To convert amperes to watts, you must multiply the current by the circuit voltage. For the most common residential baseline of 15 amps, the direct conversion is 1,800 watts at 120V (North America) or 3,450 watts at 230V (Europe/UK). The fundamental DC formula used is W = A × V, substituted as 15A × 120V = 1,800W. However, in AC circuits, this number is only your apparent power until you factor in the power factor (PF) and phase configuration.
Below is a quick-reference table for values neighboring the 15A baseline (±20%), assuming a purely resistive load (PF = 1.0).
| Current (Amps) | Watts @ 120V (US) | Watts @ 230V (EU/UK) |
|---|---|---|
| 12A | 1,440 W | 2,760 W |
| 13A | 1,560 W | 2,990 W |
| 14A | 1,680 W | 3,220 W |
| 15A | 1,800 W | 3,450 W |
| 16A | 1,920 W | 3,680 W |
| 17A | 2,040 W | 3,910 W |
| 18A | 2,160 W | 4,140 W |
The Core Formula and Assumptions That Fix Your Answer
Amperes measure the volume of electrical flow (like gallons per minute in a pipe), while watts measure the actual work being done (like the horsepower of a water wheel). You cannot convert flow to work without knowing the pressure (voltage). Therefore, the three assumptions that fix your final wattage answer are Voltage, Power Factor (PF), and Phase Configuration.
For direct current (DC) or purely resistive alternating current (AC) loads like incandescent heaters, the formula is simple:
Watts = Amps × Volts
However, most real-world electrical work involves standard breaker sizes across varying voltages and phases. The table below provides the real-world wattage capacities for common branch circuit breakers. Note: The 3-phase column assumes a 0.9 Power Factor, typical for mixed commercial lighting and HVAC loads.
| Breaker Size (A) | 120V 1-Phase (W) | 240V 1-Phase (W) | 208V 3-Phase @ 0.9 PF (W) |
|---|---|---|---|
| 15A | 1,800 | 3,600 | 4,862 |
| 20A | 2,400 | 4,800 | 6,483 |
| 30A | 3,600 | 7,200 | 9,725 |
| 40A | 4,800 | 9,600 | 12,966 |
| 50A | 6,000 | 12,000 | 16,208 |
| 60A | 7,200 | 14,400 | 19,450 |
NEC-style guidance reminder: For continuous loads (running 3 hours or more), you must derate these wattage values by 80%. A 20A breaker at 120V can handle 2,400W peak, but only 1,920W continuous.
How the Math Shifts: 120V vs 230V vs 3-Phase
The reason a single-voltage answer is never universal is that global grid standards and industrial phase configurations drastically alter the wattage output for the exact same amperage draw.
120V vs 230V Single-Phase
In North America, standard receptacles deliver 120V nominal (often measured between 114V and 126V at the outlet). In Europe, the UK, and Australia, the standard is 230V nominal. If you plug a 10A load into a US outlet, it pulls 1,200W. If that exact same 10A load is designed for and plugged into a European outlet, it pulls 2,300W. This is why high-draw appliances like EV chargers and electric dryers use 240V split-phase in the US—it halves the amperage requirement for the same wattage, allowing for smaller, cheaper copper wire.
The 3-Phase Shift
When you move into commercial or industrial spaces, you encounter 3-phase power (commonly 208V in the US or 400V in Europe). The math shifts because power is delivered across three overlapping sine waves. The formula becomes:
Watts = √3 × Volts × Amps × Power Factor
The √3 (approximately 1.732) multiplier is why a 30A 3-phase circuit delivers vastly more power than a 30A single-phase circuit. At 208V, 30A single-phase yields 6,240W. But at 208V 3-phase (assuming a 1.0 PF), it yields 10,807W. Always verify your phase configuration before sizing wire or selecting a breaker.
When Converting Amperes to Watts is Meaningless
There is a specific scenario on the jobsite where converting amps to watts using the basic formula will give you a dangerously wrong answer: when dealing with inductive AC loads and an unknown Power Factor (PF).
According to Fluke's electrical testing guidelines, Power Factor is the ratio of Real Power (Watts) to Apparent Power (Volt-Amps, or VA). Motors, transformers, and cheap LED drivers have inductive or capacitive reactance. They draw current that doesn't actually do real work, bouncing back and forth between the source and the load.
If you clamp a meter around a wire feeding an air compressor motor and read 15A at 240V, the basic math says 3,600W. But if the motor's power factor is 0.75, the actual real power doing work (and generating heat in the windings) is only 2,700W. As detailed in All About Circuits' AC power theory chapter, calculating watts without the PF gives you Volt-Amps (VA).
Frequently Asked Conversion Scenarios
- How many watts is 1 amp? At 120V, 1A = 120W. At 230V, 1A = 230W. At 12V DC (like a car battery), 1A = 12W.
- How many watts is 10 amps? 1,200W at 120V; 2,300W at 230V; 2,400W at 240V (US dryer/RV outlet).
- Does higher amps always mean higher watts? Only if voltage remains constant. A 2A load at 240V (480W) consumes more power than a 4A load at 12V (48W).
