Electrical Power Calculator
Choose the supply, enter two of voltage, current and power, and get the third. Add the hours and your unit price to see what it costs to run.
Fill in two of voltage, current and power and leave the one you want to find empty. The answer appears as you type, in the unit shown next to its box. Direct current ignores the power factor.
Worked examples
Kettle
Single phase, 230 V, 13 A, power factor 1
2,990 W
Watts = 230 x 13.
Three phase motor
400 V, 10 A, power factor 0.8
About 5,543 W
Watts = 1.732 x 400 x 10 x 0.8.
About Electrical Power Calculator
This electrical power calculator solves the relationship between power in watts, voltage in volts and current in amps for three kinds of supply: direct current, single phase alternating current and a balanced three phase load. Enter any two and it gives you the third, along with the apparent power in volt-amperes and the reactive power, which matter when you choose cables, fuses and generators.
Real equipment rarely draws current in step with the voltage, and the power factor describes how far out of step it is. Motors, fluorescent fittings and chargers often have a power factor between 0.6 and 0.95, which means the current is higher than the watts alone suggest. Add the hours of use and your price per unit and the page also gives the energy in kilowatt hours and what it costs to run.
How to use Electrical Power Calculator
- Choose the supply: direct current, single phase or three phase.
- Enter any two of voltage, current and power, and leave the third empty. Set the power factor if the label gives one.
- For three phase, use the voltage between two lines and the current in one line.
- Optionally add the hours of use and the price per kWh in pence to see the running cost.
Common uses
- Finding the current a 3 kW heater draws on a 230 volt supply
- Sizing a generator or a cable for a three phase motor
- Comparing the running cost of two appliances
- Converting a power rating in kilowatts into amps for a fuse
Frequently asked questions
How do I convert watts to amps?
Divide the power in watts by the voltage, and for alternating current also by the power factor. A 2,990 watt appliance on 230 volts with a power factor of 1 draws 13 amps.
What is power factor?
Power factor is the real power divided by the apparent power. It is 1 for a simple heater and lower for loads with coils or capacitors. A low power factor means more current for the same useful power.
Which voltage do I use for three phase?
Use the voltage between two of the lines, which is 400 volts in the UK. The line to neutral voltage is 230 volts. The three phase formula multiplies by the square root of three, about 1.732.
Is the running cost exact?
It multiplies the kilowatt hours by the price you enter, so it is as accurate as the hours and the price. Standing charges, and loads that cycle on and off, are not included.