Formula
DC: P = V × I
AC single phase: P = V × I × PF
AC three phase: P = √3 × VL-L × I × PF
Worked example
A single-phase motor drawing 10 A at 230 V with PF 0.85: P = 230 × 10 × 0.85 = 1,955 W of real power.
Reference table
Quick reference (PF = 1)
| Amps | Watts @ 120 V | Watts @ 230 V |
|---|---|---|
| 1 | 120 | 230 |
| 5 | 600 | 1,150 |
| 10 | 1,200 | 2,300 |
| 15 | 1,800 | 3,450 |
| 20 | 2,400 | 4,600 |
| 30 | 3,600 | 6,900 |
Where this shows up in the real world
Clamp meters read amps; electricity bills and generator budgets speak watts. Reading 8 A on a 240 V well-pump circuit and converting to roughly 1,900 W (at typical motor PF) tells you what that pump costs to run and what share of a backup generator it will eat — the bridge between what you can measure and what you pay for.
Common mistakes to avoid
The mistake here is multiplying volts by amps and calling it watts on an AC motor circuit — without the power factor you've calculated VA, not W, and overstated real power by 10–30%. For billing math you want watts; for sizing the supply you actually want the VA figure. Know which question you're asking.
Frequently asked questions
Why do I need power factor for AC?
In AC circuits, voltage and current can be out of phase. Real power (watts) is only the portion doing useful work: P = V × I × PF.
What if I don't know the power factor?
Use 1.0 for purely resistive loads, around 0.85 for typical motors. Nameplates on equipment often state PF (sometimes as cos φ).
Is this the same as VA?
No — V × I alone gives volt-amps (apparent power). Multiplying by power factor converts it to watts (real power).