What Exactly is Power Factor?

Power factor is an important tool for measuring overall demand on the grid, but it can be a difficult concept to understand.

Calculating power factor

Power factor is the ratio of real power — the power that does the work — and apparent power, which is the total amount of power drawn from the grid to make that work happen.

Power factor = Real power (kW) ÷ Apparent power (kVA)

But why would those numbers be different? Some equipment - such as motors, transformers and relays - requires extra power to form electromagnetic fields in order to operate.

This extra power - called reactive power - doesn't do any real work. Reactive power doesn't actually get consumed. It goes back to the grid, and new power replaces it.

The train track analogy

Let's demonstrate power factor's impact on demand using a simple analogy. Imagine you're pulling a train on the tracks with a horse. Let's say the train has to be pulled from point A to point B.

To do so, the horse has to do real work. In our power factor equation, this is real power measured in kilowatts (kW). The total power required to accomplish this work is apparent power, which is measured in kilovolt amperes (kVA). To find power factor, divide the real power by the apparent power.

If the horse could walk straight, directly on the tracks, all the power exerted would pull the train: kW would equal kVA, and our power factor would be 1.

But the horse can't walk on the uneven tracks; it has to move to the side and walk next to the tracks. The distance from the horse to the track represents reactive power (kVAR). As the horse gets further from the tracks, more reactive power is needed, and more apparent power is needed to get us from point A to point B.

That bigger difference between apparent power and real power results in a lower power factor. Power factor is not a matter of energy use; it's a matter of energy demand. So, while a low power factor won't increase the kilowatt-hours listed on your electric bill, it could significantly affect demand and any demand charges that may apply.

Understanding power factor will shed light on how certain end-use devices can add more strain on the grid than just their energy use.