Generator Running Watts vs Starting Watts: Key Differences

What are running watts

Running watts, sometimes called rated watts or continuous watts, is the amount of power a generator can deliver steadily over time. This is the figure that matters for loads that stay on for hours, such as lights, a refrigerator, a furnace blower, or a sump pump that cycles on and off. When you plan a backup power setup, the running watts figure tells you how much of your normal electric load the generator can carry hour after hour.

When you see a generator described by a single wattage number, that is almost always the running watts figure. It reflects what the alternator and engine can handle continuously without overheating or tripping an overload. In practice, that is the number you use to judge fuel consumption and how long the unit can run, because it represents the steady draw the generator will see for most of an outage.

The what does generator watts mean article explains how this number relates to rated output and load, which is useful if you are comparing models that list their output slightly differently.

What are starting watts

Starting watts, also called surge watts or peak watts, is the brief burst of extra power a generator can supply for a moment when an appliance first turns on. Many appliances, especially anything with a motor or a compressor, need this surge for a fraction of a second before they settle into their normal running draw.

That is why you will often see two numbers on a spec sheet: a running watts figure and a higher starting watts figure. The starting figure is short-duration. The generator can produce it only briefly, just long enough for the motor to spin up from rest to its working speed.

If your generator cannot deliver enough starting watts, the appliance may hum, struggle, and fail to start, or the voltage may dip enough to affect other devices on the same circuit. Getting this number right is often what separates a frustrating setup from one that works on the first try.

Why motor loads need extra surge power

Motors are inductive loads. When they start from rest, they draw a surge of current to overcome the inertia of the fan, pump impeller, or compressor and to build up their magnetic field. This surge is noticeably higher than the current they draw once they reach speed, which is why you may see lights dim for an instant when an air conditioner or well pump cycles on.

The same principle applies to power tools, refrigerators, and freezers. Any device that contains an electric motor is a candidate for a starting surge, and the larger the motor, the larger the surge. The compressor in a refrigerator cycling on is a classic everyday example of a load that needs that extra moment of power.

The generator inrush current surge page covers the mechanics of why this happens and how it relates to locked-rotor current, which is a more detailed look than this article takes.

How the two ratings appear on a generator

Generator spec sheets usually list both a running watts rating and a starting watts rating. The starting figure may be labeled as surge, peak, or max. The difference between the two numbers is the headroom the generator can supply for a moment, and that headroom is exactly what a motor needs when it starts.

Not every load behaves the same way, and knowing the two categories helps you read the ratings correctly.

Motor loads are the ones that create the surge, and they are the reason the starting watts rating exists. If your appliance list contains any motor, that rating is the one to verify.

Sizing your generator for both numbers

There is no single wattage that fits every home. The U.S. Energy Information Administration reports that the average U.S. household consumes about 10,500 kilowatt-hours of electricity per year, but what you actually need during an outage depends on the specific appliances you choose to power and which of them run at the same time.

To size a generator, check the nameplate on each appliance you plan to run and separate the motor loads from the resistive loads. You want a generator whose starting watts rating covers the largest motor you will start at the same time as everything else you plan to run. The how many watts do I need article walks through how to build that list.

The steps below give a simple order for doing the math.

• List every appliance you plan to power during an outage.
• Note the running watts for each from the nameplate.
• Identify which items have motors and treat them as starting watt loads.
• Add the running watts of everything that runs together, then confirm the generator's starting rating covers the largest motor.

Operate safely while you plan

Whatever size you settle on, use the generator outdoors only. The CPSC warns that one portable generator produces as much carbon monoxide as hundreds of cars, and CO can incapacitate and kill within minutes if the generator runs indoors, in a garage, or near an open window, door, or vent.

The CDC advises running generators outside, more than 20 feet from windows, doors, and vents, and keeping a battery-powered or battery backup CO detector in the home. The EPA adds that deadly levels of CO can build up quickly in garages, crawlspaces, and sheds and can linger for hours even after the generator is shut off.

See the generator safety checklist for the full list of safe habits to pair with your sizing decision, and check the carbon monoxide safety page for more detail.

What to pick for your use

Questions

What is the difference between running watts and starting watts?

Running watts is the power a generator delivers continuously, while starting watts is the brief surge it can supply when a motor-driven appliance first turns on. Use the running figure to cover steady loads and the starting figure to make sure motors can start.

Do all appliances need starting watts?

No. Resistive loads, like incandescent lights and electric heaters, draw about the same power at startup as they do while running. Motor loads, like refrigerators, pumps, and power tools, are the ones that need the extra surge.

Is the starting watts rating always higher than the running watts rating?

Yes. Starting watts is by definition a short-duration rating that is higher than the continuous running figure, since it must cover the momentary surge a motor draws when it starts.

What happens if my generator has enough running watts but not enough starting watts?

A motor that needs more surge than the generator can supply may hum, stall, or fail to start, and the voltage may dip enough to affect other loads. Size the generator so the starting watts rating covers the largest motor you will start.

How can I find the starting watts of an appliance?

Check the appliance nameplate for the running wattage; some nameplates also list a locked-rotor or surge figure. If the surge is not listed, treat the motor load as needing a noticeably higher figure than its running watts.

Can I rely on the running watts number alone when choosing a generator?

No. You need both ratings. The running watts tells you the continuous capacity, but the starting watts determines whether motor loads will actually start. Check both against your appliance list before you buy.