Can a Generator Run an AC Unit? Sizing by Tonnage
Why starting an AC unit requires massive starting power. Soft start kit sizing guidelines.
The Challenge of Running AC on a Generator
Air conditioners are the single most difficult appliance to run on a generator. It’s not because they consume too much power while running; it’s because the compressor requires a massive, instantaneous surge of electricity—often called Locked Rotor Amps (LRA)—to start up.
Generator Sizing by AC Tonnage (Central Air)
If you want to run a home's central AC, you usually need a large standby generator. Central AC units are measured in "tons" (1 ton = 12,000 BTU).
| Central AC Size | Running Watts | Starting (Surge) Watts | Min. Generator Size |
|---|---|---|---|
| 2 Ton (24,000 BTU) | 2,500 W | 7,500 W | 10,000 Watt Gen |
| 3 Ton (36,000 BTU) | 3,500 W | 10,500 W | 14,000 Watt Gen |
| 4 Ton (48,000 BTU) | 4,500 W | 13,500 W | 18,000 Watt Gen |
| 5 Ton (60,000 BTU) | 5,500 W | 16,500 W | 22,000 Watt Gen |
Note: Add the running watts of your fridge, lights, and other appliances to the "Min. Generator Size" to get your total house requirement.
Running Window and Portable AC Units
Window units are much smaller and can easily be run on portable generators.
- 5,000 BTU Window AC: Uses ~500W running / 1500W starting. Easily runs on a 2000W generator.
- 10,000 BTU Window AC: Uses ~1000W running / 3000W starting. Needs a 3500W generator.
- 14,000 BTU Portable AC: Uses ~1400W running / 4000W starting. Needs a 4500W generator.
The Secret Hack: Soft Start Kits
If you have a 3-ton central AC that requires 10,500 starting watts, but you only have a 7,500-watt portable generator, you can install a Soft Start kit (like Micro-Air EasyStart).
A soft starter modifies the way electricity enters the compressor, ramping it up slowly over a few seconds instead of slamming it all at once.