A watt calculator for DC, single-phase and three-phase power
This watt calculator turns volts and amps into watts — and back again — for the three kinds of supply you meet in practice: DC (batteries, solar, USB), single-phase AC (homes: 230 V in India and Europe, 230–240 V in the UK, 120 V in North America) and three-phase AC (workshops, pumps, EV chargers, industry). For AC it includes the power factor, so the answer is the real power a meter records, not just volts times amps.
Separate tabs find the current a load draws, the voltage for a given power, convert between VA and watts for UPS and generator ratings, and add up the energy and running cost of your appliances in ₹, $, £ or €.
How to use the Watt Calculator
- Choose a tab: Watts from V & A, Amps from watts, Volts from watts, VA ↔ watts or Appliance energy.
- Pick the supply: DC, AC single-phase or AC three-phase. For three-phase, say whether your voltage is line-to-line (400 V) or line-to-neutral (230 V).
- Enter the two known values and, for AC, the power factor from the rating plate (use 1 for purely resistive loads).
- Read the result with kilowatts, apparent and reactive power, and the formula with your numbers.
- For energy, list appliances with their watts, quantity and hours per day, then set the price per kWh and days per month.
Watt Calculator features
Three supply types
P = V × I for DC, V × I × PF for single-phase, and √3 × V × I × PF (or 3 × V × I × PF from phase voltage) for three-phase.
Reverse calculations
Amps from watts for choosing cables and breakers, and volts from watts and amps.
Real, apparent and reactive
Every AC answer shows watts, VA and var so you can see what the power factor costs.
VA and kVA
Convert UPS, inverter and generator ratings between VA and W at any power factor.
Energy and cost
kWh per day, month and year with the share of each appliance, at a flat price per unit.
Power formulas
The √3 (≈ 1.732) appears because the three line voltages are 120° apart, so the voltage between two lines is √3 times the voltage from a line to neutral: 230 V × 1.732 ≈ 400 V. Both forms give the same power for the same system.
| Supply | Watts | Amps from watts | Example |
|---|---|---|---|
| DC | P = V × I | I = P ÷ V | 12 V × 5 A = 60 W |
| AC single-phase | P = V × I × PF | I = P ÷ (V × PF) | 230 V × 10 A × 0.8 = 1,840 W |
| AC three-phase (line voltage) | P = √3 × V × I × PF | I = P ÷ (√3 × V × PF) | √3 × 400 V × 10 A × 0.85 = 5,889 W |
| AC three-phase (phase voltage) | P = 3 × V × I × PF | I = P ÷ (3 × V × PF) | 3 × 230 V × 10 A × 0.85 = 5,865 W |
Watts, VA and power factor
In an AC circuit with motors, compressors or switch-mode electronics, current and voltage are not perfectly in step. Volts × amps then gives the apparent power in volt-amperes (VA); only part of it, the real power in watts, does work. The ratio is the power factor: W = VA × PF. The rest is reactive power (var), which still has to be carried by wires and transformers.
That is why a 1,000 VA UPS at a power factor of 0.8 supplies at most 800 W, and why a three-phase motor drawing 10 A at 400 V and PF 0.85 takes 5,889 W of real power, not the 6,928 VA that √3 × volts × amps suggests. For generator and transformer ratings in kVA, kW, kVAR and amps, use the kVA to kW converter; for kW-to-horsepower and other unit conversions, the power converter; for circuits with resistance, the Ohm’s law calculator.
Worked examples
- How many amps is 1,000 watts? At 230 V and PF 1: 1,000 ÷ 230 = 4.35 A. At 120 V: 8.33 A. At 12 V DC: 83.3 A — which is why low-voltage systems need thick cables (check the drop with the voltage calculator).
- A 2 kW heater on 230 V draws 2,000 ÷ 230 = 8.70 A.
- An 11 kW three-phase motor at 400 V and PF 0.85 draws 11,000 ÷ (1.732 × 400 × 0.85) = 18.7 A per line (electrical input; a motor’s rated output is mechanical, so divide by its efficiency for input power).
- Energy: two 75 W fans for 10 hours and a 1,500 W air conditioner for 6 hours use 1.5 + 9 = 10.5 kWh a day, or 315 kWh in 30 days — ₹2,520 at ₹8 per unit.
Watt Calculator FAQ
How do I convert amps to watts?
Multiply volts by amps, and by the power factor for AC: watts = V × A × PF. On a 230 V single-phase supply, 10 A at PF 0.8 is 1,840 W. The watt calculator does this for DC and three-phase too.
How do I convert watts to amps?
Divide watts by volts (and by the power factor for AC): amps = W ÷ (V × PF). For three-phase, also divide by √3 when using the line-to-line voltage.
What is the difference between kW and kVA?
kW is real power that does work; kVA is apparent power, volts times amps. They are equal only at a power factor of 1. kW = kVA × PF.
What power factor should I use?
Use the value on the rating plate if there is one. Heaters, kettles and incandescent lamps are 1; motors are typically 0.8–0.9 at full load; modern electronics with power-factor correction are 0.9 or higher.
How do I calculate the running cost of an appliance?
kWh = watts × hours ÷ 1,000, and cost = kWh × price per unit. A 100 W device on for 5 hours uses 0.5 kWh; at ₹8, $0.17 or £0.25 per kWh that costs ₹4, 8.5¢ or 12.5p. These are estimates; your bill may include slabs, fixed charges and taxes.
Why is three-phase power √3 and not 3?
With the line-to-line voltage, the √3 accounts for the 120° spacing of the phases. If you use the line-to-neutral voltage instead, the factor is 3 — the calculator accepts either.