20% neighboring values around our 15A baseline anchor. This is particularly useful when sizing branch circuits, as the National Electrical Code (NEC) generally requires you to derate continuous loads to 80% of the breaker's rating (meaning a 15A breaker should only carry 12A continuously).
| Current (Amps) | Watts @ 120V (1-Phase) | Watts @ 240V (1-Phase) | NEC Continuous Load Limit? |
|---|---|---|---|
| 12A | 1,440 W | 2,880 W | Yes (Safe on 15A breaker) |
| 13A | 1,560 W | 3,120 W | No (Exceeds 80% rule) |
| 14A | 1,680 W | 3,360 W | No |
| 15A | 1,800 W | 3,600 W | No (Breaker max capacity) |
| 16A | 1,920 W | 3,840 W | Requires 20A breaker |
| 17A | 2,040 W | 4,080 W | Requires 20A breaker |
| 18A | 2,160 W | 4,320 W | Requires 20A breaker |
How Voltage and Phase Shift the Conversion
The assumption that fixes your wattage answer is the combination of system voltage, phase count, and load type. A 20-amp draw on a 120V US residential outlet yields 2,400W. That exact same 20-amp draw on a 230V European household outlet yields 4,600W. When you step up to commercial 3-phase power, the formula changes to include the square root of 3 (≈1.732): P = √3 × V × I × PF.
The data-dense matrix below maps real-world conversions across standard North American and international voltages. Note that the 3-phase columns assume a realistic industrial motor power factor of 0.85, rather than a theoretical 1.0, reflecting how induction motors actually behave on the grid.
| Amps | 120V 1-Phase (PF 1.0) | 240V 1-Phase (PF 1.0) | 208V 3-Phase (PF 0.85) | 480V 3-Phase (PF 0.85) |
|---|---|---|---|---|
| 10A | 1,200 W | 2,400 W | 3,062 W | 7,066 W |
| 15A | 1,800 W | 3,600 W | 4,593 W | 10,599 W |
| 20A | 2,400 W | 4,800 W | 6,124 W | 14,132 W |
| 30A | 3,600 W | 7,200 W | 9,186 W | 21,198 W |
| 40A | 4,800 W | 9,600 W | 12,248 W | 28,264 W |
| 50A | 6,000 W | 12,000 W | 15,310 W | 35,330 W |
When the Conversion is Meaningless (The Power Factor Trap)
A conversion from amps to watts becomes entirely meaningless if you do not know the Power Factor (PF) of an inductive AC load. Amps measure the total current flowing through the wire (Apparent Power, measured in Volt-Amps or VA), while Watts measure the actual work being done (Real Power).
This is why electrical utilities charge commercial facilities penalties for low power factor. The wires and transformers must be sized for the amps (which cause I²R heating losses in the copper), but the facility is only doing watts of useful work. If you are calculating wire gauge or breaker size, you always use the raw Amps. If you are calculating energy costs, battery bank drain, or mechanical output, you must use Watts.
Frequently Asked Conversion Questions
Does wire gauge change the wattage conversion?
No. Wire gauge (like 12 AWG vs 10 AWG THHN) dictates the maximum safe ampacity before the insulation melts or voltage drop becomes excessive. It does not alter the mathematical relationship between amps, volts, and watts. However, if a long wire run causes a severe voltage drop (e.g., dropping your 120V source down to 110V at the load), your wattage at the load will decrease proportionally because the voltage variable in the formula has shrunk.
How do I convert DC amps to watts for a 12V solar setup?
DC circuits do not have a power factor, so the formula is strictly W = A × V. If your MPPT charge controller is pushing 40A into a 12V LiFePO4 battery bank, the math is 40A × 12.8V (nominal resting voltage of a 12V LFP pack) = 512W. Always use the measured battery voltage under load, not the nominal '12V' label, for precise solar yield calculations.
Why does my 20A breaker trip on a 2,400W space heater?
A 2,400W space heater on a 120V circuit draws exactly 20A (2400 / 120 = 20). While 20A is the absolute physical limit of a standard 20A breaker, the NEC requires continuous loads (anything running for 3 hours or more) to be derated to 80%. 80% of 20A is 16A (1,920W). Running a 20A heater continuously will eventually cause the breaker's thermal bimetallic strip to heat up and trip the circuit. You must either upgrade to a 240V circuit or use a 1,500W (12.5A) heater setting.
