Lessons · Engineering · Real, reactive and apparent power
Real power, apparent power, and the gap between
Apparent power is volts times amps. Real power is what actually does work. Power factor is the ratio between them. When current lags voltage -- as it does with motors -- the supply carries current that does no work but still heats the cable and still has to be paid for.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is the commonest industrial electricity charge people do not understand. The site is billed for current it did not turn into anything, and correcting it is usually cheap.
How to think about it
Work out real power and apparent power separately, then take the ratio. If the factor is poor, look for motors running lightly loaded, because an under-loaded motor has a worse power factor than a loaded one.
Worked example
Apparent power = volts x amps, in volt-amperes.What the cable carries.
Real power is what does work, in watts.What you get.
Power factor = real / apparent.One is perfect; lower is worse.
Lightly loaded motors have poor power factor.The usual cause on a site.
Your turn
Real power 800 W, apparent power 1000 VA. Write the power factor as a decimal.
800 / 1000 =
Solve one, graded on the server
The trap
Treating volt-amperes as watts. They are equal only at unity power factor, and the gap is exactly the thing being measured.