Surge Power in Generators: Size for Motor Starts

What surge power means on a generator listing

Surge power is the short burst of extra power a generator can deliver for a few seconds when an appliance first starts. It is also called starting watts, peak watts, or surge watts. A generator listing usually shows two watt figures: running watts and starting watts. The running figure is what the generator can supply continuously. The starting figure is the higher, brief amount needed to get a motor spinning.

Motors are the reason surge power exists. A motor that runs on a certain number of watts can need several times that amount for a moment when it starts. Refrigerators, freezers, well pumps, sump pumps, air conditioners, and power tools all have motors. When you read a listing, the starting watts figure tells you whether the generator can handle that initial draw.

Why motors need a surge of power to start

A motor has to overcome inertia to start turning. It also has to build up a magnetic field. Both take extra electrical power for a brief moment. Once the motor is running, the power demand drops to its normal running level. This is why an appliance can have a running watt figure that is much lower than its starting watt figure.

The surge lasts only a few seconds, but if the generator cannot supply it, the motor may not start, the generator may bog down, or the appliance may not run properly. The starting watts figure on a listing is the number to compare against the largest motor you plan to power.

Reading running watts and starting watts on a listing

A generator listing often puts the starting watts first, followed by the running watts. For example, a title might read "7,500 starting watts, 6,000 running watts." The first figure is the surge capacity. The second is what the generator can produce continuously.

Some listings show only one watt figure. In that case, the maker's page is the place to look for the second number. The maker's specifications usually list both running and starting watts for each fuel type. If a listing does not state a starting watts figure, treat the single figure as the running watts and check the maker's page before you compare. The generator running watts vs starting watts article explains the two figures in more detail.

Which appliances have motors and need surge power

Appliances with motors are the ones that create a surge demand. Common examples in a home include the refrigerator, freezer, well pump, sump pump, furnace fan, and air conditioner. In a workshop, table saws, air compressors, and other power tools have motors. In an RV, the air conditioner is often the largest motor.

Resistive loads, like incandescent lights, a coffee maker, or a space heater, do not have a surge. They draw roughly the same power when they turn on as they do while running. The surge question is about motors, so when you are comparing generators, think about which motor-driven appliances you will connect.

Sizing for the largest motor, not the total

The key to sizing for surge power is to look at the largest motor you will start at one time. A generator must be able to supply the starting watts of that motor, plus the running watts of everything else that is already on. If you start a well pump while the refrigerator is running, the generator needs to cover the pump's surge and the refrigerator's running draw together.

This is why the starting watts figure matters more than the running figure for many buyers. A generator with a high running watts figure but a low starting figure may not start a large motor. Compare the starting watts on the listing against the surge demand of your biggest motor, and leave room for other loads that will be running at the same time. For a home backup situation, the generator for home backup sizing article covers this in more depth.

How surge power relates to generator types

Inverter generators and conventional portable generators both have starting watts figures. The difference is in how they deliver power, not whether they have surge capacity. An inverter generator produces power electronically, while a conventional generator produces it directly from the alternator. Both types list starting and running watts.

When you compare an inverter generator with a conventional one, look at the starting watts figure for each. The type affects other things, like size and how the power is delivered, but the surge figure is a separate number. The inverter vs conventional generator article explains the type differences in listing terms.

Where to find surge power information

The starting watts figure is usually in the generator listing title or in the bullet list. If it is not there, the maker's page is the source. Makers publish specifications for each model, and those pages list running and starting watts, often for each fuel type if the generator is dual fuel or tri fuel.

When you read a maker's page, look for the section that lists wattage. It may say "starting watts" and "running watts" or "peak watts" and "continuous watts." The terms mean the same thing. If a listing and the maker's page disagree, the maker's page is the more detailed source.

Common mistakes when comparing surge power

One mistake is to compare only the running watts figure and ignore the starting figure. Another is to assume the first number in a title is always the running watts. On many listings, the first figure is the starting watts. Read the label or the bullet list to see which figure is which.

A third mistake is to add up the starting watts of every appliance you might run. You do not need to start everything at once. You need to cover the largest motor's surge plus the running watts of everything else that is on. The generator overload protection article explains what happens when a generator is asked for more than its rating.

What this site cannot say about surge power

This site compares generators on the stated details in the listings and the makers' pages. It does not measure output, runtime, or fuel use. It cannot tell you the exact surge demand of a specific appliance or the exact starting watts of a specific generator model. Those numbers come from the appliance's nameplate and the maker's specifications.

For sizing a generator to your home's loads, the maker and a licensed electrician are the people to ask. They can look at your appliances and your wiring and recommend a size. The best generator guide can help you build a shortlist of models to compare, but the final sizing decision belongs to the maker and an electrician.

What to pick for your situation

Questions

What is surge power on a generator?

Surge power, also called starting watts or peak watts, is the brief extra power a generator can supply when a motor starts. It is higher than the running watts and lasts only a few seconds.

Why do motors need surge power?

Motors need extra power to overcome inertia and build a magnetic field when they start. Once running, they draw less power. The surge is the initial demand.

How do I know the starting watts of a generator?

Look at the listing title or bullet list for a starting watts figure. If it is not there, check the maker's page, which usually lists both running and starting watts, sometimes for each fuel type.

Should I size for the largest motor or the total of all motors?

Size for the largest motor you will start at one time, plus the running watts of everything else that is on. You do not need to add the starting watts of every appliance.

Do inverter generators have surge power?

Yes, inverter generators have starting watts figures just like conventional generators. The type affects how power is delivered, but both types list surge capacity.

What if a listing only shows one watt figure?

Treat the single figure as the running watts and check the maker's page for the starting watts. The maker's specifications are the more detailed source.