Soft Start vs Hard Start: Generator Motor Loads

What soft start and hard start mean

When you plug an appliance into a generator, the appliance's motor does not draw a steady amount of power the whole time it runs. At the moment the motor starts, it needs extra power to get the shaft turning. That brief extra draw is called the starting surge or starting watts. The steady draw after the motor is up to speed is the running watts.

A hard start motor reaches full speed almost instantly, so its starting surge is sharp and short. A soft start motor, or a motor with a soft start device added, ramps up over a second or two, so the surge is lower and spread out. The terms describe the motor's behavior, not the generator's.

How starting surge affects generator shopping

Generator listings usually show two watt figures: a starting (or peak) figure and a running figure. The starting figure is the one meant to cover the surge of a hard start motor. If you plan to run a motor that starts hard, like a refrigerator compressor, a sump pump, or an air conditioner, the generator needs enough starting watts to handle that surge.

A soft start device reduces the surge, which means a generator with a lower starting figure might be enough for the same appliance. But the only way to know the actual surge of your appliance is to read its nameplate or its manual. The generator listing does not tell you what your appliance draws.

This site does not size generators for specific homes or appliances. Sizing is a question for the maker and for a licensed electrician, who can look at your loads and your wiring. For more on the two watt figures, see Running vs Starting Watts.

Soft start devices on a listing

Some generators are sold with a soft start feature built in, and some are sold with a soft start device as an accessory. On a listing, you might see the words "soft start" in the title or in the bullet list. That is a feature word, not a model name.

If a listing does not mention soft start, the generator is not advertised as having it. That does not mean the generator cannot run a hard start motor; it means the listing does not claim the feature. Read the maker's page for the model to see what the manufacturer says about soft start.

A soft start device can also be added to an appliance, like an air conditioner, but that is an appliance modification, not a generator feature. The maker of the appliance and a qualified technician would decide whether that is right for your equipment.

Reading the watt figures

When you compare generators, look at the starting watts figure first if you expect to run motors. A generator with a higher starting figure can handle a larger surge, all else being equal. The running watts figure tells you what the generator can supply continuously.

The title of a listing often shows only one figure, usually the starting figure. The maker's page for the model will list both. Read both figures before you compare two generators.

For more on the difference between the two figures, see Running vs Starting Watts.

Common motor loads

Motors that start hard include compressors in refrigerators and air conditioners, well pumps, sump pumps, and power tools with large motors. These are the loads that make the starting watts figure matter.

Resistive loads, like incandescent lights, a coffee maker, or a space heater, do not have a starting surge. They draw close to their running watts from the moment they turn on.

If you plan to run a mix of motor loads and resistive loads, the starting figure needs to cover the largest motor surge plus the running watts of everything else that is on at the same time. That is a load calculation, and it is best done with the maker's guidance and an electrician's help.

What to look for on a listing

On a generator listing, the words that matter for motor loads are the watt figures, the type of generator (inverter or conventional), and any mention of soft start. The type does not tell you the surge capability; the watt figures do.

A conventional open frame generator often has a higher starting watts figure than an inverter generator of similar running watts. That is a listing detail, not a promise about performance. Compare the stated figures on the listings you are considering.

For a guide to the different types, see Inverter vs Conventional Generator.

When you need a soft start

You might look for a soft start feature if you have a large motor that is hard to start, like a central air conditioner or a deep well pump, and you want to run it from a portable generator. A soft start device lowers the surge, which can let a smaller generator handle the load.

But adding a soft start device to an appliance is a modification. The appliance maker and a qualified technician should confirm that the device is compatible and that the installation is done correctly. A generator listing that says "soft start" refers to the generator's own feature, not to your appliances.

If you are shopping for a generator to run an RV air conditioner, see Generator for RV Air Conditioner.

Safety note on generator use

Generators produce carbon monoxide, which is deadly. Always run a portable generator outside, far from windows, doors, and vents. Never run it inside a home, garage, basement, crawl space, or shed. Follow the maker's manual and read the carbon monoxide guidance from the CPSC, CDC, and EPA linked in the sources below.

What to pick for your situation

Questions

What is the difference between soft start and hard start?

Hard start means a motor draws a sharp surge of power when it turns on. Soft start means the motor ramps up gradually, so the surge is lower and spread out. The terms describe the motor's behavior, not the generator's.

Does a soft start generator use less fuel?

The listing does not state fuel use. Soft start affects the starting surge, not the running draw. Fuel use depends on the load and the generator's design, which the maker's page would describe.

Can I add a soft start device to my generator?

A soft start device is usually added to an appliance, not to a generator. If you are considering that, the appliance maker and a qualified technician should confirm compatibility. The generator listing's soft start feature refers to the generator itself.

How do I know if my appliance has a hard start?

Read the appliance nameplate or its manual. The nameplate lists the electrical ratings, and the manual may describe the starting current. If you are unsure, ask the appliance maker or an electrician.

Why does the generator title show only one watt figure?

The title often shows the starting watts figure because that is the larger number. The maker's page for the model lists both starting and running watts. Read both before comparing.