What Size Generator for Central Air Conditioning?
A typical 3-5 ton central AC system needs roughly 3,500-6,000 running watts and, depending on the compressor's Locked Rotor Amps (LRA), anywhere from 7,000 to 18,000 starting watts — far more than most portable generators supply. This is the single hardest common appliance to size a generator for, and it's a derived calculation from your specific compressor's data plate, not a fixed range.
The one thing to know: For central air conditioners specifically, don't guess the surge — the compressor's data plate lists LRA (Locked Rotor Amps), the actual current draw the moment the motor is stalled at power-on. Multiply LRA by voltage and a ~1.2 safety margin (LRA x V x 1.2) to get a real starting-watt estimate; a unit with 60A LRA at 240V needs roughly 17,000 surge watts to start, which is why central AC is usually the single hardest appliance to add to a portable generator plan without a soft-start device.
How to calculate your own number
Find the LRA figure on your outdoor condenser unit's data plate (often behind an access panel), alongside RLA (Rated Load Amps, its running current) and the system voltage. Multiply LRA × voltage × a ~1.2 safety margin to estimate real starting watts. Example: a compressor with 60A LRA at 240V needs roughly 60 × 240 × 1.2 ≈ 17,280 starting watts — well beyond a typical 3,000-5,000W portable unit, and into home-backup territory (see our home backup guide).
Soft-start devices
A soft-start (sometimes called "easy start") device installed on the compressor ramps the motor up more gradually instead of hitting it with the full locked-rotor current all at once, which can substantially reduce the surge a generator needs to supply. It changes what the GENERATOR has to deliver, not the compressor's underlying LRA rating on the data plate — professional installation is recommended, and it doesn't turn a small portable generator into a substitute for properly sizing a home-backup system.
Window and portable AC are a different, easier problem
Don't confuse central AC with window or portable AC units — those typically need only 1,800-3,600 starting watts (see the Startup Watt Index), comfortably within reach of a mid-size portable inverter generator.
This page explains a general sizing calculation and is not a substitute for consulting a licensed HVAC technician or electrician for your specific system and installation.
Frequently Asked Questions
A typical 3-5 ton central AC system needs roughly 3,500-6,000 running watts and 7,000-18,000 starting watts, based on LRA-derived estimates — well beyond most portable generators. Many homes need a 10,000W+ continuous generator, a soft-start device on the compressor, or both.
LRA (Locked Rotor Amps) is the current draw the moment a compressor motor is stalled at power-on, printed on the unit's data plate. Multiply LRA by voltage and a roughly 1.2 safety margin to estimate real starting watts — a 60A LRA compressor at 240V needs about 17,000 surge watts, which most portable generators can't supply.
Sometimes, with a soft-start (or 'easy start') device installed on the compressor, which reduces the inrush current needed at startup by ramping the motor up more gradually. This can bring a unit that nominally needs 15,000+ surge watts down to something a smaller generator can handle, but it's an added component, not a change to the compressor's underlying LRA rating.
No — window and portable units are much smaller compressors, typically needing only 1,800-3,600 starting watts, well within range of a mid-size portable generator. Central AC's whole-house compressor is a different, much larger load.
On the compressor unit's data plate (usually inside an access panel on the outdoor condenser unit), listed alongside RLA (Rated Load Amps, the running current) and voltage. If you can't locate it, an HVAC technician or your unit's installation manual can provide it.