What Is Power Factor? Watts, Volt-Amps, and Why They Differ

Multiply the voltage at an outlet by the current flowing through it and you get a number in volt-amperes. For a heater, that number is its wattage. For a refrigerator compressor, a pump or a laptop power supply, it is not — the watts are smaller. Power factor is the ratio between the two, and it explains three things people run into without a name for them: why a motor needs more wiring than its wattage suggests, why a home electricity bill never mentions it, and why a solar inverter can be told by the utility to give up part of its output.

The definition

There are three kinds of power in an AC circuit:

• Real power, in watts (W) — the part that becomes heat, light or motion, and the part an electricity meter bills.
• Apparent power, in volt-amperes (VA) — the voltage times the current, whatever that current is doing.
• Reactive power, in volt-amperes reactive (VAR) — current that flows back and forth each cycle, building and collapsing a magnetic or electric field, without delivering energy.

Power factor = watts ÷ volt-amperes. It runs from 0 to 1. The three are related like the sides of a right triangle: VA² = W² + VAR². A load of 40 kW with 30 kVAR of reactive power is √(40² + 30²) = 50 kVA, a power factor of 0.8.

A load drawing 5 A at 120 V and using 510 W is 600 VA at a power factor of 0.85.

What causes it

Displacement: motors and other inductive loads

An induction motor needs a magnetic field to run, and building that field every cycle makes the current lag the voltage. For part of each cycle, current is flowing while voltage is pushing the other way, and that portion delivers nothing. The motors in compressors, pumps, fans and air handlers all do this. A fully loaded induction motor is commonly around 0.8 to 0.9; the same motor lightly loaded is much lower, because the field current stays while the useful work falls.

Capacitive loads do the opposite, drawing current that leads the voltage. Because most buildings are net inductive, power factor correction almost always means adding capacitance.

Distortion: electronics

A simple electronic power supply charges a capacitor from the peaks of the voltage wave and draws current in short pulses there rather than smoothly across the cycle. Even when those pulses are in step with the voltage, the current is not a clean sine wave, and power factor falls. Equipment with active power factor correction shapes its input current to follow the voltage and typically operates above 0.9.

True power factor is the product of the two effects — displacement and distortion — and it is what a meter that measures watts and volt-amps directly reports.

Loads that sit at 1

Resistive loads — water heater and baseboard elements, a range, a toaster, incandescent lamps — draw current exactly in step with the voltage. Their power factor is 1, and for them watts and volt-amps are the same number. That is why the volts to watts calculator can re-rate a heating element for a different voltage using resistance alone, and why it declines to do the same for a motor.

Why it matters: wiring carries volt-amps

A conductor, a breaker, a transformer and a generator are all limited by current, and current follows apparent power, not real power. A motor drawing 510 W at a power factor of 0.85 needs the same current as a 600 W heater. Everything upstream has to be sized for the 600.

That is the whole reason two conversions that look alike are not:

• kW to amps needs a power factor, because kW leaves out the reactive part of the current. The kW to amps calculator applies it — and, for a motor, the efficiency as well, because a motor's kW rating is its shaft output.
• kVA to amps takes none, because kVA already is volts times amps. Applying a power factor to a kVA figure is a common mistake, and the kVA to amps calculator explains why.

It is also why transformers and many commercial generators are rated in kVA: their limit is current, whatever the load does with it.

At home, the practical example is a refrigerator. How many amps a refrigerator uses works from the rating label to the running draw, and the step from amps to watts there includes the compressor's power factor.

Are homeowners charged for it?

Almost certainly not. Residential electricity is billed on kilowatt-hours, which count real energy, and residential tariffs generally carry no power factor or kVA demand charge. The reactive current flows in the house wiring and the utility's lines, and the utility absorbs it across its residential customers.

That is why a plug-in device sold to homeowners as a power factor corrector does not reduce the bill. Correcting power factor at an outlet can slightly reduce the current in the house wiring between that outlet and the panel, and with it a small resistive loss, but the meter was never charging for that current.

Commercial and industrial customers are different. Many tariffs add a charge based on kVA demand, or an adjustment when the power factor falls below a threshold — commonly around 0.9 or 0.95, set by the individual utility. That is where correction capacitors at a motor or a service pay for themselves, and where the figure belongs in an electrical design rather than in a sales claim.

What it does to a solar inverter

A grid-connected inverter normally exports at a power factor close to 1. But IEEE 1547-2018, the interconnection standard that current inverter certifications test against, requires inverters to be capable of supplying or absorbing reactive power, and a utility can specify how an inverter must operate — a fixed power factor, or a mode that adjusts reactive power with voltage — as a condition of interconnection.

An inverter's output is limited by its current, so its capacity is really a volt-amp rating. When it is held below unity, part of that capacity goes to reactive power:

On a sunny afternoon when the array could drive the inverter to its limit, that difference is clipped production. The setting is therefore a design input: it belongs in the production estimate and in the plan set, and it arrives with the utility's interconnection requirements rather than from the installer. The interconnection layer is where those requirements are received, and design and permitting is where they become settings an inspector and a commissioning technician can follow.

For partners who would rather not carry that layer themselves, Seamless Home runs design, engineering, permitting and interconnection on the projects it delivers — talk to Seamless about how that works. Coverage is confirmed per service area rather than promised as blanket availability.

The short version

• Power factor is watts ÷ volt-amps, from 0 to 1.
• Motors lower it by making current lag voltage; electronics lower it by drawing current in pulses; heaters leave it at 1.
• Wiring, breakers, transformers and generators are limited by current, which follows volt-amps.
• Homes are billed on kWh and not charged for it; some commercial tariffs are.
• A solar inverter held below unity delivers less real power from the same volt-amp rating.