Watts to Amps Calculator

About the Watts to Amps Calculator

Converting watts to amps is essential for determining what circuit, breaker, and wire size an electrical load requires. While watts tell you how much power a device uses (and therefore your energy cost), amps determine the physical infrastructure needed: conductor size, overcurrent protection, and switch ratings.

The fundamental formula is Amps = Watts ÷ (Volts × Power Factor). For resistive loads like heaters and incandescent bulbs, the power factor is 1.0, simplifying the formula to Amps = Watts ÷ Volts. For motor-driven and electronic loads, the power factor (typically 0.75–0.95) means the actual current draw is higher than the simple W/V calculation.

This calculator converts watts to amps at any voltage, with optional power factor for AC circuits. It's used by electricians, homeowners, and engineers for circuit design, generator sizing, and load balancing.

Integrating this calculation into regular energy reviews ensures that conservation strategies are grounded in measured data rather than assumptions about building performance and usage patterns.

When This Page Helps

Knowing the amp draw of a load is critical for selecting the right breaker, wire size, and outlet type. This calculator converts known wattage to amps at your specific voltage, ensuring safe and code-compliant installations.

How to Use the Inputs

1. Find the wattage of the device (check the nameplate or manual).
2. Enter the wattage in the calculator.
3. Enter the circuit voltage (120V or 240V typical).
4. Enter the power factor (1.0 for heaters/resistive, 0.85 for motors).
5. View the current draw in amps.
6. Select a breaker and wire size that exceeds the calculated amps.

Example Calculation

Tips & Best Practices

• Always size breakers above the calculated amp draw.
• For continuous loads (3+ hours), limit to 80% of breaker rating.
• A 1,500W heater draws 12.5A at 120V but only 6.25A at 240V.
• Motor starting current (inrush) can be 5–7x the running amps.
• Use 14 AWG wire for 15A, 12 AWG for 20A, 10 AWG for 30A circuits.
• When in doubt about power factor, use 0.80 for a conservative (higher) amp estimate.

Wire Gauge Quick Reference

14 AWG copper: 15A max at 60°C. 12 AWG: 20A. 10 AWG: 30A. 8 AWG: 40A. 6 AWG: 55A. Long runs require upsizing to compensate for voltage drop. For runs over 50 feet, consider using the next larger gauge.

Common Wattage to Amp Conversions (120V)

100W = 0.83A. 500W = 4.17A. 1,000W = 8.33A. 1,500W = 12.5A. 1,800W = 15A (max for 15A circuit). 2,400W = 20A (max for 20A circuit). These assume PF = 1.0 (resistive loads).

NEC Code Requirements

The National Electrical Code requires circuit conductors be rated for the maximum overcurrent device (breaker) protecting them. Continuous loads must not exceed 80% of the breaker rating. This means a 20A breaker supports 16A continuous — or 1,920W at 120V.

Frequently Asked Questions

• How many amps does a 1,500W heater draw?

  At 120V: 1,500 ÷ 120 = 12.5 amps. At 240V: 1,500 ÷ 240 = 6.25 amps. The 120V version draws twice the current and needs a larger circuit. Most 1,500W portable heaters use 120V.

• What breaker size do I need for a certain wattage?

  Calculate amps (W ÷ V), then choose a breaker rated above that value. For continuous loads, the breaker must be rated at 125% of the load current. A 12.5A continuous load needs a 16A minimum breaker — use a 20A.

• Why does the same wattage draw different amps at different voltages?

  Power (watts) equals volts times amps. If voltage doubles, amps are halved for the same wattage. This is why high-power appliances use 240V — the lower current allows smaller, cheaper wires and breakers.

• What if the power factor is less than 1?

  A lower power factor means the device draws more amps than W/V suggests. For example, 1,000W at 120V with PF = 0.80 draws 10.4A, not 8.3A. The extra current doesn't do useful work but still heats wires and loads breakers.

• How do I find the wattage of an appliance?

  Check the nameplate label (usually on the back or bottom) for watts or amps and volts. If only amps and volts are listed, multiply them for VA (apparent power). For the wattage, you'd also need the power factor.

• Can I plug multiple devices into one circuit?

  Yes, as long as the total amp draw doesn't exceed the circuit rating (80% for continuous loads). Add up all device amps. For example, a 15A circuit can handle a 8A + 5A combination (13A total) but not three 8A devices (24A total).