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Power factor: what it means for the Texas exam

Bonding Jumper editorial teamSources checked 3 min read

Power factor is the ratio of true power, in watts, to apparent power, in volt-amperes. It tells you how much of the current a load draws is actually doing work. A power factor of 1.0 means voltage and current rise and fall together. Motors and other inductive loads pull the two out of step.

Two sine waves on one axis: voltage, and current shifted to the right by an angle labeled theta, with a note that power factor equals the cosine of theta, beside a small right triangle with kW on the base, kVAR on the upright and kVA on the slope
The further current lags voltage, the smaller the cosine and the lower the power factor.
Use this diagram

Where it comes from

In a purely resistive load, such as a heater, current and voltage peak together, and every volt-ampere becomes a watt. Inductance, in motors, transformers and ballasts, stores energy in a magnetic field and returns it each cycle, which delays the current behind the voltage. Capacitance does the opposite and pushes current ahead. The energy that sloshes back and forth is reactive power, measured in VAR. It does no work but still flows in the conductors.

Power factor equals the cosine of the angle between voltage and current. It also equals the circuit's resistance divided by its impedance.

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Why low power factor costs

For the same true power, a lower power factor means more current. A worked example with invented numbers: a single-phase 240-volt load uses 8 kW.

  • At a power factor of 1.0, apparent power is 8 kVA, and current is 8,000 ÷ 240 = about 33.3 amperes.
  • At a power factor of 0.8, apparent power is 8 ÷ 0.8 = 10 kVA, and current is 10,000 ÷ 240 = about 41.7 amperes.

The second load does the same work and draws about 25% more current. That current heats conductors, adds voltage drop and uses up transformer capacity. It is also why utilities charge some large customers for low power factor.

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Fixing it

Capacitors installed near inductive loads supply the reactive current locally, so less of it travels from the source. That is power factor correction. Synchronous motors and some drives can do the same job.

Common mix-ups

  • Watts and volt-amperes are not interchangeable. They match only at a power factor of 1.0. See apparent vs true power.
  • Using power factor to size a transformer. Transformers are rated in kVA, which already includes it.
  • Power factor above 1.0. It cannot be; the ratio tops out at 1.

On the Texas journeyman exam

Power factor belongs to definitions, theory and plans, a small area of 5 scored items across both portions under the PSI bulletin, and appears inside motor and load calculations. Study to convert between watts, volt-amperes and current with a given power factor.

Practice it

The Ohm's law calculator works the power formulas, including three-phase. Every term in this set is in the electrical terms glossary.

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Not affiliated with or endorsed by TDLR, PSI or NFPA. NEC and National Electrical Code are registered trademarks of NFPA.

Sources

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Facts checked 26 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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