What Size Generator Do You Need to Run a Central Air Conditioner?
By the Generator Size Calculator Team · Updated · 6 min read
Quick answer: Honda lists a central air conditioner at 1,500 running and 2,200 starting watts for 10,000 BTU, 2,500 and 3,300 for 20,000, 3,800 and 4,950 for 24,000 and 6,000 and 7,800 for 40,000. That points to about 3,000, 4,000, 5,000 and 8,000 watts. Central AC is hardwired, so it needs a transfer switch or interlock installed by an electrician.
Central AC watts
| Central air conditioner | Running watts | Starting watts | Extra start-up surge |
|---|---|---|---|
| 10,000 BTU | 1,500 | 2,200 | 700 |
| 20,000 BTU | 2,500 | 3,300 | 800 |
| 24,000 BTU | 3,800 | 4,950 | 1,150 |
| 40,000 BTU | 6,000 | 7,800 | 1,800 |
These are Honda's typical figures. Real units vary with age and efficiency, so check the data plate on the outdoor unit. A window unit is much smaller, as covered in what size generator runs a window AC.
Which generator size to choose
| Central AC | Generator for the AC alone |
|---|---|
| 10,000 BTU (1,500 / 2,200 W) | 3,000 W |
| 20,000 BTU (2,500 / 3,300 W) | 4,000 W |
| 24,000 BTU (3,800 / 4,950 W) | 5,000 W |
| 40,000 BTU (6,000 / 7,800 W) | 8,000 W |
Sizes keep the running load under 80% of the rating and assume a starting capacity about 25% above it. Add the indoor fan, a fridge and lights to the AC's watts. See what a 6,000-watt generator runs and what a 10,000-watt generator runs. The AC LRA calculator and HVAC load calculator help with an exact unit.
Example: AC, fridge and lights
| Load | Running watts |
|---|---|
| Central AC, 24,000 BTU | 3,800 |
| Refrigerator | 700 |
| Lights and fans | 200 |
| Running total | 4,700 |
The refrigerator's extra start-up load is 1,500 watts (2,200 minus 700), bigger than the AC's 1,150, so the peak is about 6,200 watts. A 6,000-watt generator has an 80% mark of 4,800 watts, just above the 4,700 total, and its approximate 7,500-watt starting capacity covers the peak. That is tight, so avoid adding more loads. The lights and fans are example values.
Add the AC, fridge and the rest of the house.
The hookup you need
A central air conditioner is wired into the house, so you cannot plug it into a generator. It needs a transfer switch or interlock installed by a licensed electrician. See how to connect a generator to your house and the transfer switch calculator. Never backfeed through a dryer outlet. Compare with a whole-house setup in what size generator runs a whole house.
Run the generator outdoors, at least 20 feet from doors, windows and vents. Never in a garage or shed, even with the door open. See how far a generator should be from your house.
Frequently asked questions
How many watts does a central air conditioner use?
Honda lists 1,500 running watts for 10,000 BTU, 2,500 for 20,000, 3,800 for 24,000 and 6,000 for 40,000 BTU, with higher starting watts.
What size generator runs a 3-ton central AC?
A 3-ton unit is about 36,000 BTU, between Honda's 24,000 and 40,000 BTU rows, so expect roughly 5,000 to 6,000 running watts. Check the data plate and size toward 8,000 watts.
Can a 5000-watt generator run a central air conditioner?
A 24,000 BTU unit, yes, on its own. Add a fridge and lights and a 6,000-watt generator is safer.
Can I plug a central AC into a generator?
No. It is hardwired, so it needs a transfer switch or interlock installed by an electrician.
Can a portable generator run central air?
Yes, if it is big enough and properly connected. Many people use a window unit instead because it needs far less power.
Related calculators
Sources: Running and starting watts from the Honda generator wattage guide (central air conditioners, refrigerator). Generator sizes apply the 80% running rule and an assumed starting capacity about 25% above the running rating. The lights and fans in the example are assumed values.