Watt Calculator (Volts × Amps to Watts)
Convert volts and amps to watts for DC, single-phase AC and three-phase AC with power factor, plus kVA, daily kWh and optional running cost.
Watts measure the real power a device uses. For DC it is simply volts × amps, but on AC the power factor matters, and three-phase supplies add a factor of √3 or 3 depending on which voltage you measure. This watt calculator handles all four cases, shows running apparent power in VA for comparing generators and UPS units, and turns the result into daily kWh and an optional running cost.
How to use this tool
- Choose DC, single-phase AC or three-phase AC (line-to-line or line-to-neutral voltage).
- Enter the voltage and the current in amps, from the nameplate or a clamp meter.
- For AC, enter true power factor for the operating condition, using RMS voltage and current.
- Enter hours of use per day and, if you like, your electricity price per kWh.
Formula
DC: P = V × I 1-phase: P = PF × V × I 3-phase: P = √3 × PF × V_LL × I or 3 × PF × V_LN × I
P is real power in watts, V voltage, I current per line in amps and PF the power factor (0 to 1). Apparent power S in volt-amperes is the same formula without PF. Three-phase formulas assume a balanced load. Daily energy is P × hours ÷ 1,000 kWh. Cost uses your own currency units, excluding fixed charges and tariffs.
Example
A space heater on a 120 V single-phase circuit draws 10 A with a power factor of 1: P = 1 × 120 × 10 = 1,200 W (1.2 kW). Used 3 hours a day it consumes 3.6 kWh daily and 108 kWh over 30 days. At a price of 0.15 per kWh that month would cost 16.20.
Frequently asked questions
What is the difference between watts and VA?
VA (apparent power) is volts × amps. Watts measure net energy transfer, including heat and losses. With a power factor of 0.8, a 1,000 VA load uses 800 W. Check both W and VA equipment ratings, along with starting surges; running VA alone is insufficient for sizing.
Which three-phase option should I choose?
Pick line-to-line if your voltage is measured between two phases (400 V, 480 V, 208 V systems). Pick line-to-neutral if it is measured from one phase to neutral (230 V, 277 V, 120 V).
Can I calculate watts from a device's fuse rating?
A fuse or breaker rating is a protection rating, not a measurement of normal draw or an instantaneous limit. Use matched operating measurements or nameplate input values; allow for startup and duty cycle.
Last updated: 2026-10-30