There is no universal constant for how many watts make an amp because watts measure real power and amps measure electrical current. However, assuming a standard US residential voltage of 120V and a purely resistive load, 1 amp equals exactly 120 watts. The governing formula is Watts = Amps × Volts. Substituting our baseline values: 120W = 1A × 120V. If you are on a 230V European circuit, 1 amp equals 230 watts. Without knowing the system voltage, converting between the two is physically impossible.
Below is a data-dense reference table for common household and workshop loads, demonstrating how the amperage draw shifts dramatically depending on whether you are wiring for North American (120V) or European/Asian (230V) mains.
| Device / Load | Rated Watts | Amps at 120V (US) | Amps at 230V (EU/UK) | Recommended Min. Wire (US) |
|---|---|---|---|---|
| Space Heater | 1500W | 12.5A | 6.5A | 14 AWG (15A breaker) |
| Countertop Microwave | 1000W | 8.3A | 4.3A | 14 AWG (15A breaker) |
| Level 1 EV Charger | 1440W | 12.0A | 6.2A | 14 AWG (15A breaker) |
| Window AC Unit | 1200W | 10.0A | 5.2A | 14 AWG (15A breaker) |
| LED Shop Light | 40W | 0.33A | 0.17A | 14 AWG (15A breaker) |
Note: Amperage values assume a purely resistive load (PF=1.0). Motorized appliances like the Window AC will draw higher startup amps and require power factor adjustments, detailed below.
The Core Assumption: Why Voltage Dictates the Answer
To understand why voltage is the mandatory assumption that fixes the watt-to-amp answer, use the standard hydraulic analogy. Voltage is the water pressure in the pipe, amps are the volume of water flowing, and watts represent the total mechanical work the water can do when it hits a turbine. You cannot calculate the total work (watts) just by knowing the flow rate (amps); a high-pressure, low-flow stream can do the exact same work as a low-pressure, high-flow stream.
In electrical terms, this relationship is defined by Joule’s Law and Ohm’s Law. When sizing branch circuits under NEC-style guidance, we use this assumption to prevent conductor overheating. A 1500W heater draws 12.5A on a 120V circuit, requiring 14 AWG copper wire. That exact same 1500W heater, if designed for a 240V dedicated circuit, draws only 6.25A, meaning it could theoretically run on thinner wire (though 14 AWG remains the practical minimum for branch circuits).
How the Conversion Shifts: 120V vs 230V vs 3-Phase
The conversion formula shifts depending on the phase configuration of your electrical supply. While single-phase DC or AC (with a PF of 1) uses the simple W = A × V formula, three-phase power introduces the square root of 3 (approximately 1.732) to account for the phase angle differences between the conductors.
- Single-Phase 120V (US Standard): Watts = Amps × 120
- Single-Phase 230V (EU/UK Standard): Watts = Amps × 230
- Three-Phase 208V (US Commercial): Watts = Amps × 208 × 1.732 × PF
- Three-Phase 400V (EU Industrial): Watts = Amps × 400 × 1.732 × PF
To visualize how sensitive this conversion is to minor fluctuations in current draw, here is a micro-conversion table showing a ±20% range around a baseline of 1 Amp. This is particularly useful when calculating tolerance thresholds for sensitive electronics or PCB trace sizing.
| Current (Amps) | Variance from Baseline | Watts at 120V | Watts at 230V |
|---|---|---|---|
| 0.8A | -20% | 96W | 184W |
| 0.9A | -10% | 108W | 207W |
| 1.0A | Baseline | 120W | 230W |
| 1.1A | +10% | 132W | 253W |
| 1.2A | +20% | 144W | 276W |
When the Conversion Becomes Meaningless: Power Factor
If you are working with inductive AC loads—such as induction motors, air compressors, transformers, or fluorescent ballasts—and you do not know the Power Factor (PF), the watt-to-amp conversion is practically meaningless.
Power factor is the ratio of Real Power (Watts, which does actual work) to Apparent Power (Volt-Amps or VA, which is what the utility must supply). A motor might be rated for 746W of mechanical output (1 Horsepower). However, if the motor is 80% efficient and has a power factor of 0.75, the actual electrical current drawn from the wall will be vastly higher than a simple 746W ÷ 120V calculation suggests.
According to US Department of Energy guidelines on estimating appliance energy use, motor-driven appliances have high startup surges (Locked Rotor Amps) and running power factors that deviate heavily from 1.0. To calculate amps for an inductive load, you must use the adjusted formula:
Amps = Watts ÷ (Volts × Efficiency × Power Factor)
Example: A 1 HP (746W) well pump on a 240V circuit, with 85% efficiency and a 0.80 PF.
Amps = 746 ÷ (240 × 0.85 × 0.80) = 746 ÷ 163.2 = 4.57 Amps.
If you had blindly used the basic formula (746 ÷ 240), you would have calculated 3.1 Amps, leading you to undersize the breaker and wire, resulting in nuisance tripping or a fire hazard.
Frequently Asked Questions
How many watts is a 15-amp breaker good for?
On a standard 120V circuit, a 15-amp breaker can theoretically handle 1,800 watts (15A × 120V). However, under NEC Article 210.20(A), if the load is continuous (running for 3 hours or more), you must derate the breaker by 80%. Therefore, a 15-amp breaker is only good for 1,440 continuous watts.
How many watts is a 20-amp breaker good for?
At 120V, a 20-amp breaker has an absolute maximum capacity of 2,400 watts. For continuous loads, applying the 80% safety derating factor limits the safe continuous draw to 1,920 watts. This is why high-draw kitchen appliances are often split across multiple 20A small-appliance branch circuits.
Does wire gauge change the watt-to-amp conversion?
No. Wire gauge (AWG) does not alter the physics of the watt-to-amp conversion. However, the wire gauge dictates the maximum allowable amps (ampacity) before the insulation melts. 14 AWG THHN copper is generally limited to 15A, while 12 AWG is limited to 20A, regardless of whether the voltage is 12V, 120V, or 600V.
