Generator Power Factor: What It Means for Sizing

What is power factor?

Power factor is a term you may see on a generator's specification sheet. It is a number between 0 and 1 that describes how effectively the electrical current from the generator is converted into useful work. A power factor of 1 means all the current is doing useful work, while a lower number means some current is not contributing to the output.

In simple terms, power factor matters most when you run motor-driven equipment, such as pumps, compressors, or air conditioners. These loads can have a power factor below 1, which means the generator must supply more current to deliver the same amount of real power. Understanding this helps you read a listing and compare generators, but it does not replace the maker's sizing guidance. For a broader look at how to compare generator specs, see how to read generator specs.

Real power versus apparent power

On a generator listing, you will often see two wattage figures: starting watts and running watts. These are measures of real power, the power that actually does work. Power factor relates to another concept called apparent power, which is the combination of real power and reactive power. Reactive power is the portion that does not do useful work but is still drawn by inductive loads like motors.

When a generator's spec sheet lists a power factor, it tells you the ratio between real power and apparent power. A generator with a power factor of 0.8, for example, can deliver 80 percent of its apparent power as real power. The rest is reactive. This is why a generator rated for a certain wattage may not run a motor load that draws the same wattage if the motor has a low power factor. To understand the difference between starting and running watts, read running watts vs starting watts.

Why it matters for motor loads

Motor loads, such as well pumps, sump pumps, and air conditioner compressors, have a power factor that is often below 1. This means they draw more current than a resistive load of the same wattage, such as a heater or incandescent light bulb. When you compare generators, a unit with a higher power factor rating can supply more real power to these loads.

The practical effect is that a generator with a lower power factor may struggle to start or run motor-driven equipment, even if the wattage rating seems sufficient. This is why reading the power factor on the maker's page is part of understanding a listing. It is not a substitute for the maker's sizing recommendations, but it helps you ask the right questions. For more on how generators handle surge power, see generator surge power.

How to read power factor on a listing

Power factor is not always stated in the title of a generator listing. It is more commonly found in the detailed specifications on the maker's page. When you see a number like 0.8 or 1.0, that is the power factor. A rating of 1.0 means the generator can deliver all its apparent power as real power, which is typical for resistive loads. A rating of 0.8 is common for generators that may power inductive loads.

If a listing does not state the power factor, that does not mean the generator lacks one. It simply means the seller has not included that detail. The maker's specification sheet is the place to look. This article cannot tell you which power factor is right for your situation, because that depends on the loads you plan to run and the generator's design. When comparing models, you might also want to review inverter vs conventional generators.

Power factor and sizing

Sizing a generator involves matching the generator's output to the loads you want to run. Power factor is one part of that equation, but it is not the only factor. The starting watts of motor loads, the running watts, and the total load all play a role. This site does not provide sizing calculations, because that is a question for the maker and, for home wiring, a licensed electrician.

When you compare generators, consider the types of loads you expect to run. If you plan to power motor-driven equipment, ask the maker about the power factor and how it affects the generator's real power output. The maker's page and manual are the authoritative sources for this information. For home backup considerations, see home backup sizing.

What this site cannot say about power factor

This site compares generators on stated details, not on measured performance. That means the site cannot tell you which generator has a better power factor in practice, or how a specific model will perform with your loads. Those are questions for the maker, who publishes the specifications and can explain how the generator is designed to be used.

The site also cannot advise on connecting a generator to your home's wiring or on sizing a generator for a whole house. Those tasks involve electrical work that must follow local codes and the maker's instructions. A licensed electrician and the local permit office are the people to ask. For a general overview of choosing a generator, start with the best generator guide.

What to pick for your situation

Questions

What is a good power factor for a generator?

A power factor of 1.0 means all the current is doing useful work, which is ideal for resistive loads. Many generators are rated at 0.8, which is common for motor loads. The right power factor depends on what you plan to run, so check the maker's specifications.

Does power factor affect starting watts?

Starting watts are the extra power a motor needs to start, and power factor affects how much real power is available. A motor with a low power factor may require more current, which can affect whether a generator can start it. The maker's page is the best source for this information.

Why is power factor not on every listing?

Power factor is a technical specification that sellers may not include in the title or bullet list. It is often found in the detailed specifications on the maker's page. If it is not stated, you can ask the maker directly.

Can I use a generator with a power factor of 0.8 for my home?

A power factor of 0.8 is common and can work for many loads, but sizing a generator for a home involves more than power factor. You need to consider the total load and the starting watts of motor-driven appliances. Consult the maker and, for wiring, a licensed electrician.