For the most common DIY and jobsite queries, 15 amps equals 1,800 watts at 120V (or 1,440W for continuous loads) and 20 amps equals 2,400 watts at 120V (1,920W continuous). At 240V, those values double to 3,600W and 4,800W respectively. The underlying formula substituting these exact values is: Watts = 120V × 15A × 1.0 (Power Factor) = 1,800W. This amperage to watts calculator guide gives you the exact numbers without the fluff, but remember: amperage alone is meaningless without knowing your system voltage, phase, and power factor. Below are the quick-reference tables and the critical assumptions that fix your final number.
The Core Formula and Fixed Assumptions
The conversion from amperage to wattage is not a fixed constant; it is a relationship dictated by three fixed assumptions: Voltage, Power Factor (PF), and Phase. Change any one of these, and your wattage shifts dramatically.
For DC circuits or purely resistive AC loads (like incandescent heaters or toasters), the formula is straightforward:
Watts (W) = Volts (V) × Amps (A)
However, for standard AC circuits with inductive or capacitive components, you must account for Power Factor:
Watts (W) = Volts (V) × Amps (A) × Power Factor (PF)
When you are measuring AC current for an inductive load (like an air compressor or table saw motor) and the Power Factor is unknown. A clamp meter reads total current (apparent power, measured in Volt-Amps or VA). If you measure 10A on a 120V circuit, you get 1,200 VA, not true Watts. Without the motor's nameplate PF (typically 0.75 to 0.85), any wattage calculation is a guess. True power (Watts) is what your utility bills you for and what generates real work and heat. For deep diagnostics on this, refer to Fluke's guide on measuring power factor.
Amperage to Watts Calculator Reference Tables
Use the data-dense table below to find your exact wattage across standard North American and light-commercial voltages. The 3-phase calculations assume a standard industrial Power Factor of 0.80.
| Amps (A) | 120V 1-Phase (W) | 240V 1-Phase (W) | 208V 3-Phase (W) |
|---|---|---|---|
| 10A | 1,200W | 2,400W | 2,882W |
| 15A | 1,800W | 3,600W | 4,323W |
| 20A | 2,400W | 4,800W | 5,764W |
| 30A | 3,600W | 7,200W | 8,646W |
| 40A | 4,800W | 9,600W | 11,528W |
| 50A | 6,000W | 12,000W | 14,410W |
If you are sizing a circuit specifically for a 15-amp breaker, you need to look at the neighboring values to understand how slight voltage drops or load spikes affect your total wattage. Here is the ±20% range for a 15A baseline at 120V:
| Current (A) | Watts (W) | Context / Note |
|---|---|---|
| 12A (-20%) | 1,440W | NEC 80% continuous load limit for a 15A breaker |
| 13A | 1,560W | Safe intermittent load |
| 14A | 1,680W | Approaching thermal trip threshold over time |
| 15A (Base) | 1,800W | Maximum absolute breaker rating (non-continuous) |
| 16A | 1,920W | Overload; breaker will trip eventually |
| 17A | 2,040W | Severe overload; rapid thermal trip |
| 18A (+20%) | 2,160W | Immediate magnetic or rapid thermal trip |
How the Answer Shifts: 120V vs 230V vs 3-Phase
The numbers above assume North American split-phase standards. If you are working with international mains or industrial equipment, the math shifts based on the following rules:
- 120V vs 230V (EU/UK Standard): In regions using 230V nominal mains (like the UK and EU), a 10A draw yields 2,300W, not 1,200W. Always check the nominal voltage printed on the device nameplate, as actual measured voltage can fluctuate between 216V and 253V depending on grid load and transformer tap settings.
- Single-Phase vs 3-Phase: For 3-phase systems, you cannot simply multiply Volts × Amps. You must use the formula: W = √3 × Voltage(Line-to-Line) × Amps × PF. The √3 (approximately 1.732) accounts for the phase angle displacement between the three conductors. This is why a 20A draw on a 208V 3-phase system yields 5,764W (assuming 0.8 PF), rather than the 4,160W you would get if you just multiplied 208 × 20.
Frequently Asked Questions: Code Compliance and Sizing
Q: Do I need to calculate watts differently for continuous loads?
A: Yes. According to NFPA 70 (National Electrical Code) Article 210.20, if a load is expected to run for 3 hours or more (like a space heater, server rack, or EV charger), it is considered 'continuous'. You must derate the circuit to 80%. Therefore, a 15A breaker can only safely handle 12A (1,440W at 120V) continuously. If your calculated load is 1,500W continuous, you must step up to a 20A breaker and use 12 AWG wire.
Q: Why does my inverter say 2000W but trips my 15A RV pedestal?
A: Inverters are rated by their output wattage, but the pedestal breaker sees the input amperage. If your inverter is pulling 1,800W from the grid at 120V, it is drawing 15A. However, inverters are not 100% efficient (usually 85-90%). To output 1,800W, it might pull 2,000W from the wall, which equals 16.6A. This exceeds the 15A breaker limit, causing a trip. Always calculate based on input wattage, not output rating.
Q: Can I use this amperage to watts calculator for DC battery banks?
A: Yes, but drop the Power Factor from your math (DC PF is always 1.0). Keep in mind that battery voltage sags under load. A '12V' LiFePO4 battery might sit at 13.2V at rest but drop to 11.8V when pulling 100A. For accurate wire and fuse sizing, calculate your watts using the lowest expected voltage under load (e.g., 11.8V × 100A = 1,180W) to ensure your conductors are sized for the highest possible amperage required to deliver your target wattage.
