Watts / Volts / Amps Calculator
Calculate power, voltage or current for DC circuits, single-phase AC and three-phase AC. Enter any two known values — the third is calculated instantly. Includes power factor for AC loads.
Last updated: May 2026
Select circuit type, choose what to solve, then enter the two known values.
DC: P = V × I
DC vs single-phase vs three-phase power
The relationship between watts, volts and amps differs by circuit type. Getting the formula wrong — especially on three-phase — leads to undersized wiring, tripped breakers or incorrect energy cost estimates.
Formula reference
| Circuit | Power formula | Notes |
|---|---|---|
| DC | P = V × I | No power factor. Used for battery systems, 12/24 V loads. |
| AC single-phase | P = V × I × PF | PF = 1.0 for pure resistive loads (heaters, incandescents). Motors, switching PSUs typically 0.6–0.95. |
| AC three-phase | P = √3 × VL × IL × PF | VL = line voltage (e.g. 400 V). √3 ≈ 1.732. Used for industrial motors, EV fast chargers. |
Typical use cases
- Sizing a fuse or circuit breaker for a known load
- Estimating current draw from a rated power figure on a nameplate
- Checking if a supply is large enough for a motor or inverter
- Converting between apparent power (VA) and real power (W) using power factor
Frequently Asked Questions
What power factor should I use for a motor?
Induction motors typically have PF between 0.7 and 0.9 at full load, and significantly lower at partial load. The nameplate usually lists both kW and kVA — divide the two to get PF. For rough estimation when the datasheet is unavailable, use 0.85 for motors 1 kW and above, 0.75 for smaller motors.
Why is three-phase more efficient than single-phase?
Three-phase delivers the same total power at a lower current per conductor because the load is distributed across three phases. A 10 kW load on 230 V single-phase pulls 43.5 A per conductor. The same load on 400 V three-phase at PF 0.9 pulls only 16.1 A per conductor — about 37% of the single-phase current. This reduces conductor size and resistive losses.
What is the difference between W (watts) and VA (volt-amperes)?
Watts measure real power — the work actually done. VA measures apparent power — the product of RMS voltage and current regardless of phase. The ratio is power factor. A 1000 VA UPS with PF = 0.8 only delivers 800 W to the load. Always check both figures when sizing backup power or generator capacity.
How do I check the current draw of a 400 V three-phase motor?
Use I = P ÷ (√3 × V × PF). For a 7.5 kW motor at 400 V and PF 0.87: I = 7500 ÷ (1.732 × 400 × 0.87) = 12.45 A. The three-phase mode in this calculator handles the √3 factor automatically — enter the kW figure in the Watts field (as watts, e.g. 7500), set voltage to 400 and power factor to 0.87.