If you are using a watt converter to calculate the maximum power of a standard US 15-amp branch circuit at 120V AC (single-phase, resistive load), the exact answer is 1,800 watts (or 1,440 watts for continuous NEC loads). The foundational formula used is Watts = Amps × Volts × Power Factor (PF). Substituting our baseline values: 1800W = 15A × 120V × 1.0 PF.
This answer assumes three fixed variables: a nominal 120V supply, a purely resistive load (PF = 1.0), and single-phase power. Change any of these, and the output shifts drastically. Below is a quick-reference table for neighboring values within a ±20% range of that 15A baseline.
| Amps (A) | Watts (W) | Typical Application |
|---|---|---|
| 12A | 1,440W | Standard 15A circuit continuous load (80% rule) |
| 13A | 1,560W | High-draw space heater on low setting |
| 14A | 1,680W | Heavy-duty portable table saw |
| 15A | 1,800W | Maximum theoretical 15A breaker limit |
| 16A | 1,920W | 20A circuit mid-range load |
| 17A | 2,040W | Large window AC unit startup surge |
| 18A | 2,160W | 20A circuit continuous load (80% rule) |
The Core Watt Converter Formulas (and When They Fail)
To convert electrical current to power, you must know your circuit topology. The formulas shift depending on whether you are dealing with direct current (DC), single-phase alternating current (AC), or three-phase AC.
W = A × VSingle-Phase AC (Inductive/Capacitive):
W = A × V × PFThree-Phase AC:
W = √3 × A × V × PF (where √3 ≈ 1.732)
When the Conversion is Meaningless
A watt converter output is entirely meaningless if you do not know the Power Factor (PF) of an inductive load. If you measure 10A on a 230V motor using a standard clamp meter and multiply them (10 × 230 = 2,300), you have calculated Volt-Amps (VA), or apparent power, not true Watts. According to Fluke's guide on power factor, motors and transformers draw reactive current that does no real work. A 230V motor drawing 10A with a lagging PF of 0.75 is only consuming 1,725 true Watts. Without a True-RMS meter capable of measuring phase angle, or a known PF from the equipment nameplate, any V × A calculation for inductive loads is a guess.
Amps to Watts Data Table (120V, 230V, and 3-Phase)
The assumption that 'Watts = Amps × 120' is a dangerous shortcut. As voltage increases, the current required to deliver the same wattage drops, which is why heavy appliances use 230V/240V circuits and industrial shops use 480V 3-phase. The table below demonstrates how the exact same amperage yields vastly different wattage depending on the supply voltage and phase configuration.
| Amps (A) | 120V 1-Phase (PF = 1.0) |
230V 1-Phase (PF = 1.0) |
208V 3-Phase (PF = 0.85) |
480V 3-Phase (PF = 0.85) |
|---|---|---|---|---|
| 10A | 1,200W | 2,300W | 3,055W | 7,066W |
| 20A | 2,400W | 4,600W | 6,110W | 14,133W |
| 30A | 3,600W | 6,900W | 9,165W | 21,199W |
| 50A | 6,000W | 11,500W | 15,275W | 35,332W |
| 100A | 12,000W | 23,000W | 30,550W | 70,664W |
Note: 3-Phase calculations assume a line-to-line voltage (V_LL). The Electronics Tutorials AC Power Factor resource provides deeper math on how line-to-neutral voltages shift these calculations in wye vs. delta configurations.
Mechanical and Thermal to Watts (HP and BTU)
Electricalflux readers frequently need to cross the boundary from electrical power to mechanical or thermal power. When sizing a generator or a solar inverter, you must convert the mechanical output of a motor or the thermal draw of an HVAC system into electrical Watts.
Horsepower (HP) to Watts
The strict electrical equivalent of one mechanical horsepower is 746 Watts. However, motors are not 100% efficient. When calculating the electrical input wattage required to drive a motor, you must divide by the motor's efficiency (η). For a standard NEMA Premium efficiency motor, η is typically around 0.90.
- 1/2 HP Motor: 373W mechanical → ~450W electrical input
- 1 HP Motor: 746W mechanical → ~880W electrical input
- 5 HP Motor: 3,730W mechanical → ~4,380W electrical input
For exact nameplate compliance and locked-rotor amperage (LRA) surge calculations, always defer to the NEMA MG 1 Motors and Generators standard.
BTU/hr to Watts
HVAC systems are rated in BTU/hr, but inverters and breakers care about Watts. The conversion factor is fixed: 1 BTU/hr = 0.29307 Watts. Therefore, a 12,000 BTU/hr mini-split air conditioner has a nominal thermal output equivalent to roughly 3,516 Watts. Because heat pumps have a Coefficient of Performance (COP) greater than 1, the actual electrical draw will be lower (often around 1,200W to 1,500W depending on the SEER rating).
Watt Converter FAQ
Why does my 15A breaker trip at 1,700 watts when the converter says 1,800W?
Under NEC Article 210.20(A), if a load is expected to run continuously for 3 hours or more (like a space heater or server rack), the circuit must be derated to 80% of its maximum capacity. 80% of 1,800W is 1,440W. If you pull 1,700W continuously on a 15A breaker, the thermal element inside the breaker will eventually heat up and trip, even though you haven't exceeded the instantaneous 1,800W absolute limit.
How do I convert Watts back to Amps?
Reverse the formula: Amps = Watts / (Volts × PF). If you have a 2,000W resistive water heater on a 230V circuit, the math is 2000 / (230 × 1.0) = 8.69 Amps. You would safely wire this with 14 AWG copper (rated 15A) on a 15A double-pole breaker.
Does voltage sag change my wattage?
Yes. If your utility delivers 114V instead of a nominal 120V at the end of a long feeder run, a 15A resistive load will only produce 1,710W (15 × 114). For constant-power switching supplies (like LED drivers or PC power supplies), a voltage sag causes the device to draw more amps to maintain the same wattage, which can inadvertently trip breakers if the wire is sized only for nominal voltage.
