A 15-amp circuit at 120V delivers exactly 1,800 watts. At 240V, that same 15 amps delivers 3,600 watts. The universal amp to watts conversion formula for DC and purely resistive AC loads is Watts = Amps × Volts. For a standard 15A load on a US 120V branch circuit, the math is substituted as: 15A × 120V = 1,800W. If you are sizing a breaker or selecting wire, this baseline number is your starting point before applying safety derating.
The Core Assumptions: Voltage, Phase, and Power Factor
The amp to watts conversion is not a single universal constant; it is entirely dependent on three fixing assumptions: voltage, phase configuration, and power factor (PF).
Voltage is your primary multiplier. A 10A draw on a 12V DC battery system yields only 120W, while 10A on a 240V AC dryer circuit yields 2,400W. However, when dealing with AC circuits containing motors, transformers, or compressors, you must account for Power Factor. The true AC formula is:
- Single-Phase AC: Watts = Amps × Volts × Power Factor
- Three-Phase AC: Watts = Amps × Volts × Power Factor × √3 (1.732)
Amp to Watts Conversion Table (12A to 18A Range)
The table below maps the ±20% range around the most common residential breaker threshold (15A). This covers the typical operating window for standard 120V receptacle circuits and small 240V appliances.
| Amps (A) | Watts @ 120V (1Φ, PF 1.0) | Watts @ 240V (1Φ, PF 1.0) | Watts @ 208V (3Φ, PF 0.9) |
|---|---|---|---|
| 12A | 1,440 W | 2,880 W | 3,892 W |
| 13A | 1,560 W | 3,120 W | 4,216 W |
| 14A | 1,680 W | 3,360 W | 4,541 W |
| 15A | 1,800 W | 3,600 W | 4,865 W |
| 16A | 1,920 W | 3,840 W | 5,189 W |
| 17A | 2,040 W | 4,080 W | 5,514 W |
| 18A | 2,160 W | 4,320 W | 5,838 W |
How the Math Shifts: 120V vs 230V vs 3-Phase
Regional grid standards and industrial configurations drastically alter the wattage output for the exact same amperage draw.
120V (North American Standard)
Used for general lighting and receptacles. Because the voltage is relatively low, current (amps) must be higher to deliver useful power. A 1,500W space heater pulls 12.5A, which is why plugging two of them into a single 15A/120V circuit (1,800W max) instantly trips the breaker.
230V / 240V (European & NA Heavy Appliance)
By doubling the voltage, you halve the current required for the same wattage. A 3,600W EV charger pulls 30A at 120V (requiring massive, stiff wire), but only 15A at 240V (allowing standard 14 AWG or 12 AWG wire). In Europe, the nominal 230V standard means a 16A Schuko plug delivers roughly 3,680W.
Three-Phase Power (Industrial / Workshop)
Three-phase systems deliver power continuously rather than in pulses. The inclusion of the √3 multiplier (1.732) means a 208V 3-phase circuit delivers roughly 73% more power than a single-phase 208V circuit at the same amperage. For a deep dive on measuring this in the field, refer to the Fluke guide on clamp meter usage for 3-phase panels.
Decision Path: Sizing Your Breaker and Wire
Do not stop at the raw wattage calculation. The National Electrical Code (NEC) requires derating for continuous loads (anything running for 3 hours or more). Use this decision tree to select your exact hardware.
| Scenario | Calculation Step | NEC Rule Applied | Concrete Hardware Pick |
|---|---|---|---|
| 1500W Space Heater (120V, Continuous) | 1500W ÷ 120V = 12.5A | Continuous load × 1.25 = 15.6A | Pick: 20A Breaker, 12 AWG NM-B wire |
| 3600W EV Charger (240V, Continuous) | 3600W ÷ 240V = 15.0A | Continuous load × 1.25 = 18.75A | Pick: 20A Breaker, 12 AWG THHN in conduit |
| 4500W Water Heater (240V, Non-Continuous) | 4500W ÷ 240V = 18.75A | Non-continuous (100% rating allowed) | Pick: 20A Breaker, 12 AWG NM-B wire |
| 1800W Microwave (120V, Non-Continuous) | 1800W ÷ 120V = 15.0A | Non-continuous, but requires dedicated circuit | Pick: 20A Breaker, 12 AWG NM-B wire |
When the Conversion Fails (and What to Do Instead)
Theoretical math falls apart in two specific real-world scenarios:
- Inrush Current (Locked Rotor Amps): A 120V table saw might draw 1,800W (15A) while running, but the startup surge can pull 45A for a fraction of a second. The amp to watts conversion will not predict this. You must size the breaker for the running watts, but ensure it has a magnetic trip curve (like a standard thermal-magnetic breaker) that tolerates the brief inrush without tripping.
- Harmonic Distortion: Cheap LED drivers and switching power supplies draw current in sharp spikes rather than smooth sine waves. This inflates the RMS current without increasing true wattage. If your clamp meter reads 14A but your Kill-A-Watt meter reads only 1,200W, trust the wattage meter for thermal load calculations, but trust the clamp meter for sizing the physical copper wire.
When in doubt, bypass the formula entirely. Clamp a true-RMS meter around the hot conductor under maximum load, measure the actual amperage, and size your breaker to 125% of that measured value.
