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Watts to Amps Calculator: The Electrical Formulas You Actually Need
The Formula
Amps = Watts / Volts
That’s it for single-phase resistive loads. A 1,500-watt space heater on a 120V circuit draws 12.5 amps.
For motors and inductive loads, power factor enters the picture:
Amps = Watts / (Volts x Power Factor)
A motor rated 1,500 watts with a 0.85 power factor on 120V draws 14.7 amps, not 12.5. Power factor accounts for the fact that motors use some energy to maintain their magnetic field and that energy doesn’t do useful work. Resistive loads like heaters and light bulbs have a power factor of 1.0, so the simple formula works.
The Quick Reference Table
These come up constantly. Memorize the ones you use, look up the rest.
| Appliance | Watts | Volts | Amps |
|---|---|---|---|
| 15A circuit capacity | 1,800 | 120 | 15 |
| 20A circuit capacity | 2,400 | 120 | 20 |
| Space heater | 1,500 | 120 | 12.5 |
| Microwave | 1,200 | 120 | 10 |
| Hair dryer | 1,875 | 120 | 15.6 |
| Circular saw | 1,400 | 120 | 11.7 |
| Table saw (contractor) | 1,725 | 120 | 14.4 |
| Window AC (10,000 BTU) | 1,200 | 120 | 10 |
| Electric dryer | 5,400 | 240 | 22.5 |
| Electric range | 12,000 | 240 | 50 |
| Welder (small MIG) | 4,800 | 240 | 20 |
| EV charger (Level 2) | 7,680 | 240 | 32 |
| Tankless water heater | 24,000 | 240 | 100 |
Notice that 240V circuits draw half the amps for the same wattage. That’s why dryers, ranges, and EV chargers run on 240V. A 5,400-watt dryer on 120V would need 45 amps, which is impractical for household wiring. On 240V it’s a manageable 22.5.
The 80% Rule
NEC 210.20(A): continuous loads (running 3+ hours) can’t exceed 80% of the circuit breaker rating.
A 20-amp circuit has 2,400 watts of capacity at 120V, but only 1,920 watts (16 amps) for continuous loads. A 15-amp circuit: 1,440 watts continuous.
This matters for space heaters, grow lights, and anything else that runs for hours. A 1,500-watt space heater draws 12.5 amps for hours at a time, which is more than 80% of a 15-amp breaker (12 amps). Heaters are made to plug into 15-amp receptacles, but share that circuit with anything else and you’re over. A dedicated 20-amp circuit is the better home for one.
Three-Phase
For three-phase (commercial and industrial):
Amps = Watts / (Volts x 1.732 x Power Factor)
The 1.732 is the square root of 3. It comes from the phase angle between the three legs of power.
A 10 HP motor on 480V three-phase (PF = 0.85): 10 HP = 7,460 watts of output 7,460 / (480 x 1.732 x 0.85) = 10.6 amps
That formula ignores motor efficiency, so it runs low. For sizing conductors and protection, the NEC doesn’t use it at all: NEC 430.6 sends you to the full-load current tables. Table 430.250 lists a 10 HP three-phase motor at 14 amps at 460V and 30.8 amps at 208V.
Voltage matters. Higher voltage means lower amperage, which means smaller wire, which means less copper, which means less money. That’s the whole reason commercial buildings run 480V.
“Can I Run This on That Circuit?”
The question that actually brings people to this formula. Some common ones:
“Can I run a welder on a 20-amp 120V circuit?” Many small 120V flux-core welders want a dedicated 20-amp circuit and can still trip it at full output. Welder circuits are sized by NEC Article 630 using the nameplate current and duty cycle, so follow the welder’s manual. For heavier work, a 240V welder on its own circuit is the usual answer.
“Can I run my table saw and shop vac on the same 20-amp circuit?” Table saw: 14.4 amps startup (higher surge, maybe 18-20 amps). Shop vac: 10 amps. Together: 24+ amps. No. Separate circuits.
“How many LED can lights on a 15-amp circuit?” A typical LED recessed light uses 12 watts. 15 amps x 120V x 0.80 (continuous derating) = 1,440 watts. 1,440 / 12 = 120 lights. You’ll run out of wire capacity and junction boxes long before you hit the electrical limit. LED loads on 15-amp circuits are practically unlimited for residential rooms.
“Can I charge my EV on a regular outlet?” A standard 120V, 15-amp outlet (Level 1) delivers about 1.4 kW. That adds roughly 4-5 miles of range per hour. An average commute of 30 miles needs 6-8 hours of charging. It works, it’s just slow. A Level 2 charger on a 240V, 50-amp circuit delivers 9.6 kW and adds 25-30 miles per hour.
VA vs Watts
On data sheets and nameplates, you’ll see both watts (W) and volt-amps (VA). For resistive loads they’re the same. For motors, transformers, and UPS systems, VA is higher than watts by the power factor.
A UPS rated 1,500 VA with a 0.6 power factor only delivers 900 watts of real power. You can’t run 1,500 watts of equipment on it. Always check the watt rating, not just the VA rating, for UPS systems and generators.
Watts-to-amps conversions are a building block for bigger electrical work. If you’re sizing a whole panel, the electrical load calculation per NEC 220 uses these same VA numbers to determine whether you need 100-amp, 200-amp, or larger service.
Running the Numbers
Trussly doesn’t have a standalone watts-to-amps converter; the formula above is quick by hand. Once you have amps, its Breaker Sizing calculator applies the 125% continuous-load rule, and the wire sizing and voltage drop calculators take it from there.