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Tool Belt Calc

Electrical

Watts to Amps Calculator

Convert watts to amps at any voltage, with power factor.

Your measurementsEvery assumption editable01

1.0 for heaters and incandescent lamps. 0.8 is typical for motors.

Current drawCalculated live02

Result

15A

1800 W at 120 V draws 15 A

Power
1800 W
Voltage
120 V
Power factor
1
CurrentW ÷ (V × PF)
15 A
Apparent power
1800 VA
Equivalent resistance
8 Ω

Power factor is what separates watts from volt-amps. A resistive load has a power factor of one and the two figures match; a motor draws more current than its wattage implies, which is why conductors and breakers are sized on current rather than on watts.

A 15 amp circuit is not a 15 amp budget. Continuous loads — anything running three hours or more — are conventionally limited to eighty per cent of the breaker rating, so 12 amps on a 15 amp circuit.

Appliance nameplate wattage is the rated draw, not necessarily the peak. Motors pull several times their running current for the moment they start.

Watts divided by volts gives amps for a resistive load. For anything with a motor or a switching supply, power factor matters — and current, not wattage, is what conductors and breakers are sized on.

Why use this tool?What it does differently03

Why use this tool?

Power factor included

A motor draws more current than its wattage implies, which is exactly why current governs sizing.

Apparent power shown

Volt-amps alongside watts, so the difference between the two is visible.

The 80% rule flagged

Continuous loads are conventionally limited to 80% of breaker rating.

Startup surge noted

Motors pull several times running current at the moment they start.

How this worksThe method04

How this watts to amps calculator works

Current is power divided by voltage times power factor. With a power factor of one — heaters, incandescent lamps — it reduces to the familiar watts over volts.

Power factor is what separates watts from volt-amps. A motor with a power factor of 0.8 draws twenty-five per cent more current than its wattage alone suggests, which is why conductors and overcurrent protection are sized on current rather than on watts.

A fifteen amp circuit is not a fifteen amp budget. Continuous loads — anything running three hours or more — are conventionally limited to eighty per cent of the breaker rating, so twelve amps on a fifteen amp circuit.

How to use itStep by step05

How to use it

  1. Step 1: Enter the power

    From the appliance nameplate, in watts.

  2. Step 2: Enter the voltage

    The supply voltage the appliance runs on.

  3. Step 3: Set the power factor

    1.0 for resistive loads, around 0.8 for motors, 0.9 to 0.95 for modern electronics.

  4. Step 4: Apply the 80% rule

    For anything running continuously, compare against 80% of the breaker rating.

Example usageWorked figures06

Example usage

A resistive heater
1,800 W at 120 V with unity power factor draws 15 A — right at the limit of a 15 A circuit and over the continuous-load allowance.
A motor load
1,800 W at 120 V with a 0.8 power factor draws 18.75 A, needing a 20 A circuit despite identical wattage.
A 240 V appliance
4,800 W at 240 V draws 20 A — half the current of the same power at 120 V, which is the whole argument for higher voltage on large loads.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert watts to amps?

Divide watts by volts for a resistive load. For motors and electronics, divide by volts times the power factor, which raises the current draw.

What is power factor?

The ratio of real power in watts to apparent power in volt-amps. Resistive loads are 1.0; motors are typically around 0.8, meaning they draw more current than their wattage suggests.

How many watts can a 15 amp circuit handle?

1,800 W at 120 V in theory, but continuous loads are conventionally limited to 80% — so 1,440 W for anything running three hours or more.

Why do motors need bigger circuits than their wattage suggests?

Two reasons: power factor raises running current above what wattage implies, and motors draw several times their running current for the moment they start.

Does higher voltage reduce current?

Yes, proportionally. The same power at 240 V draws half the current it would at 120 V, which is why large appliances use the higher voltage.

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