To convert 15 amps to watts at a standard US 120V single-phase supply, the answer is exactly 1,800 watts. The foundational DC and purely resistive AC formula is P(W) = I(A) × V(V). Substituting our anchor values: 15A × 120V = 1,800W. However, if you are running that same 15A load on a 240V split-phase circuit (like a dryer), it jumps to 3,600W. On a 208V 3-phase system with a 0.8 power factor, it yields 4,322W. The exact wattage is entirely dependent on three fixed assumptions: system voltage, phase configuration, and power factor (PF).
Here is how the wattage shifts across a ±20% current range (12A to 18A) for standard single-phase residential voltages:
| Current (Amps) | Watts @ 120V (1Φ) | Watts @ 240V (1Φ) |
|---|---|---|
| 12A (-20%) | 1,440 W | 2,880 W |
| 13A | 1,560 W | 3,120 W |
| 14A | 1,680 W | 3,360 W |
| 15A (Base) | 1,800 W | 3,600 W |
| 16A | 1,920 W | 3,840 W |
| 17A | 2,040 W | 4,080 W |
| 18A (+20%) | 2,160 W | 4,320 W |
How the Answer Shifts: 120V vs 230V vs 3-Phase Systems
Presenting a single-voltage conversion as universal is a critical error in electrical design. The assumption that fixes your answer changes based on regional grid standards and load types. In North America, standard branch circuits operate at 120V or 240V. In the UK, EU, and Australia, the standard single-phase voltage is 230V. When you introduce 3-phase power (common in commercial panels and heavy machinery), the formula incorporates the square root of 3 (≈1.732) and the Power Factor.
According to NFPA 70 (NEC) Article 210, branch circuit sizing must account for the actual wattage and continuous load multipliers. Use the data-dense table below to map common breaker sizes to their maximum theoretical wattage capacities across different global and commercial voltage systems.
| Breaker Size (Amps) | 120V 1Φ (Watts) | 230V 1Φ (EU/UK Watts) | 208V 3Φ @ 0.8 PF (Watts) | 400V 3Φ @ 0.8 PF (Watts) |
|---|---|---|---|---|
| 15A | 1,800 W | 3,450 W | 4,322 W | 8,313 W |
| 20A | 2,400 W | 4,600 W | 5,763 W | 11,085 W |
| 30A | 3,600 W | 6,900 W | 8,644 W | 16,627 W |
| 40A | 4,800 W | 9,200 W | 11,525 W | 22,170 W |
| 50A | 6,000 W | 11,500 W | 14,407 W | 27,712 W |
Note: 3-Phase calculations use the formula: W = A × V × √3 × PF. The 0.8 PF assumption represents a standard industrial inductive motor load.
When Converting to Watts is Meaningless: The Power Factor Trap
If you are looking at a nameplate for an inductive load—such as an AC compressor, a UPS system, or a fluorescent lighting ballast—and it only lists Amps and Volts, converting directly to Watts is mathematically invalid. You are actually calculating Volt-Amps (VA), which is Apparent Power, not Real Power (Watts).
As detailed in All About Circuits, the Power Factor (PF) represents the phase shift between voltage and current waveforms caused by inductance or capacitance. A purely resistive load (like a space heater) has a PF of 1.0, meaning VA = Watts. But a motor might have a PF of 0.75.
A 10A, 120V motor nameplate yields 1,200 VA (10 × 120).
If the PF is 0.75, the real power is only 900 Watts (1,200 × 0.75).
Hazard: If you size a solar inverter assuming 1,200W, you over-spec it. If you size a breaker based on 900W (7.5A), you will trip the breaker because the wire must still carry the full 10A of apparent current.
When the Power Factor is unknown, you must stop at VA. Never assume a 1.0 PF for motors, transformers, or switching power supplies unless explicitly stated on the spec sheet.
Mechanical and Apparent Power: HP and kVA to Watts
Beyond Amps, makers and electricians frequently need to convert mechanical output (Horsepower) or transformer ratings (kVA) into Watts to calculate thermal dissipation, battery bank drain, or generator sizing.
By international standard, 1 mechanical Horsepower (HP) is exactly 745.7 Watts (often rounded to 746W in field calculations). However, because motors are not 100% efficient, the electrical input wattage required to produce 1 HP of mechanical shaft work will always be higher than 746W.
| Motor Rating (HP) | Mechanical Output (Watts) | Electrical Input @ 85% Efficiency (Watts) |
|---|---|---|
| 0.5 HP | 373 W | 439 W |
| 1.0 HP | 746 W | 878 W |
| 1.5 HP | 1,119 W | 1,316 W |
| 2.0 HP | 1,492 W | 1,755 W |
| 5.0 HP | 3,729 W | 4,387 W |
For kVA to Watts, the conversion relies entirely on the system's power factor. A 5 kVA UPS system does not automatically provide 5,000 Watts of real power. Below is a quick reference for common kVA ratings at standard power factors:
- 5 kVA @ 1.0 PF (Unity): 5,000 W (Rare, usually only seen in pure DC or resistive AC testing)
- 5 kVA @ 0.9 PF (Modern IT Loads): 4,500 W (Standard for modern server rack UPS systems)
- 5 kVA @ 0.8 PF (Industrial Standard): 4,000 W (Standard for older motors and mixed commercial loads)
FAQ: Common 'To Watts' Conversion Edge Cases
Why does my 1500W space heater draw 12.5A instead of the 13.6A I calculated?
Your calculation likely assumed a nominal 110V (1500W / 110V = 13.6A). However, modern US residential grids deliver a nominal 120V at the panel. Under load, the voltage at the receptacle might read 120V to 122V. At 120V, a 1500W resistive heater draws exactly 12.5A (1500 / 120 = 12.5). Always measure actual line voltage with a multimeter before finalizing breaker sizing.
Can I convert battery mAh directly to watts?
No. Milliamp-hours (mAh) is a unit of electric charge, while Watts is a unit of power (energy over time). To find the energy capacity, you must multiply mAh by the battery's nominal voltage to get Watt-hours (Wh). For example, a 3.7V 3000mAh Li-ion cell holds 11.1 Wh of energy. To find Watts, you must then divide that energy by the time (in hours) over which it is discharged.
How do I convert LED lumens to watts?
You cannot convert lumens directly to watts without knowing the luminous efficacy of the specific LED chip. Efficacy is measured in lumens per watt (lm/W). A high-end 2026 Cree or Osram LED might achieve 180 lm/W, meaning a 10,000-lumen fixture requires roughly 55 Watts. A cheaper, older diode at 80 lm/W would require 125 Watts for the same light output. Always check the manufacturer's datasheet for the specific bin and efficacy rating.
