Apprentice path · Stage 23: Motors · Lesson 86

Power factor, gently

Real power is the part doing work. Apparent power is what voltage and current look like together. Power factor is the ratio between them.

Why this matters on the job

Lesson 9 said the power wheel was for DC resistive circuits and that AC power may also require power factor. This is that caveat, cashed in.

How to think about it

  1. Real power performs useful work or becomes heat.
  2. Apparent power combines RMS voltage and current.
  3. Power factor compares real power with apparent power.

Worked through

800 watts of real power against 1,000 volt-amperes of apparent power gives a power factor of 800 / 1,000 = 0.8. The watts are what the load is actually doing. The volt-amperes are what the supply has to carry to deliver it. When they are not the same number, the difference is what power factor describes.

Check yourself: A load shows 800 W and 1,000 VA. What is its power factor?

Answer: 0.8. Real power divided by apparent power: 800 / 1,000 = 0.8.

Power factor is defined as the ratio of real power to apparent power, so it falls straight out of the two numbers with nothing else needed.

Watch out for: Treating watts and volt-amperes as the same unit written differently. They are the same only when power factor is 1, and a nameplate in VA is not a nameplate in W.
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Written from the owner's poster for this lesson. Nothing added beyond what the poster states.

Written for Texas apprentices as free study material. The Texas apprentice licence is an application, not an exam (Occupations Code 1305.161), and nothing here counts as apprenticeship hours or continuing-education credit. VoltMark is not affiliated with, endorsed by, or sponsored by NFPA, PSI, TDLR, or any state or local licensing authority. Always follow your employer's safety programme and your supervising electrician.