Watts to Amps Converter — Free Online Calculator
Convert watts to amps instantly. Works for single-phase and three-phase AC, plus DC circuits. Enter watts, voltage, and power factor.
How to Use This Calculator
Enter the power in watts, the voltage, and select DC, single-phase AC, or three-phase AC. For AC circuits, you can also enter the power factor (default 1.0 for resistive loads). The calculator converts watts to amps using the appropriate formula.
The Formula Explained
For DC: Amps = Watts / Volts. For single-phase AC: Amps = Watts / (Volts × Power Factor). For three-phase AC: Amps = Watts / (Volts × √3 × Power Factor). Power factor accounts for the phase difference between voltage and current in AC circuits — resistive loads like heaters have PF = 1, while motors typically have PF = 0.8-0.9.
Common Watts to Amps Conversions (120V, PF=1)
| Watts | Amps @ 120V | Amps @ 240V | Breaker Size |
|---|---|---|---|
| 500W | 4.17A | 2.08A | 15A |
| 1000W | 8.33A | 4.17A | 15A |
| 1500W | 12.50A | 6.25A | 15A |
| 2000W | 16.67A | 8.33A | 20A |
| 3000W | 25.00A | 12.50A | 30A |
| 5000W | 41.67A | 20.83A | 50A |
| 7500W | 62.50A | 31.25A | 70A |
| 10000W | 83.33A | 41.67A | 100A |
Watts to Amps: Sizing Loads for Circuits
Converting watts to amps is the most common sizing calculation in electrical work. You have a load specified in watts (from a data plate, specification, or energy rating), and you need to know how many amps it will draw to select the correct wire, breaker, and outlet. The math is simple — amps equals watts divided by volts, adjusted for phase configuration and power factor — but the details trip up even experienced electricians.
The calculation also reveals why North America uses 240V for high-power appliances and Europe uses 230V for nearly everything. At the same wattage, higher voltage means lower current. A 3,000W water heater at 120V would draw 25 amps (needing 10 AWG wire and a 30A breaker). The same heater at 240V draws 12.5 amps (14 AWG wire, 20A breaker). The voltage doubling cuts the wire cost in half and the breaker cost by about a third. Europe's standard 230V lets most residential loads run on smaller wire than equivalent 120V North American circuits.
Worked Example: Sizing a Microwave Circuit
A 1,200-watt microwave oven at 120V single-phase. Current: 1,200 / 120 = 10 amps. Microwaves are technically not continuous loads (most people use them for minutes at a time), so no 125% adjustment is needed. A dedicated 20A branch circuit with 12 AWG wire handles this comfortably.
But most kitchens feed the microwave from a small-appliance circuit that NEC requires to be 20A per 210.11(C)(1). You could run a 15A circuit (14 AWG, 15A breaker) and the microwave would work, but code requires 20A for receptacle circuits in kitchens. Dedicated over-the-range microwaves should have their own 20A circuit separate from the countertop receptacles.
Nameplate power can be misleading: the 1,200W figure is usually cooking power (microwave energy output). Actual electrical input might be 1,500-1,800W due to magnetron efficiency of 65-80%. Check the nameplate "input power" or "line current" for accurate sizing. A "1,200W" microwave might actually draw 14 amps on a 120V circuit, not 10.
Worked Example: Commercial LED Lighting Retrofit
A warehouse retrofits 30 metal halide high-bay fixtures (400W each, 12,000W total) with 30 LED high-bays (150W each, 4,500W total). The existing circuit was designed for the original load: at 277V single-phase, 12,000W / 277V = 43.3 amps, on a 50A circuit with 8 AWG wire.
After retrofit: 4,500W / 277V = 16.2 amps. That uses only 32% of the existing 50A circuit capacity. Options: leave the existing infrastructure (safe but wasteful of breaker capacity) or add more fixtures to the existing circuit (cost-effective use of installed capacity). The existing 50A circuit could handle 2.5x the new load, so up to 75 LED fixtures could be added without changing wire or breaker.
Many LED retrofits miss this opportunity to consolidate or expand lighting on existing circuits. With smart planning, a retrofit can increase lighting coverage while reducing total energy consumption — a win-win.
Five Watts-to-Amps Pitfalls
1. Forgetting the 125% continuous load rule. NEC 210.19 and 215.2 require sizing branch circuits and feeders at 125% of continuous load (operating 3+ hours). EV chargers, water heaters, commercial lighting, and HVAC all trigger this rule.
2. Using nominal voltage instead of actual. Residential voltage varies from 114V to 126V per ANSI C84.1. At low end, a 1,500W heater draws 1500/114 = 13.2 amps instead of the nominal 12.5 amps. Design with margin.
3. Ignoring inrush current. Motors, capacitive power supplies, and incandescent bulbs (cold filament has 10x lower resistance) draw huge inrush currents for brief periods. Breakers must ride through these without tripping.
4. Not accounting for power factor on LED drivers. Cheap LED bulbs can have PF as low as 0.5, making the amps much higher than watts / volts suggests. Quality LEDs have PF 0.9+ and behave like resistive loads for calculation purposes.
5. Using watts to size wire for a three-phase load. Three-phase formula includes sqrt(3). Missing this factor makes your wire sizing 73% off.
Quick Reference: Watts to Amps
At 120V (residential outlet): 100W = 0.83A. 500W = 4.17A. 1000W = 8.33A. 1500W = 12.5A. 1800W = 15A (max on 15A circuit). 2400W = 20A (max on 20A circuit).
At 240V (residential large appliance): 1000W = 4.17A. 3600W = 15A. 4800W = 20A. 7200W = 30A (typical dryer). 9600W = 40A (EV charger, water heater). 12000W = 50A (range).
At 208V three-phase: 1 kW per phase ≈ 2.78 amps. 10 kW ≈ 27.8A. 50 kW ≈ 139A.
At 480V three-phase: 1 kW per phase ≈ 1.20 amps. 10 kW ≈ 12A. 100 kW ≈ 120A. 500 kW ≈ 601A.
Multiply by 1/PF for inductive loads. A 100 kW motor load at 0.85 PF on 480V three-phase draws 120 / 0.85 = 141 amps per phase, not 120.
Code and Standards Context
NEC Article 220 covers load calculations. NEC Table 220.12 gives general lighting load values in VA per square foot for various occupancy types. NEC 210.19 and 215.2 govern the 125% continuous load rule. NEC 210.23 covers permissible loads on general-purpose branch circuits.
For specific appliance circuits: NEC 422 covers appliance installation requirements. NEC 424 covers fixed electric space-heating equipment. NEC 625 covers EVSE. Each section has specific rules that modify the basic watts-to-amps calculation with load-specific safety factors and continuous-load treatment.
Watts to amps: the practical math behind P = V x I
The watts-to-amps conversion is the most-used calculation in residential and light commercial work. You know the wattage of an appliance from its nameplate, and you need amps to size a breaker, conductor, or cord. The math is straightforward for DC and single-phase AC, slightly more involved for three-phase or reactive loads.
The calculator handles all three cases: DC (I = P/V), single-phase AC (I = P / (V x PF)), and three-phase AC (I = P / (sqrt(3) x V x PF)). Power factor defaults to 1.0 for resistive loads (heaters, incandescent bulbs, EV charging) and to lower values for motors and electronics with cheap power supplies.
The formula and what it does
P is real power in watts. V is RMS voltage (120, 240, 208, 480). PF is power factor: 1.0 for resistive, 0.85-0.95 for motors at full load, 0.5-0.9 for switching electronics without PFC. The sqrt(3) factor in three-phase math is approximately 1.732 and comes from the geometric relationship of three voltage phases offset 120 degrees.
Worked example
Scenario: 1800 W microwave on a 120 V branch circuit.
Microwave PF is around 0.95 (the magnetron input is near-resistive). I = 1800 / (120 x 0.95) = 15.8 A. On a 20 A breaker, that is fine. On a 15 A breaker (12.5 A continuous limit per NEC 210.20(A)), it is over the continuous threshold and needs its own 20 A dedicated circuit.
Three-phase example: 10 kW commercial water heater on 480 V three-phase. I = 10,000 / (1.732 x 480 x 1.0) = 12.0 A. Tiny conductor for substantial wattage, which is why commercial buildings run three-phase whenever practical.
Common mistakes to avoid
Using nameplate wattage at startup. Motors draw 4-7x running current during locked-rotor startup. The breaker may need to be a time-delay type to ride through, even if the running amps are modest.
Forgetting power factor on motors. A 1500 W motor at PF 0.8 draws 15.6 A on 120 V, not 12.5 A. The breaker has to handle apparent current, not real wattage.
Treating 240 V loads as 120 V. A 4800 W water heater at 240 V draws 20 A. The same 4800 W at 120 V would draw 40 A and need much heavier wire.
Frequently asked questions
What is the difference between watts and VA?
Watts is real power, energy that does work. VA is apparent power, the product of RMS volts and amps the source must supply. They differ by the power factor. For resistive loads (heaters, incandescent lighting) they are equal.
Do I use 120 V or 110 V?
Use 120 V. The 110 V nominal is a holdover from earlier US standards. Modern utility service is 120/240 V nominal, with actual delivered voltage from 114 to 126 V per ANSI C84.1.
How do I find power factor?
Resistive loads: 1.0. Motors at full load: 0.85-0.95 (often marked on nameplate). Switching electronics with PFC: above 0.9. Cheap electronics without PFC: 0.5-0.7. When in doubt, use 0.85 for general mixed loads.
Why does 3-phase use sqrt(3)?
Three-phase power transmits over three lines spaced 120 degrees apart. The total power sums geometrically across all three lines, and the math works out to V_line x I_line x sqrt(3) x PF. sqrt(3) is approximately 1.732.
Can I use this for DC solar systems?
Yes. Set PF to 1.0 and use the DC voltage of the string (400-600 V is typical). The formula collapses to I = P / V.
Why is my measured current higher than the calculation?
Most likely your device has higher startup current (motor/compressor) or lower power factor than assumed. Clamp meters measure RMS amps including reactive current; nameplate watts are real only.