If you are looking for the formula amps to watts for a standard US 15-amp circuit at 120V, the direct answer is 1,800 watts (or 1,440 watts for continuous 80% loads). For a standard 20-amp circuit, the answer is 2,400 watts. The base formula used is Watts = Amps × Volts. Substituting the values for a 15-amp breaker: 15A × 120V = 1,800W. However, treating this single-voltage calculation as a universal truth is a fast track to tripped breakers and melted wire insulation. The actual wattage your circuit can safely deliver shifts dramatically based on three hidden assumptions: your system voltage, whether the current is single-phase or three-phase, and the power factor of the load.
The Core Formula Amps to Watts (and the Hidden Assumptions)
The fundamental equation for electrical power is derived from Ohm's and Joule's laws. For direct current (DC) circuits or purely resistive alternating current (AC) loads—like incandescent heaters or toasters—the formula is straightforward:
Real Power (W) = Current (A) × Voltage (V)
This formula assumes a Power Factor (PF) of 1.0. Power factor is the ratio of real power (which does actual work) to apparent power (which the utility must supply). When you plug a purely resistive space heater into a 120V outlet drawing 12 amps, it consumes exactly 1,440 watts of real power.
According to NEC-style guidance (Article 210.20), if a load runs continuously for 3 hours or more, you must derate the breaker capacity to 80%. Therefore, a 15-amp breaker is only good for 12 amps continuous (1,440W at 120V), and a 20-amp breaker is limited to 16 amps continuous (1,920W at 120V). Always size your wire and breaker for the continuous load, not just the peak.
When you introduce inductive or capacitive loads—like AC motors, compressors, or fluorescent lighting ballasts—the current and voltage waveforms fall out of phase. The formula must be adjusted to account for this inefficiency:
Real Power (W) = Current (A) × Voltage (V) × Power Factor (PF)
If you are sizing an inverter or a generator, ignoring the PF means you are calculating apparent power (Volt-Amps, or VA), not real watts. A 15A motor on a 120V circuit with a PF of 0.8 doesn't consume 1,800W; it consumes 1,440W of real power, but the wiring must still be sized to carry the full 1,800 VA of apparent power.
Quick Reference Conversion Table (±20% Range)
Below is a spec-sheet-table covering the ±20% neighboring values around the most common residential breaker sizes (15A and 20A). This assumes a perfect 1.0 Power Factor. Use this to quickly estimate heat dissipation or resistive load limits.
| Current (Amps) | 120V Single-Phase (Watts) | 240V Single-Phase (Watts) | 208V 3-Phase (Watts)* |
|---|---|---|---|
| 12A | 1,440 W | 2,880 W | 4,323 W |
| 13A | 1,560 W | 3,120 W | 4,684 W |
| 14A | 1,680 W | 3,360 W | 5,045 W |
| 15A (Standard) | 1,800 W | 3,600 W | 5,405 W |
| 16A | 1,920 W | 3,840 W | 5,766 W |
| 17A | 2,040 W | 4,080 W | 6,126 W |
| 18A | 2,160 W | 4,320 W | 6,487 W |
| 20A (Standard) | 2,400 W | 4,800 W | 7,207 W |
*Note: 3-Phase calculations assume a line-to-line voltage of 208V and a Power Factor of 1.0. Formula used: W = A × V × √3 (1.732).
How Voltage, Phase, and Power Factor Shift the Answer
The most common mistake DIYers make is assuming the 120V calculation applies universally. The wattage an amp delivers is entirely dependent on the electrical pressure (voltage) pushing it. Here is how the answer shifts across different global and industrial standards:
- 120V (North American Standard Receptacles): 1 Amp delivers 120 Watts. A 15A circuit yields 1,800W. This is your baseline for standard household electronics and lighting.
- 230V / 240V (European/UK/AU Standard & US Heavy Appliances): 1 Amp delivers 230 to 240 Watts. That same 15A current now yields 3,450W to 3,600W. This is why a 240V baseboard heater can output massive heat on a relatively small 20A double-pole breaker compared to a 120V plug-in heater.
- 3-Phase Power (Industrial & Commercial): In a 3-phase system, power delivery is continuous rather than pulsing. The formula shifts to include the square root of 3 (approximately 1.732). The equation becomes: Watts = Amps × Volts × 1.732 × PF. For a 15A load on a 480V 3-phase system with a 0.9 PF, the calculation is: 15 × 480 × 1.732 × 0.9 = 11,223 Watts.
When is the Amps to Watts Conversion Meaningless?
The conversion becomes practically meaningless for sizing wire and breakers if the Power Factor is unknown. If you are measuring the current draw of a large induction motor with a clamp meter and read 20 amps, you cannot simply multiply 20A × 240V to get 4,800 watts. The motor might have a PF of 0.65, meaning it's only doing 3,120 watts of real mechanical work. However, the wires still have to carry the full 20 amps (the apparent power). Sizing a generator based on the real wattage rather than the apparent VA will result in a stalled generator and severe voltage sag. For a deeper dive into the physics of reactive power, refer to the All About Circuits guide on true, reactive, and apparent power.
Frequently Asked Questions
How do I convert amps to watts for a 3-phase motor?
Use the 3-phase power formula: Watts = Amps × Volts × √3 × Power Factor. First, find the line-to-line voltage (e.g., 208V or 480V) and the motor's power factor (usually printed on the nameplate, typically between 0.80 and 0.90). Multiply the measured amps by the voltage, then multiply that result by 1.732, and finally multiply by the PF. If you are sizing a VFD or generator, calculate the VA (skip the PF step) to ensure the source can handle the reactive current.
Why does my 15-amp breaker trip at 1,800 watts?
If your 15-amp breaker trips right at the theoretical 1,800W limit (15A × 120V), it is doing exactly what it was designed to do. Breakers are thermal-magnetic devices; the thermal bimetallic strip is calibrated to trip at 100% of the rated current to prevent wire insulation from melting. Furthermore, if that 1,800W load runs for more than 3 hours, it violates the NEC 80% continuous load rule. For continuous operation, you must upgrade to a 20-amp breaker (with 12 AWG wire) to safely handle 1,920W continuously.
Is the formula amps to watts the same for DC and AC?
Yes and no. For DC circuits (like a 12V car battery or a 48V solar bank), the formula is strictly W = A × V because there is no phase shift or power factor to worry about. For AC circuits, the basic formula W = A × V only applies to purely resistive loads (PF = 1.0). For inductive AC loads, you must factor in the PF. For more on how AC waveforms affect power calculations, see the Georgia State University HyperPhysics AC Power module.
How many watts is 1 amp at 12 volts?
Using the DC formula (W = A × V), 1 amp at 12 volts equals exactly 12 watts. This is a common calculation for automotive and off-grid solar setups. For example, a 12V LED light bar drawing 5 amps consumes 60 watts. When sizing the fuse for this DC circuit, you would use a 7.5A or 10A blade fuse to protect the 16 AWG or 14 AWG wiring feeding the light.
