What size generator for a central air conditioner?
Central AC sizes on the compressor's start, and that start is bigger than most charts admit. A 3 ton unit runs about 3,500 watts, but starting it bare pulls roughly 8,571W for a fraction of a second, its locked-rotor draw. A soft starter cuts that to about 6,000W. Both are in the table by tonnage. And starting the compressor is not the same as backing up your house: central AC is home-standby work, wired through a transfer switch.
Running and start watts by tonnage
| Compressor | Running | Hard start (bare) | Soft-start | Class to start it (bare) | Class with soft starter |
|---|---|---|---|---|---|
| 2 ton | 2,333 W | 5,714 W | 4,000 W | 5,000W portable | 3,500W portable |
| 3 ton | 3,500 W | 8,571 W | 6,000 W | 7,500W portable | 5,000W portable |
| 4 ton | 4,667 W | 11,429 W | 8,000 W | 9,500W portable | 6,500W portable |
| 5 ton | 5,833 W | 14,286 W | 10,000 W | 12,000W portable | 9,500W portable |
Running watts scale straight with tonnage (about 1,167W a ton). The start is the number that moves the generator class, and a soft starter is worth one to two classes of it. The class columns are the smallest portable whose peak covers the start with running headroom; they size the generator to start the compressor, not to run the whole house.
Why central AC is a standby question, not a portable one
Three things push central AC out of the portable aisle. It's 240V and hardwired, so it connects only through a transfer switch or interlock a licensed electrician installs, never an extension cord and never by backfeeding an outlet. Its start is large enough that even the soft-started figure wants a 5,000W portable for a 3 ton, before anything else in the house draws a watt. And you don't run the AC alone; you run it with the fridge, the furnace fan, the kitchen, and the lights, which is the whole-house load, not one appliance.
That's why a central-AC backup is almost always an air-cooled home standby unit (roughly 18 to 26 kW for a typical house) sized to a load calculation, with load management shedding the AC for minutes at a time so a smaller unit covers the house. Size that in the whole-house calculator, and read the transfer switch guide for how the connection is done safely. If you're set on a large portable and a soft starter for AC only, the table's bare-start column is the honest place to size from.
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Questions people ask
What size generator do I need to run a 3 ton central air conditioner?
To start a 3-ton compressor cold, the peak is about 8,571W (its locked-rotor draw), which is a 7,500W portable just for the AC. Add a soft starter and the start falls to 6,000W, a 5,000W portable. But that only starts the compressor. Running central air plus the rest of the house is home-standby territory, and either way the hardwired 240V unit connects through a transfer switch, not a cord.
Why does the surge on this page look higher than other charts?
Because most charts (and our own calculators) quote the soft-started number. The 6,000W figure for a 3 ton is what you get with a soft starter or hard-start kit installed. Bare, an unaided single-stage compressor slams the line for roughly 8,571W in the first fraction of a second, which is why a generator sized only to the running watts trips the instant the AC calls.
Will a soft starter let a smaller generator run central AC?
Yes, by about a class. A soft starter ramps the compressor instead of letting it lock-rotor, cutting the start draw around 30%. On a 3 ton that moves the start from 8,571W to 6,000W, and the generator that starts it from a 7,500W portable to a 5,000W portable. It does nothing for the running watts, so a unit that has to hold the AC for hours still needs the continuous headroom.
Can a portable generator run central air at all?
It can start and run the compressor if the peak and the 240V outlet are there, but that is rarely the real goal. Central AC backup almost always means a home standby unit sized to the house, wired to a transfer switch by a licensed electrician. Use the whole-house calculator to size that, and treat the portable-plus-soft-starter route as the exception, not the plan.