About the Watts, Amps and Volts Calculator
This watts, amps and volts calculator converts between the three basic electrical quantities. Choose what you want to find — current in amps, power in watts or voltage in volts — pick the circuit type, and enter the two values you know. It works for DC systems such as 12 V vehicles and solar batteries, for ordinary single-phase household AC, and for three-phase commercial supplies.
Use it to check how many amps an appliance draws before plugging it into a circuit, to size a fuse or inverter, to convert a motor nameplate current to watts, or to compare equipment rated in different units. It also reports apparent power in volt-amperes, which is what generators, UPS units and transformers are rated in.
For AC loads, real power equals volts × amps × power factor. Heaters, kettles and incandescent lamps have a power factor close to 1; motors, compressors and many power supplies are lower (0.7–0.95). For three-phase circuits enter the line-to-line voltage (for example 208, 400 or 480 V).
How to use the watts, amps and volts calculator
- 1Choose whether you want amps, watts or volts.
- 2Select DC, single-phase AC or three-phase AC.
- 3Enter the two values you know (from the appliance label or nameplate).
- 4For AC motors or electronics, enter the power factor if you know it.
- 5Read the result and the apparent power in VA.
Formula and method
Power is the product of voltage and current. In DC circuits that is the whole story. In AC circuits the current and voltage can drift out of step, so real power (watts) is volts × amps × power factor, while apparent power (VA) is simply volts × amps.
Three-phase circuits carry power on three conductors, so with the line-to-line voltage the formula gains a factor of √3 ≈ 1.732. Rearranging gives current I = P ÷ (V × PF) for single-phase or I = P ÷ (√3 × V × PF) for three-phase, and voltage V = P ÷ (I × PF).
- P
- Real power in watts
- V
- Voltage (line-to-line for three-phase)
- I
- Current in amps (per line)
- PF
- Power factor, 0–1
Worked examples
1800 W appliance on 120 V
I = 1800 ÷ 120 = 15 A. That is the full rating of a 15 A circuit, so nothing else should share it while the appliance runs.
10 kW motor on 480 V three-phase, PF 0.85
I = 10,000 ÷ (1.732 × 480 × 0.85) ≈ 14.15 A per line. The supply sees 10,000 ÷ 0.85 ≈ 11,765 VA of apparent power.
8 A from a 12 V battery
For DC, P = V × I = 12 × 8 = 96 W.
Voltage for 2400 W at 10 A
V = P ÷ I = 2400 ÷ 10 = 240 V, which tells you this is a 240 V appliance.
Frequently asked questions
How many amps is 1500 watts?+
At 120 V, 1500 W draws 1500 ÷ 120 = 12.5 A. At 230 V it draws about 6.5 A. Higher voltage means less current for the same power.
How do I convert amps to watts?+
Multiply amps by volts (and by power factor for AC motors or electronics). A 10 A load on 240 V with a power factor of 1 uses 2,400 W.
What is the difference between watts and VA?+
Watts measure real power that does work; volt-amperes (VA) measure apparent power, volts × amps. They are equal only when the power factor is 1. Generators and UPS units are rated in VA.
What power factor should I use?+
Use 1.0 for heaters, kettles and incandescent lights. Motors typically run at 0.8–0.9, and older electronics without power-factor correction can be 0.6–0.7. The nameplate often lists it.
Why is there a √3 in the three-phase formula?+
In a balanced three-phase system the line-to-line voltage is √3 times the phase voltage, so total power is √3 × V(line) × I × PF rather than 3 × V(phase) × I × PF — the two give the same answer.