If you are asking how to convert amps to watts for a standard US 15-amp household circuit, the direct answer is 1,800 watts at 120V, or 3,600 watts at 240V. The foundational formula is Watts = Amps × Volts (W = A × V). Substituting the values for a standard 120V branch circuit: 15A × 120V = 1,800W. However, treating this single calculation as a universal rule is a fast track to tripped breakers or melted wire insulation. The true wattage depends entirely on your system's voltage, phase configuration, and power factor.
Below is a quick-reference table showing the wattage for currents within a ±20% range of our 15A baseline, calculated for standard US single-phase voltages.
| Current (Amps) | Watts @ 120V (Single-Phase) | Watts @ 240V (Single-Phase) |
|---|---|---|
| 12A | 1,440W | 2,880W |
| 13A | 1,560W | 3,120W |
| 14A | 1,680W | 3,360W |
| 15A | 1,800W | 3,600W |
| 16A | 1,920W | 3,840W |
| 17A | 2,040W | 4,080W |
| 18A | 2,160W | 4,320W |
The Core Formula: Why Voltage Fixes Your Wattage
Amps (current) and Watts (power) are fundamentally different physical dimensions. You cannot convert between them without a bridging variable: Voltage (electrical pressure). Think of it like a water system: amps represent the flow rate (gallons per minute), volts represent the water pressure (PSI), and watts represent the total hydraulic power delivered to the waterwheel. Without knowing the pressure, the flow rate tells you nothing about the work being done.
For DC circuits or purely resistive AC loads (like a toaster, incandescent bulb, or resistive water heater), the math is strictly linear: W = A × V. But in practical wiring, you must also account for the National Electrical Code (NEC) continuous load rule. According to NEC Article 210.20, if a load is expected to run for three hours or more, the branch circuit must be derated to 80% of its maximum capacity.
This means a 15-amp breaker protecting 14 AWG NM-B wire should not see a continuous draw of 1,800W. The safe, code-compliant continuous limit is 12 amps, which equates to 1,440 watts at 120V. If you are sizing a solar inverter or a UPS system for a continuous server load, always use the 80% derated wattage, not the theoretical maximum.
AC vs DC, Single-Phase vs 3-Phase, and Power Factor
The assumption that fixes your wattage answer changes dramatically the moment you leave standard US residential single-phase DC/resistive territory. Here is how the math shifts across different global and industrial standards:
- 120V vs 230V Regions: In North America, standard outlets are 120V nominal. In the UK, EU, and Australia, standard outlets are 230V nominal. A 15A draw in London yields 3,450W (15A × 230V), nearly double the 1,800W it would yield in New York. This is why European appliances can be physically smaller and use thinner cords for the same wattage—they operate at higher voltage and lower current.
- 3-Phase Power Shifts: In commercial and industrial settings, 3-phase power is standard. The formula shifts to include the square root of 3 (approx. 1.732). The equation becomes: W = A × V × √3 × PF. For a 15A draw on a 208V 3-phase system with a unity power factor (1.0), the wattage is 15 × 208 × 1.732 = 5,403W.
When the Conversion is Meaningless: The Power Factor Trap
If you are measuring an inductive load—like an AC compressor, a shop vacuum motor, or a transformer—and you do not know the Power Factor (PF), a simple A × V calculation is effectively meaningless for determining real work. As detailed in All About Circuits' AC Power guide, inductive loads cause the current waveform to lag behind the voltage waveform.
Multiplying Amps by Volts in this scenario gives you Volt-Amps (VA), also known as apparent power, not true Watts (real power). If your motor has a power factor of 0.80, 15A at 120V yields 1,800 VA of apparent power, but only 1,440W of real, usable power (1,800 × 0.80). According to Fluke's electrical testing guidelines, you must use a true power meter or clamp meter with PF measurement capabilities to find the real wattage. However, your wire and breaker must still be sized for the full 1,800 VA, because the wires carry the total current regardless of whether it is doing real work or just magnetizing a coil.
Real-World Amp-to-Watt Conversion Reference Chart
The following comparison-table maps standard breaker sizes to their theoretical maximum wattage and their NEC-compliant continuous wattage (80% rule) across the most common voltage configurations. Use this when sizing feeders, selecting inverters, or calculating solar array requirements.
| Breaker Size | Wire Size (Copper) | Max Watts @ 120V (1Φ) | Continuous Watts @ 120V | Max Watts @ 240V (1Φ) | Max Watts @ 208V (3Φ) |
|---|---|---|---|---|---|
| 15A | 14 AWG | 1,800W | 1,440W | 3,600W | 5,403W |
| 20A | 12 AWG | 2,400W | 1,920W | 4,800W | 7,205W |
| 30A | 10 AWG | 3,600W | 2,880W | 7,200W | 10,807W |
| 40A | 8 AWG | 4,800W | 3,840W | 9,600W | 14,410W |
| 50A | 6 AWG | 6,000W | 4,800W | 12,000W | 18,013W |
Frequently Asked Questions
How many watts can a 20-amp breaker handle continuously?
A 20-amp breaker can handle a theoretical maximum of 2,400 watts at 120V. However, for continuous loads (running 3 hours or more), the NEC requires an 80% derating. Therefore, the safe continuous limit is 1,920 watts. This is why high-draw appliances like window AC units or space heaters often trip 15A breakers but run fine on 20A circuits when kept under the 1,920W threshold.
Can I use a DC amp-to-watt calculator for my AC refrigerator?
No. A refrigerator compressor is a highly inductive AC motor with a low starting power factor and a massive inrush current (often 5 to 7 times the running amps). While the running wattage might be 700W (approx. 6A at 120V with a PF of ~0.9), the instantaneous startup VA can exceed 3,500VA. DC calculators ignore inrush current and power factor, which will lead you to severely undersize your inverter or UPS.
Why does my multimeter show 125V when the formula uses 120V?
120V is the nominal system voltage used for standardized calculations and NEC ampacity tables. In reality, utility transformers are often tapped to deliver 125V or even 128V at the panel to compensate for voltage drop at the end of long feeder runs. If you measure 125V and draw 15A, your actual real-time wattage is 1,875W. Always use nominal voltage (120V/240V) for breaker sizing and wire ampacity, but use measured voltage when calculating exact battery drain or solar inverter efficiency.
