Generator Power Factor: What It Means for Your Loads

What power factor means on a generator

Power factor is a technical term that appears in a generator's spec sheet or manual more often than in a listing title. It describes how much of the electricity a generator supplies turns into useful work in the connected load.

The U.S. Energy Information Administration explains that electricity is measured in units of power called watts. A generator's stated wattage counts the real power a load uses. A separate measure, volt-amperes, counts the total power the generator supplies, including reactive power that magnetic fields in motors and transformers draw.

Power factor is the ratio of real power to total power. A resistive load, such as a heating element, has a power factor near 1. A motor-driven load also draws reactive power, so its power factor sits below 1.

What the wattage figure assumes

A generator listing states its output in watts, and that figure is calculated for a load near a power factor of 1. The running watts figure and the starting watts figure are the two numbers to read on a title. Starting watts reflects the extra current a motor needs for a moment at startup; the guide on running versus starting watts explains the difference.

Power factor is not a feature that most listings state, so it does not drive the comparisons on this site. The guides compare the details a listing does state: running watts, starting watts, fuel type and outlets. The seller may place a figure in the title that represents starting watts rather than running watts; reading the bullet list and the image set on the listing tells you which figure appears where. The nameplate specs, with the maker's continuous and surge ratings, are the figures you take from the manual, not the title.

Why motor-driven appliances change the picture

Refrigerators, freezers, window air conditioners, well pumps and sump pumps are motor-driven. Their motors create magnetic fields when they run, and those fields draw reactive power. The effect is a power factor below 1 for the appliance while the motor runs.

When a load has a power factor below 1, a generator reaches its current limit before it reaches the wattage figure on the listing. That means the generator can supply fewer watts to that motor-driven appliance than the rating suggests for a resistive load of the same labeled watts. The exact figures belong to the appliance and the generator, so the appliance label and the maker's page are the place to read them.

This site does not size a home and does not publish wattage worksheets. Understanding power factor helps you see why a motor-driven appliance behaves differently from a heating element, and why the label is what you read next to the generator's continuous output.

Reading the maker's page next to a listing

The maker's page and the owner's manual state output terms a seller may leave out of a title. For power factor, read what the maker says about volt-amperes and about the rating under inductive loads. Ask the maker directly when the manual is unclear.

The kVA versus kW article covers the related difference between apparent power and real power on a spec sheet. For a portable generator, the listing states watts in the title and the manual states the continuous and surge figures.

For a home standby generator, the rating and the installation come from the maker and the installer, so talk with an authorized installer about the loads you expect. When you compare, read the appliance label for its stated watt figure and read each generator's continuous output figure, then compare the best 3000 to 5000 watt generators for hurricanes and the best inverter generators for Florida neighborhoods on those stated figures.

Power stations and their output ratings

A battery power station lists its capacity in watt-hours and its AC output in watts. The same current-limit idea applies to the inverter inside the station. Motor-driven appliances draw reactive power from a power station as well, so the rated AC output is the figure to compare. A station's stated AC output is what the maker publishes, and the manual may state how the output behaves under inductive loads.

The guides for power stations compare stated capacity, continuous AC output and features such as expansion batteries and solar inputs. For a unit for a home or a unit for an apartment, read the maker's page for output figures and the appliance label for the load.

Storm season questions to ask

Before the hurricane season, which runs from June 1 to November 30, the National Weather Service advises checking emergency equipment such as flashlights, generators and storm shutters. That is the time to read the manual and the maker's page for the output terms and to keep both documents with the unit.

The Florida Division of Emergency Management notes that each Florida county has a designated emergency management program and asks residents to know their local office. That office, along with the NWS pages listed in the sources, is the place to look for locally relevant preparation guidance.

What to pick for your situation

Questions

What is power factor on a generator?

Power factor is the ratio of real power, measured in watts, to total power, measured in volt-amperes. A resistive load has a power factor near 1, while a motor-driven load draws reactive power and has a power factor below 1.

Does a lower power factor mean a generator cannot run my refrigerator?

This article does not promise what any generator can run. A lower power factor means a generator reaches its current limit before the wattage rating, so it delivers fewer watts to a motor-driven load than the rating suggests. Read the appliance label and the maker's page for the figures, and ask the maker for the generator's rating at the power factor of the appliance you plan to connect.

Where can I read about watts and kilowatt-hours?

The source list includes the EIA's 'Measuring electricity' page, which explains how electricity is measured in units of power called watts. That page is a good starting point for the terms behind a power factor.

Should I compare generators on power factor?

Most generator listings do not state power factor, so this site compares generators on the details a listing does state: running watts, starting watts, fuel and outlets. Use the maker's page and the manual for power factor and volt-amperes.

Do battery power stations have a power factor?

A power station's inverter has the same current-limit idea. Motor-driven appliances draw reactive power from a power station, so compare the station's rated AC output and read the appliance label.