What size generator to run HVAC?
Reviewed 2026-07-04 · Sources listed below
A 3-ton central AC needs about a 10,000W generator, because the compressor surges to roughly 9,000W at startup even though it only runs at 3,500W. Add a soft start kit (~$300) and a 6,500W generator can do the same job. Smaller homes with 2-ton units: about 8,000W without soft start, 5,000W with it.
The gap between running watts and starting watts is why this question fills forum threads. Here's the full table, the soft start math, and how to read your own unit's real number.
Generator size by AC tonnage
| Central AC | Running W | Starting W | Generator (no soft start) | Generator (with soft start) |
|---|---|---|---|---|
| 2 ton (24k BTU) | 2,500 | 6,500 | 8,000 W | 5,000 W |
| 2.5 ton (30k BTU) | 3,000 | 7,800 | 9,500 W | 5,500 W |
| 3 ton (36k BTU) | 3,500 | 9,000 | 10,000 W | 6,500 W |
| 4 ton (48k BTU) | 4,500 | 12,000 | 13,000 W | 8,500 W |
| 5 ton (60k BTU) | 5,500 | 14,500 | 16,000 W | 10,000 W |
These generator sizes assume the AC starts while a normal essentials load (fridge, lights, furnace fan, electronics, roughly 1,500W) is already running. If you plan to run more at the same time, add it in the generator size calculator.
Why the surge is the whole problem
An AC compressor is a big motor, and motors pull 2.5 to 4 times their running current for the first second. Your generator has to swallow that spike on top of everything else it's already powering. That's why a "7,500W" generator stalls trying to start a "3,500W" air conditioner. It's not broken. It's math.
The soft start option most people don't know about
A soft start kit ramps the compressor up over a fraction of a second instead of slamming it on. Manufacturer data and measured installs show starting current dropping 65 to 75 percent (one documented install: 81.4A locked rotor down to 21 to 26A). That turns a 9,000W surge into roughly 2,300 to 3,200W, which is why RV owners run 3-ton units off small generators. For home backup, a $300 to $400 kit is usually cheaper than stepping up two generator classes. It also makes the start easier on the compressor itself.
Frequently asked
Can a 7,500 watt generator run central air?
Usually not a 3-ton unit on its own. The compressor surge (around 9,000W) exceeds the generator before the rest of the house draws anything. With a soft start kit installed, a 7,500W generator handles most 2 to 3 ton systems with room to spare.
What is a soft start kit and how much does it cost?
A device wired into the AC compressor that ramps the motor up instead of slamming it on. It cuts starting current by 65 to 75 percent per manufacturer data and measured installs. Kits run about $300 to $400 plus an hour of HVAC labor. For generator use it is usually cheaper than buying a bigger generator.
Does the generator need to be 240V for central AC?
Yes. Central AC runs on 240V, so you need a 240V capable generator connected through a transfer switch or interlock installed by a licensed electrician. A regular 120V extension cord cannot power it, no matter the wattage.
How do I find my exact AC surge instead of using typical numbers?
Read the nameplate on the outdoor condenser. Find LRA (locked rotor amps) and multiply by voltage. Example: LRA 58 at 240V is 13,920W of surge. That number beats any table, including this one.
Can I just run a window AC on a small generator instead?
Yes, and during an outage it is often the smart move. A 10,000 BTU window unit surges around 3,600W, so a 4,000W generator cools one room while also keeping the fridge going. Whole house comfort is a luxury during outages; one cool room is a plan.
Related tools
Sources
- Generator sizing worksheets: Generac, Champion Power Equipment.
- Soft start manufacturer test data (Micro-Air EasyStart, Hyper Engineering SureStart) for surge reduction ranges.
- Condenser nameplate LRA method for exact surge.