Quick Answer
Window AC (5,000-8,000 BTU): A 3,000-3,500W inverter generator like the Champion 3500W.
Central AC (2-3 ton): You need 7,500-10,000W like the DuroMax XP12000EH.
Our Top Picks
Westinghouse iGen4500DF
Handles 12,000 BTU window AC plus essentials with dual fuel
DuroMax XP12000EH
Powers 3-ton central AC plus whole house with 240V ready
Champion 3500W Inverter
Great value for smaller window AC units with CO Shield safety
AC Wattage Requirements by Type
Air conditioners require significant starting wattage—often 2-3x their running watts. Here's what each type needs:
| AC Type | BTU Rating | Running Watts | Starting Watts | Recommended Generator |
|---|---|---|---|---|
| Small Window AC | 5,000 BTU | 450-500W | 1,400-1,800W | 2,500W+ |
| Medium Window AC | 8,000 BTU | 700-900W | 2,100-2,700W | 3,500W+ |
| Large Window AC | 12,000 BTU | 1,000-1,200W | 3,000-3,600W | 4,500W+ |
| Portable AC | 10,000 BTU | 900-1,200W | 2,700-3,600W | 4,500W+ |
| Central AC (1.5 ton) | 18,000 BTU | 1,500-2,000W | 4,500-6,000W | 7,500W+ |
| Central AC (2 ton) | 24,000 BTU | 2,000-2,500W | 6,000-7,500W | 9,000W+ |
| Central AC (3 ton) | 36,000 BTU | 3,000-3,500W | 9,000-10,500W | 12,000W+ |
Important: Central AC Usually Requires 240V
Most central AC systems run on 240V power. You'll need a generator with a 240V outlet AND a proper transfer switch installed by an electrician. This is not a DIY project.
Tips for Running AC on a Generator
1. Size Up for AC
AC units have massive startup surges. Always get a generator rated for at least 1.5x the AC's starting watts to have safe headroom.
2. Start AC First
Turn on the AC before plugging in other devices. Once the compressor is running, you can add refrigerator and other loads.
3. Consider a Soft Start Device
A "soft start" device reduces AC startup surge by 60-70%. This lets you run larger AC units on smaller generators. They cost $300-500 but can save thousands on generator size.
4. Central AC Needs Professional Installation
Never backfeed power into your panel. A licensed electrician must install a transfer switch. This protects utility workers and your equipment.