What Is Generator Power Factor and Why It Matters

Power factor is the gap between useful power and total current

Power factor is the relationship between the power that does useful work and the total power a generator must deliver. In an alternating-current circuit, voltage changes direction many times each second. Some loads draw current in step with that changing voltage, while other loads draw current on a slight delay. The degree of mismatch between voltage and current is what power factor measures.

A purely resistive load draws current that rises and falls with the voltage. As a result, almost all of the current is converted into work. A reactive load, such as a motor, draws current out of phase with the voltage. Part of the current is used to build a magnetic field, then flows back to the generator. The generator still has to carry that current, even though it does not appear in the load's watt rating.

• Real power, expressed in watts, is the portion that performs useful work.
• Apparent power, expressed in volt-amperes, is the total current the generator must handle.
• Power factor is the ratio between real power and apparent power.

Resistive loads: current and voltage in step

Resistive loads are the simplest category to power. They include incandescent light bulbs, electric heaters, toasters, coffee makers, and hair dryers. These loads turn electricity directly into heat or light. The current follows the voltage closely, so their power factor is effectively unity. A watt rating on a resistive device is a fair representation of the volt-ampere load it places on a generator.

Most emergency and recreational load lists start with resistive loads because lighting and heating are usually the largest share. Adding up the watt ratings of these devices gives you a reliable baseline. For a fuller walkthrough of watt ratings, see what generator watts mean.

Reactive loads: motors ask for extra current

Reactive loads are where generator sizing gets more complicated. Motors, compressors, pumps, fans, and air conditioners use a magnetic field to rotate or move things. That magnetic field causes the current to reach its peak after the voltage does. Some of the current returns to the generator during every cycle instead of being turned into mechanical work.

The generator must still supply that returning current. It flows through the alternator windings, creates heat, and reduces the capacity left for other loads. A motor's nameplate watt rating can be much lower than the apparent-power draw on the generator. This is why running watts vs starting watts is only one part of motor-load planning. You also need to account for lower power factor.

How power factor changes generator sizing

To size a generator correctly, look at both the watt rating and the volt-ampere rating. The watt rating describes the continuous working power the generator can supply. The volt-ampere rating describes the full current load the alternator and output wiring can carry. For a resistive load, these values line up closely. For a motor load, the volt-ampere requirement is higher than the working watts.

If you size by watts alone, a motor can push the generator toward its current limit while the watt reading still looks comfortable. That can cause voltage to sag and heat to build. Electricity use in homes varies by region and by type of home, so a load list based on your own appliances is more reliable than a broad rule. For steps to build that list, see how many watts do I need.

Resistive and reactive loads at a glance

Use the table below as a quick way to sort your own loads.

In a real home, the load mix is almost always mixed. Lights, heating, and electronics add to the real-power total. Motors add to the apparent-power total. Both totals must stay within the generator's limits. The best way to choose a class of machine is to compare ratings on portable generators.

Power factor, inverters and sensitive electronics

Inverter generators produce a cleaner, more stable output by converting AC to DC and then back to AC. That process is helpful for electronics and for steady frequency, but it does not change the power factor of a connected load. A motor still behaves like a reactive load on an inverter generator, and the generator still has to deliver the motor's apparent power. See how inverter generators work for the full picture.

Electronics with switch-mode power supplies such as computers and televisions draw current in short pulses rather than smoothly. This is more closely related to harmonic distortion than to power factor. If you plan to run sensitive equipment, read total harmonic distortion and the guide on THD and sensitive electronics.

Sizing safely and choosing a generator

Power factor also matters because it affects how close you can run a generator to its maximum rating. An overloaded generator may sag in voltage, hunt in speed, and build up heat. That is hard on the machine and can shorten its service life. Leave headroom between your estimated load and the generator's rated output, especially if motors are part of the mix.

Operating safety is separate from sizing, but no sizing article should skip it. Portable generators produce carbon monoxide, a colorless and odorless gas that can build up quickly indoors. The CPSC says to run generators outside, far from windows, doors, and vents, and the EPA notes that deadly CO levels can linger even after the generator is shut off. Install battery-operated CO alarms with battery backup on every level of the home. Use the generator safety checklist before you start any machine. Models with built-in sensors are covered in the CO sensor generators guide.

For general use, compare models in the best portable generators guide. If your home has a well pump, central air, or large shop equipment, look at the larger portable class described in the larger portable generators guide. For permanent backup, home standby generators are typically sized by an installer who takes apparent-power and motor-starting needs into account.

What to pick for your use

Questions

What is generator power factor in simple terms?

Power factor is the ratio between the power that performs useful work and the total power that flows through the generator. A purely resistive load uses almost all of the current for useful work, while a motor uses some of the current to build a magnetic field and then sends it back.

Why does low power factor matter for generator sizing?

It matters because the generator must supply apparent power, not just working watts. A motor may have a low watt draw but still force the generator to deliver extra current. If you size only by wattage, you can overload the generator's alternator even while the watt reading is under the limit.

Do I need to know the exact power factor of my appliances?

Usually not. You need to recognize which loads are resistive and which have motors, and you need to include starting watts and check the generator's volt-ampere capacity. When in doubt, leave headroom between your running load and the generator's maximum rating.

Does an inverter generator improve power factor?

No. An inverter changes the shape and stability of the generator's output, but the power factor is a property of the load. A refrigerator motor on an inverter generator still draws reactive current, so you should size it the same way you would for a conventional generator.

How should I size for a mix of lights, heater, and a pump?

Add the watts of the resistive loads, then add the running and starting requirements of the motor load. Make sure the generator can deliver both the total working watts and the full volt-ampere draw. A guide on how many watts you need walks through the steps.