Convert freely between watts, amps and volts — DC, single-phase or three-phase, with power factor handled — and see what the load costs to run.
V
120 V or 240 V US · 230 V UK/EU · 12 V or 24 V DC · 208/400/480 V three-phase.
A
W
1.0 for heaters, kettles and incandescent lamps. 0.85–0.95 for motors and LED drivers. 0.6–0.8 for older motors.
Running cost (optional)
Power
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Watts—
Amps—
Volts—
Kilowatts—
Apparent power (VA)—
kWh per day—
Cost per month—
Cost per year—
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The three formulas
Everything here comes from one relationship — power equals voltage times current — adjusted for how the current and voltage line up in AC systems.
System
Power
Current
DC
W = V × A
A = W ÷ V
AC single-phase
W = V × A × PF
A = W ÷ (V × PF)
AC three-phase
W = √3 × V × A × PF
A = W ÷ (√3 × V × PF)
In three-phase, V is the line-to-line voltage and √3 (about 1.732) accounts for the 120° phase separation between the three legs.
Power factor, and why VA is not W
In an AC circuit with motors, transformers or switching supplies, current and voltage fall out of step. The apparent power in volt-amps (V × A) is what the wiring, breaker and generator must carry; the real power in watts is what does useful work and appears on your bill. Power factor is the ratio between them.
A 0.7 power factor motor drawing 1,000 W pulls 1,430 VA, so the cabling and breaker must be sized for the higher figure even though you are billed for the lower one. This is why generator and UPS ratings are given in kVA rather than kW — and why sizing a generator by watts alone leaves you short.
An LED bulb is 6–12 W, a laptop 30–65 W, a fridge 100–200 W while the compressor runs, a desktop PC 300–600 W under load, a microwave 800–1,200 W, a kettle or toaster 1,500 W, and an electric shower 8,500–10,500 W. At 120 V a 15 A circuit tops out around 1,800 W — and the continuous-load rule means you should not plan more than 1,440 W of it for anything running over three hours.
Sizing rather than converting
When you are choosing a circuit rather than just converting units, add a 25% headroom for any load that runs continuously: a 16 A continuous load needs a 20 A circuit. Then check the cable is sized for that breaker over the distance involved, which is where the wire size calculator and voltage drop calculator take over.
Divide watts by volts for DC. For single-phase AC divide by volts times power factor, and for three-phase divide by 1.732 times volts times power factor. A 1,200 W load on 120 V at unity power factor draws 10 A.
How do I convert amps to watts?
Multiply amps by volts, then by power factor for AC. Ten amps at 240 V with a power factor of 0.9 gives 2,160 W. Without the power factor you would overstate it as 2,400 W.
What is power factor and what should I use?
It is how well current and voltage stay in step in an AC circuit. Use 1.0 for purely resistive loads — heaters, kettles, incandescent bulbs. Use 0.85 to 0.95 for modern motors and LED drivers, and 0.6 to 0.8 for older induction motors.
What is the difference between watts and VA?
Watts is the real power that does work and gets billed. VA is the apparent power the wiring actually has to carry. They are equal only at a power factor of 1. Generators and UPS units are rated in kVA because that is the limit their hardware faces.
Why do three-phase calculations include √3?
The three legs of a three-phase supply are 120 degrees apart, so their contributions do not simply add. The factor √3 — about 1.732 — is what falls out of combining them, and it applies whenever you work from line-to-line voltage.