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Watts to Amps Converter

Convert watts to amps (or amps to watts) for any voltage, ideal for sizing generators, planning solar systems, and checking circuit loads.

Convert watts and amps

Enter watts or amps along with your voltage to get the conversion instantly.

Conversion inputs

Select the voltage of the circuit or system.
Enter wattage to calculate amps. Leave blank to calculate from amps.
Enter amps to calculate watts. Leave blank to calculate from watts.
Live results update as you type
Conversion Result

10.0 Amps

1,200W at 120V draws 10.0 amps.

10.0 A Amps
1.20 kW Kilowatts
15A Min. Breaker Size

How to use this converter

Select your system voltage (or enter a custom value), then enter either the wattage or the amp draw, the tool calculates the other direction automatically. The result also shows kilowatts and the minimum recommended circuit breaker size. To go from amps to watts, clear the watts field and fill in the amps field instead.

Quick tips

  • Amps = Watts ÷ Volts. That's the whole formula, this tool just does the math for you.
  • Always size your breaker at 125% of continuous load. A 10A load needs at least a 15A breaker.
  • Generator output is rated in watts. Divide by your voltage to know how many amps you have available.
  • 12V systems draw much higher amps than 120V, a 1,200W inverter on a 12V battery pulls 100A.
  • Check your wire gauge too, amps determine the correct wire size, not just the breaker.
This calculator assumes a purely resistive (DC or single-phase AC) load with a power factor of 1.0. For motors, compressors, or large AC loads, actual amp draw at startup can be 3–6× higher than the running figure. Always account for surge wattage when sizing breakers and generators.

Watts, amps, and volts, the basics

  • Watts measure power, the rate at which energy is used. Most appliances list their wattage on the label or spec sheet.
  • Amps measure current, the flow of electricity through a wire. Breakers and wires are rated in amps.
  • Volts measure electrical pressure. Standard US outlets are 120V; large appliances and dryers typically use 240V.
  • The relationship is always: Watts = Amps × Volts. Rearrange to get whichever value you need.
  • Kilowatts (kW) are simply watts ÷ 1,000, used by utility bills and larger equipment ratings.

FAQ

How many amps does a 1,500W space heater use at 120V?

12.5 amps. That's close to the limit of a standard 15A circuit, which is why running other devices on the same breaker while using a space heater often trips it. A 20A circuit is a better fit.

My generator is rated at 3,500 watts. How many amps is that?

At 120V: 3,500 ÷ 120 = 29.2 amps. At 240V: 3,500 ÷ 240 = 14.6 amps. Many generators provide both 120V and 240V outlets, so the available amps depend on which outlet you're using.

Why does voltage matter so much?

The same wattage at lower voltage requires proportionally higher amps. A 1,200W load at 120V draws 10A, manageable with standard wiring. That same load at 12V draws 100A, requiring much heavier cable and connectors. This is why high-power inverters need thick battery cables.

What breaker size do I need?

The NEC (National Electrical Code) requires that continuous loads not exceed 80% of the breaker rating. So divide your amp draw by 0.8 to get the minimum breaker size, then round up to the nearest standard size (15, 20, 30, 40, 50A, etc.). This tool calculates that for you automatically.

What's the difference between running watts and starting watts?

Running (or rated) watts is the steady power a device needs during normal operation. Starting watts (or surge watts) is the brief spike when a motor or compressor first kicks on, often 2–3× the running figure. Always size your generator and breaker to handle the starting watts of any motor-driven appliance.