Generator for Central Air Conditioner: How to Size It From the Nameplate (2026)
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How we evaluated: WildJoule has not bench-tested any air conditioner or generator here. The rule-of-thumb wattage table is Generator Source’s published power consumption chart, which the publisher itself says is an example rather than a strict guide. The soft-starter figure is Micro-Air’s manufacturer claim. Generator specs come from Westinghouse and DuroMax. The worked example uses illustrative label values, not a specific product. Checked October 12, 2026.
Picking a generator for central air conditioner use goes wrong in one predictable way: people size for running watts and forget the compressor’s first half-second. A 3-ton condenser might run happily on 3,000–4,000 W and still trip a 7,500 W generator at start-up, because the locked-rotor current on its label can be five or six times the running current. The good news is that the label tells you almost everything, and a soft starter changes the maths a lot.
Short answer
You need a 240 V generator connected through a transfer switch or interlock — 120 V-only inverter generators can’t run a standard central AC condenser. As a rough guide, a 2-ton system with a soft starter is realistic on a 7,500 W class portable feeding a 30A inlet; 3 tons and up, or no soft starter, points to a 10,000 W+ machine on a 50A inlet. The condenser’s RLA and LRA numbers decide it, not the tonnage.
Quick note on where this comes from: I haven’t wired a generator to a central AC, and I’m not an HVAC tech. I worked through the nameplate maths, a published wattage chart and the generator spec sheets so you can sanity-check your own system. Hot outages are the ones that worry me most — I’ve sat through enough power cuts to know heat is what makes them miserable.
Summer shutoffs are increasingly planned, too: utilities in fire-prone areas cut power during high-risk weather, which is exactly when you want AC. Our wildfire power outages guide covers how those shutoffs work.
Generator for central air conditioner: read the condenser label first
Go to the outdoor unit and find the data plate. Three numbers matter:
- Voltage — almost always 208/230 V single-phase on a home condenser. That’s why you need a 240 V generator.
- RLA (rated load amps) for the compressor, plus the FLA of the outdoor fan motor. Together they’re your running current.
- LRA (locked rotor amps) — the current the compressor pulls for a moment as it starts.
Multiply by 240 to get a working figure. Running watts ≈ (RLA + fan FLA) × 240. Worst-case starting load ≈ LRA × 240. These are approximations; power factor and how quickly the motor comes up to speed both shift the real number, and a generator’s peak rating only holds for a short time.
A worked example (illustrative numbers)
Say a condenser label reads RLA 14, fan FLA 1.2 and LRA 76. Those are made-up but realistic numbers for a mid-size unit; use your own.
| Step | Maths | Result |
|---|---|---|
| Running load | (14 + 1.2) × 240 | ≈ 3,650 W |
| Starting load, no soft starter | 76 × 240 | ≈ 18,200 VA (momentary) |
| Starting load, soft starter at a 75% cut* | 18,200 × 0.25 | ≈ 4,550 VA |
| Indoor blower (separate circuit) | From the furnace or air handler label | Add it to running load |
*Micro-Air claims its EasyStart Flex can “reduce start current by up to 75%.” That’s the best case as stated by the manufacturer; real reduction varies by compressor.
The table shows the whole problem. Without help, that compressor briefly asks for more than any portable generator can deliver. Generators do tolerate a short overload, and the full LRA rarely lasts long, which is why some units start anyway. But “sometimes starts” isn’t a backup plan. With a soft starter, the same compressor fits under the 9,500 W peak of a 7,500 W class machine with room left for the fridge.
Rule-of-thumb wattage by system size
If you can’t read the label, a published chart gives a starting point. These figures are from Generator Source’s power consumption chart, which the publisher describes as an example of how wattage varies, not a sizing guarantee:
| Central AC size | Running W | Starting W |
|---|---|---|
| 10,000 BTU | 1,500 | 2,200 |
| 20,000 BTU | 2,500 | 3,300 |
| 24,000 BTU (2 ton) | 3,800 | 4,950 |
| 32,000 BTU | 5,000 | 6,500 |
| 40,000 BTU | 6,000 | 6,700 |
Our honest read: those starting figures look optimistic next to typical LRA values, which is why we’d trust your label over any chart. Use the chart for ballpark running load and the nameplate for start-up.
Which size generator for central air conditioner loads
About 2 tons, with a soft starter: a 7,500 W class dual-fuel portable on a 30A inlet. The Westinghouse WGen7500DF is rated 7,500 running / 9,500 peak on gas and has the L14-30R a 30A transfer switch expects. Expect to keep the electric dryer, oven and water heater off while the AC runs.
3 tons and up, or no soft starter: step up to a 10,000 W+ machine on a 50A inlet. The DuroMax XP13000EH is rated 10,500 running / 13,000 peak on gas, keeps 10,000 / 12,500 on propane, and has a 14-50R. The Westinghouse WGen9500DF is the other common choice.
4–5 tons, or AC plus electric cooking and water heating: realistically a permanently installed standby generator. That’s outside what we cover on portable units, and it’s a job for a licensed installer.
Why an inverter generator usually can’t do it
Popular inverter generators like the Honda EU2200i and the Honda EU3000iS are 120 V only. A home condenser is a 240 V load, so they can’t run it however many watts they make. A window or portable 120 V AC is a different story; for those, the generator for RV air conditioner numbers are closer to what you need. If you’re unsure whether a generator puts out 240 V, look for an L14-30R or 14-50R outlet. Our split phase guide explains the 120/240 V setup.
Connecting it safely
A central AC runs off the panel, so any generator for central air conditioner duty has to feed the panel through a transfer switch or an interlock with an inlet box, installed by an electrician. Never backfeed through a dryer outlet; it can energise the utility line and injure line workers. Our generator transfer switch guide covers manual switches, interlocks and the 30A vs 50A decision. Then keep the generator well away from the house; our carbon monoxide safety guide has the placement rules.
One practical habit: start the AC after the generator has warmed up and before anything else with a motor kicks in. Staggering big loads is free and often the difference between a clean start and a tripped breaker. The same logic applies to a well pump and a sump pump on the same generator.
Frequently asked questions
What size generator do I need for a 3 ton AC unit?
Usually 10,000 W or more on a 50A inlet without a soft starter. With a soft starter, a smaller 240 V unit may work. Check the condenser’s LRA: multiply it by 240 for the worst-case start.
Can a 7,500 watt generator run central air?
A smaller system, often yes, particularly with a soft starter and little else running. A 3-ton or larger unit without one is a stretch.
Do I need a soft starter to run AC on a generator?
Not always, but it’s the cheapest way to make a portable generator start a compressor reliably. Micro-Air claims up to a 75% cut in start current for its EasyStart Flex.
Can I run central air on an inverter generator?
Only if it outputs 240 V. Most small inverter generators are 120 V only and can’t run a home condenser.
Does a heat pump work the same way?
The compressor maths is the same. Electric backup heat strips are not: they draw several kilowatts each and usually need to stay off on a portable generator.
Sources: Generator Source power consumption chart; Micro-Air EasyStart Flex product page; manufacturer spec pages for each generator named.
Last updated: October 12, 2026. Sizing a generator for central air conditioner loads is an estimate; confirm with your condenser’s label and an electrician. We do not display live prices.