Power Size Calculator
Enter the voltage, current, phase and power factor, and the running hours if you want the energy. The calculator gives the real, apparent and reactive power and the energy used.
- Real power
- Apparent power
- Energy
Heaters and kettles have a power factor of 1. Motors are often 0.8 to 0.9.
Real power = volts × amps × power factor. At 230 V and 10 A with a power factor of 0.9, a load uses 2.07 kW of real power and 2.3 kVA of apparent power. Size a generator or UPS for the kVA, with headroom.
Power chart
Power factor 1. Search for a current.
| Current (A) | 120 V (kW) | 230 V (kW) | 400 V three phase (kW) |
|---|---|---|---|
| 1 | 0.12 | 0.23 | 0.69 |
| 5 | 0.6 | 1.15 | 3.46 |
| 10 | 1.2 | 2.3 | 6.93 |
| 13 | 1.56 | 2.99 | 9.01 |
| 16 | 1.92 | 3.68 | 11.09 |
| 20 | 2.4 | 4.6 | 13.86 |
| 32 | 3.84 | 7.36 | 22.17 |
| 50 | 6 | 11.5 | 34.64 |
| 100 | 12 | 23 | 69.28 |
kW.
Current and power
230 V, power factor 0.85. Slide the current.
The power triangle grows.
How to work out power
Volts and amps to kW. Step through it.
Volts
Supply.
Measure current with a clamp meter under real load.
kW, kVA or kVAR
Power types. Compare them.
Real.
Apparent.
Reactive.
Generators and UPS units are rated in kVA because they must carry the full current.
Before you size
Power checks. Tick each item.
Tick each item as you go.
Add headroom for motor starting current.
What you get
Power sizing. Each tile explains one result.
Real.
Apparent.
Reactive.
Motors.
Energy.
A low power factor means more current for the same useful power.
Frequently Asked Questions
No question matches that search.
How do I work out power from volts and amps?
Multiply them, and multiply by the power factor for real power.
How many watts is ten amps at two hundred and thirty volts?
Two thousand three hundred watts at a power factor of one.
What is the difference between kW and kVA?
kW is real power, kVA includes the reactive part.
How do I work out three phase power?
Multiply root three by volts, amps and power factor.
What is power factor?
The share of apparent power that does useful work.
What size generator do I need?
At least the load in kVA plus about twenty five per cent.
How do I convert kW to horsepower?
Divide the watts by seven hundred and forty five point seven.
Power you can plan with
Turn volts and amps into kW and kVA and size cables, generators and supplies with confidence.
More size calculators
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