Watts to Amps Calculator
A 1,500-watt space heater on a standard 120-volt outlet draws 12.5 amps (1,500 ÷ 120). Enter your own watts, voltage, and circuit type below for an exact result — single-phase or three-phase.
Current draw
12.50 A
1500W ÷ 120V
How the conversion works
For a single-phase circuit — the kind that powers nearly every US home — the formula is simple: amps equal watts divided by volts. This comes straight from the power equation watts = volts × amps, rearranged to solve for amps.
For a three-phase circuit — common in commercial buildings, workshops, and some standby generators — the math adds two more factors: the square root of 3 (about 1.732, from how three offset AC waveforms combine) and the power factor. The formula is amps = watts ÷ (√3 × volts × power factor).
Power factor measures how efficiently a load turns the power it draws into useful work, on a scale from 0 to 1. A purely resistive load, like a heater or incandescent bulb, has a power factor of 1.0. Motors, HVAC equipment, and other inductive loads usually run lower — typically 0.8 to 0.9. This calculator defaults to 0.85 for three-phase circuits, a reasonable estimate for mixed commercial loads. Check your equipment's nameplate for its exact power factor when precision matters.
Worked examples
Single-phase: a window air conditioner rated at 1,200 running watts on a standard 120-volt household circuit draws 1,200 ÷ 120 = 10 amps. That's within a typical 15- or 20-amp circuit's limit, but close enough that running another large appliance on the same circuit could trip the breaker.
Three-phase: a 10,000-watt piece of shop equipment on a 208-volt three-phase circuit with a 0.85 power factor draws 10,000 ÷ (1.732 × 208 × 0.85) ≈ 32.66 amps. Leave out the power factor and the result looks like a lower 27.75 amps — enough of a gap to undersize a breaker or wire.
Common US voltages
- 120V — standard wall outlets, lamps, electronics, and most small appliances.
- 240V — dedicated circuits for clothes dryers, electric ranges, and other large appliances.
- 208V — line-to-line voltage in many commercial buildings' three-phase systems.
- 480V — heavy commercial and industrial three-phase equipment.
Common mistakes
Using the single-phase formula on a three-phase circuit. Skipping the √3 factor overstates the amps by roughly 73%.
Forgetting power factor. Leaving power factor out of a three-phase or motor-driven calculation understates the true current draw, which can lead to an undersized breaker or wire.
Confusing running watts with surge watts. Many motor- or compressor-driven appliances need far more power to start than to run. If you're sizing a circuit, battery, or backup generator to guarantee an appliance always starts, use surge watts, not running watts — see our generator size calculator.
Mixing up volts and amps. Watts is a rate of energy use, volts is electrical pressure, and amps is current. You can't get amps from watts alone — voltage is always part of the equation. If you're working with a DC battery system that's rated in amp-hours instead of watts, use our amp-hours to watt-hours calculator to convert first.
Appliance amps at 120V
These are published running-watt specs for common household appliances, converted to amps at a standard 120-volt outlet using the single-phase formula above. Based on published specs — actual draw varies by model. Want to know what that amp draw costs to run? Use our electricity cost calculator.
| Appliance | Running watts | Amps at 120V |
|---|---|---|
| Refrigerator (standard, 18-20 cu ft, frost-free) | 150W | 1.25 A |
| Microwave Oven (countertop, ~1,000-1,200W cooking power) | 1,000W | 8.33 A |
| Window Air Conditioner (8,000-10,000 BTU) | 1,200W | 10.00 A |
| Hair Dryer | 1,500W | 12.50 A |
| Portable Electric Space Heater | 1,500W | 12.50 A |
| Toaster (2-slice/4-slice) | 1,000W | 8.33 A |
| Drip Coffee Maker | 1,000W | 8.33 A |
| Dishwasher (standard, with heated dry option) | 1,800W | 15.00 A |
| Washing Machine (top-load or front-load, standard, no internal water heater) | 500W | 4.17 A |
FAQ
How do you convert watts to amps?
Divide watts by volts for a single-phase (or DC) circuit: amps = watts ÷ volts. For a three-phase circuit, divide watts by voltage times the square root of 3 (about 1.732) times the power factor: amps = watts ÷ (√3 × volts × power factor).
What's the difference between single-phase and three-phase for this calculation?
Single-phase circuits — what almost every US home uses — only need watts and voltage. Three-phase circuits, common in commercial and industrial buildings, also need the √3 factor and a power factor, because three offset AC waveforms combine to deliver the power instead of one.
What is power factor and why does it matter?
Power factor measures how much of the power drawn actually does useful work, on a scale from 0 to 1. Purely resistive loads, like heaters and incandescent bulbs, have a power factor of 1.0. Motors and other inductive loads typically run 0.8 to 0.9. Skipping power factor on a three-phase or motor-driven load understates the true amps.
How many amps does a 1,500-watt appliance draw at 120 volts?
12.5 amps (1,500 ÷ 120). That is close to the 15-amp rating of a standard household circuit, which is why many portable space heaters and hair dryers are capped at 1,500 watts by design.
What is the difference between line-to-line and line-to-neutral voltage in three-phase power?
Line-to-line voltage is measured between two of the three power wires and is the higher figure (for example, 208V or 480V). Line-to-neutral voltage is measured between one power wire and the neutral wire and is lower — line-to-line divided by √3. Most three-phase amp calculations, including this one, use line-to-line voltage.
Data sources
Last updated 2026-09-22
- EcoFlow — Appliance Wattage Chart & Energy Usage Calculator — accessed 2026-09-22
- Generatorist — Power Consumption of Household Appliances — accessed 2026-09-22
- Virginia Cooperative Extension — Appliance Energy Use Chart — accessed 2026-09-22
- Unbound Solar — Power Table — accessed 2026-09-22
Written by GoSolarIndex Team