How Many Amps to Watts: Exact Conversion Formula & Calculator Chart

There is no single universal answer to how many amps to watts because amperes (current) and watts (power) measure different electrical properties; the exact conversion strictly depends on your system's voltage. However, to give you an immediate baseline: 10 amps equals 1,200 watts on a standard US 120V circuit, and 2,400 watts on a 240V circuit.

The foundational formula for DC and single-phase AC resistive loads is:

Watts (W) = Amps (A) × Volts (V)

Substituting our baseline values: 10A × 120V = 1,200W. If you are sizing a breaker or selecting wire gauges for a specific appliance, you must first lock in your system voltage before the math means anything.

The Core Formula: Converting Amps to Watts

To convert amps to watts accurately, you must know the operating voltage of the circuit. Below is a reference chart showing a ±20% range around our 10-amp baseline, calculated for the three most common global residential voltages. This assumes a purely resistive load (Power Factor = 1.0), such as a baseboard heater or incandescent lighting.

Current (Amps)	Watts at 120V (US Standard)	Watts at 230V (EU/UK Standard)	Watts at 240V (US Split-Phase)
8A (-20%)	960W	1,840W	1,920W
9A (-10%)	1,080W	2,070W	2,160W
10A (Baseline)	1,200W	2,300W	2,400W
11A (+10%)	1,320W	2,530W	2,640W
12A (+20%)	1,440W	2,760W	2,880W

Note: Always use the 240V column for large US appliances like electric dryers and ranges, and the 120V column for standard receptacles. According to the U.S. Department of Energy, verifying the nameplate voltage before calculating wattage is critical to avoid undersizing branch circuits.

How Voltage and Phase Shift the Calculation

The assumption that fixes your answer is always the voltage and the phase configuration. If you take a 10-amp measurement on a jobsite, the wattage shifts dramatically depending on what panel you are pulling from:

• 120V Single-Phase (US): Standard branch circuits. 10A = 1,200W. This is why a standard 15-amp breaker (1,800W max) easily handles a 1,500W space heater, but tripping occurs if you add a 300W vacuum to the same circuit.
• 230V Single-Phase (EU/UK/AU): Standard wall outlets in most of the world. 10A = 2,300W. This higher baseline voltage allows thinner wire gauges to deliver the same power, which is why European homes often use 2.5mm² or 1.5mm² cable for circuits that would require 12 AWG or 10 AWG in the US.
• 208V / 480V Three-Phase (Commercial/Industrial): For 3-phase systems, the formula changes to account for the phase angle. The equation becomes W = A × V × √3 × Power Factor. For a 10A load on a 208V 3-phase system with a 0.90 PF, the math is: 10 × 208 × 1.732 × 0.90 = 3,242W.

When the Conversion is Meaningless: The Power Factor Trap

The standard W = A × V formula completely fails when you are measuring inductive loads like AC compressors, well pumps, or industrial motors without accounting for the Power Factor (PF).

Inductive components create a phase shift between voltage and current waveforms. This results in a difference between Apparent Power (measured in Volt-Amps, or VA) and Real Power (measured in Watts). If you clamp a multimeter around the THHN wire feeding a 240V well pump and read 15 amps, multiplying 15 × 240 gives you 3,600 VA. But if the motor's power factor is 0.80, the actual wattage (the real work being done and the heat being generated) is only 2,880 Watts.

As detailed in All About Circuits' AC power textbook, sizing breakers and generators based purely on the VA calculation without checking the motor nameplate for PF or LRA (Locked Rotor Amps) will lead to nuisance tripping or severe voltage drop during startup. When PF is unknown, your amp-to-watt conversion is technically meaningless for real power calculations.