How to convert watts, volts and amps
- Choose DC, AC single phase (household sockets) or AC three phase, and what you want to Work out.
- Enter the two values you know. For three phase, enter the voltage between two lines (such as 400 V or 480 V).
- For AC, set the Power factor: 1 for heaters, kettles and filament bulbs, lower for motors (the nameplate often gives it). Read the answer, the kW and kVA, and the formula.
The formulas
| Circuit | Amps | Watts | Volts |
|---|---|---|---|
| DC | I = P ÷ V | P = V × I | V = P ÷ I |
| AC single phase | I = P ÷ (V × PF) | P = V × I × PF | V = P ÷ (I × PF) |
| AC three phase | I = P ÷ (√3 × V × PF) | P = √3 × V × I × PF | V = P ÷ (√3 × I × PF) |
P is power in watts, V voltage in volts, I current in amps and PF the power factor. For three phase, V is the line-to-line voltage and I the current in each line. √3 is about 1.732.
Watts, VA and kW
Watts measure the power a device actually uses. Volt-amperes (VA) are volts times amps, the power the wiring has to carry. With a power factor of 1 they are equal; with a power factor of 0.8 a 1,000 W load needs 1,250 VA. Generators and UPS units are often rated in kVA for this reason, so check both figures when sizing one.
Checking a circuit
The current a load draws is what decides the wire and breaker size. In the US the electrical code limits continuous loads to 80% of the breaker rating, so a 15 A circuit should carry no more than 12 A for long periods. Rules differ between countries, so ask a qualified electrician before adding big loads. For resistance and power together, the Ohm’s law calculator solves V = I × R.