How Many Watts Is a Standard Outlet? (120V, 230V & 3-Phase Limits)

A standard US 15-amp, 120-volt outlet can deliver a maximum of 1,800 watts (120V × 15A). However, under NEC guidelines for continuous loads (running 3 hours or more), that limit derates to 1,440 watts (80% of the breaker rating). The formula used here is Watts = Volts × Amps × Power Factor, substituting 120 × 15 × 1.0 = 1,800W. This is the absolute ceiling before the breaker's thermal mechanism trips.

The Formula and the Assumptions That Fix the Answer

The 1,800W figure is not a universal constant; it is the result of three fixed assumptions. Change any of these variables, and the available wattage shifts dramatically:

• Voltage (120V Nominal): We assume 120V, but actual measured voltage at the receptacle under load often drops to 114V–126V due to transformer taps and voltage drop across the branch circuit wire. At 114V, your max wattage drops to 1,710W.
• Phase (Single-Phase): Standard residential outlets are single-phase. Multiphase systems use a different multiplier (√3) in the power formula.
• Power Factor (PF = 1.0): The calculation assumes a purely resistive load (like an incandescent bulb or a space heater) where all drawn current performs real work.

For a deeper look at how the National Electrical Code handles branch circuit loading and continuous duty limits, reference the Mike Holt Enterprises guide on Continuous Loads, which breaks down NEC Article 210.20(A) for practical jobsite application.

Neighboring Values: Wattage vs. Amperage at 120V

To understand the margins of error and thermal limits on a standard 15-amp circuit, here is a spec-sheet table showing a ±20% range around the 1,800W baseline. This illustrates exactly where the breaker will hold, where it will thermally degrade over time, and where it will instantly trip.

How the Answer Shifts: 120V vs 230V vs 3-Phase

Assuming a standard outlet means a US 15A 120V receptacle is a geographic blind spot. If you are wiring a workshop, traveling, or importing equipment, the wattage capacity changes based on the regional standard and phase configuration:

• US 120V 20A (NEMA 5-20R): Common in kitchens and garages. Delivers 2,400W peak (1,920W continuous). Requires 12 AWG wire.
• UK 230V 13A (BS 1363): The standard British 3-pin plug contains an internal 13A fuse. Max wattage is 2,990W, limited by the plug fuse, not the 32A ring main breaker.
• EU 230V 16A (Schuko): Standard European grounded outlet. Delivers 3,680W peak. This is why high-draw appliances like kettles and EV granny-cables boil water or charge faster in Europe than on a standard US 15A circuit.
• US 208V 3-Phase 30A (NEMA L6-30R): Common for commercial shop equipment. The formula shifts to W = √3 × V × A × PF. Substituting values: 1.732 × 208 × 30 × 1.0 = 10,808W.

When sizing wire for these higher capacities, always consult the NFPA 70 (National Electrical Code) ampacity tables, specifically adjusting for terminal temperature ratings (60°C vs 75°C columns).

When the Conversion Becomes Meaningless: The Power Factor Trap

The conversion from Volts and Amps to Watts becomes entirely meaningless when the Power Factor (PF) is unknown on highly inductive or capacitive loads.

Watts measure real power (the work actually done, like heat or mechanical rotation). Volt-Amps (VA) measure apparent power (the total current pushed through the wires). If you plug a cheap, uncorrected LED driver or an undersized AC motor into a 120V outlet, it might draw 14 Amps (1,680 VA), but due to a poor power factor of 0.65, it only consumes 1,092 Watts of real power.

If you are sizing a backup UPS or a solar inverter, you must size for the VA (apparent power) to prevent the wiring and breakers from melting, even if the actual Wattage (real power) seems low. The US Department of Energy provides excellent baseline data on real-world appliance wattage versus nameplate VA ratings.