What Size Generator Runs Central Air Conditioning?
Air conditioning is the single load that pushes most households from a portable generator to a standby unit — and the reason is the start-up surge, not the running draw.
Alex Rivers
Home Improvement Editor
Last Updated
September 2, 2026
In This Guide
A three-ton central air conditioner runs on 3,000 to 3,800 watts, which sounds manageable. It starts on 9,000 to 12,000, which does not — and that momentary surge is what decides the generator you need.
1. Generator Size by Air Conditioner Tonnage
| AC Size | Running Watts | Starting Watts | Generator With Other Loads |
|---|---|---|---|
| 2 ton (24,000 BTU) | 2,000-2,800 | 6,000-8,000 | 12,000W minimum |
| 2.5 ton (30,000 BTU) | 2,500-3,200 | 7,500-10,000 | 12,000-14,000W |
| 3 ton (36,000 BTU) | 3,000-3,800 | 9,000-12,000 | 14,000-18,000W |
| 4 ton (48,000 BTU) | 4,000-5,000 | 12,000-16,000 | 18,000-22,000W |
| 5 ton (60,000 BTU) | 5,000-6,000 | 15,000-20,000 | 22,000W or more |
The final column assumes the air conditioner is not the only thing running, which it never is. A refrigerator, freezer, lights and internet add roughly 2,000 running watts before the compressor even starts, and the generator has to carry that load while absorbing the surge.
Why This Is Standby Territory
Most portables top out around 9,500 to 12,000 watts.
A three-ton unit alone can surge to 12,000, which is the entire output of a large portable generator with nothing else connected. Once you add household essentials, the arithmetic lands outside what portable machines deliver. That is why central air conditioning is the load that decides between a portable and a standby installation.
2. Why the Start-Up Surge Is So Much Larger
Air conditioner compressors use induction motors that have to overcome both mechanical inertia and the pressure differential already in the refrigerant system. At the instant power is applied the rotor has not moved, so the motor draws what is effectively a locked-rotor current — several times its running draw — until it comes up to speed.
That period lasts a second or two, but a generator has to supply it. If it cannot, voltage sags, the compressor fails to start, and the generator either trips its breaker or stalls. Worse, a compressor that repeatedly attempts and fails to start can be damaged by the sustained high current, so undersizing risks the air conditioner as well as the generator.
Reading Your Own Unit
- The data plate on the outdoor condenser lists RLA (rated load amps) and LRA (locked rotor amps).
- Multiply LRA by the supply voltage for a conservative starting wattage figure.
- Multiply RLA by voltage for the running figure.
- LRA is the honest worst case and is generally higher than published typical ranges.
- Older units tend to draw more than modern high-efficiency equipment of the same tonnage.
Using your own data plate rather than a generic table matters more here than for any other household appliance, because air conditioners vary enormously between models and ages, and because this is the load that determines your entire generator purchase.
3. The Soft Starter: How to Avoid Buying a Bigger Generator
This is the most useful thing on this page and it is frequently overlooked. A soft starter is a device fitted to the air conditioner that ramps the compressor up gradually rather than applying full voltage instantly, which dramatically reduces the start-up surge.
| Without Soft Starter | With Soft Starter | |
|---|---|---|
| 3-ton starting draw | 9,000-12,000 watts | Often 3,000-5,000 watts |
| Generator needed | 14,000-18,000W | Frequently 7,500-9,500W |
| Class of machine | Standby installation | Large portable |
| Added cost | None | Device plus installation |
The reduction is substantial enough to change what class of generator you need. A household that could not run its air conditioning on any portable can often run it on a large portable once a soft starter is fitted, and the cost of the device plus installation is typically a fraction of the difference between a large portable and a standby installation.
Worth Pricing Before You Buy a Generator
A soft starter can be cheaper than the generator upgrade it avoids.
If air conditioning is the only reason you are considering a standby unit, get a quote for a soft starter on your existing condenser first. It is a small device, it is fitted by an HVAC technician, and it may let a 9,500 watt portable do what would otherwise need a 18,000 watt standby.
Two caveats worth knowing. Soft starters are not universally compatible with every compressor, particularly some older units and certain variable-speed systems, so the installer needs to confirm it suits your equipment. And the device reduces the surge, not the running draw — you still need to carry 3,000 to 3,800 watts continuously for a three-ton unit.
4. Worked Examples With and Without Air Conditioning
Essentials Only, No Air Conditioning
Refrigerator 700, freezer 500, gas furnace blower 900, sump pump 1,050, lights and electronics 600. Running total 3,750. The largest surge is the sump pump at about 1,550 above its running draw. Peak demand roughly 5,300, and with twenty percent headroom around 6,400. A 7,500 watt portable covers it comfortably.
The Same Home Plus a 3-Ton Central AC
Add 3,800 running watts and the total climbs to 7,550. The largest surge is now the air conditioner at roughly 8,200 above its running draw. Peak demand is about 15,750, and with headroom close to 19,000 watts. That is a standby unit — a 22,000 watt machine in practice.
The Same Home, AC With a Soft Starter
Running total is unchanged at 7,550, because a soft starter does not reduce running draw. But the surge drops to perhaps 1,500 above running, and the largest surge on the list may now be the sump pump again. Peak demand falls to roughly 9,100, and with headroom about 11,000 watts. A 12,000 watt portable becomes viable.
Those three scenarios differ by one appliance and one small device, and they span a 7,500 watt portable to a 22,000 watt standby installation. It is the clearest illustration available of why the surge, rather than the running load, drives generator sizing.
5. Alternatives Worth Considering First
Before committing to standby-scale capacity, it is worth asking whether whole-house cooling is genuinely what you need during an outage.
- A window air conditioner at 1,000-1,200 running and 2,200-3,000 starting cools one room for a fraction of the capacity.
- A portable air conditioner is similar and needs no permanent installation.
- Ceiling and box fans draw 50-200 watts and make a substantial difference to comfort.
- A soft starter on the existing condenser, as above, often bridges the gap entirely.
- Load shedding lets a smaller standby unit run the AC by dropping other circuits when it starts.
The window unit route deserves serious thought. Cooling one bedroom through a summer outage costs perhaps 3,000 watts of surge capacity rather than 12,000, and for most families that is the difference between a comfortable night and an uncomfortable one — which is what they actually wanted from the air conditioning.
Load Shedding on Standby Units
If you do go standby but want to avoid the largest unit, load management modules automatically shed lower-priority circuits when the air conditioner calls for power, then restore them once it is running. That allows a mid-size standby generator to serve a house that would otherwise need full capacity, at the cost of some circuits pausing briefly.
6. Heat Pumps and Mini-Splits Behave Differently
Not all cooling equipment presents the same problem, and the differences matter enough to change your generator decision.
| System | Starting Behaviour | Generator Implication |
|---|---|---|
| Conventional central AC | Hard start across the line | Largest surge; standby territory |
| Heat pump, single stage | Similar to central AC | Same as above, and it runs in winter too |
| Two-stage or variable speed | Starts in low stage | Noticeably lower surge |
| Inverter mini-split | Ramps up electronically | Very low surge; often portable-friendly |
| Window unit | Modest hard start | 2,200-3,000 starting watts |
Inverter Mini-Splits Are the Quiet Advantage
An inverter-driven mini-split ramps its compressor up electronically rather than starting it across the line, so it has very little start-up surge at all. A household with mini-splits rather than a conventional central system can often cool one or two zones on a mid-size portable generator, which is simply not possible with a conventional condenser of similar capacity.
Heat Pumps Cut Both Ways
A heat pump is an air conditioner that also runs in reverse for heating, which means the surge problem applies in winter as well as summer. That is a stronger argument for standby capacity than cooling alone, because losing heat in a winter outage is a more serious matter than losing air conditioning in a summer one. Households on heat pumps with resistance backup heat should also check what that backup strip draws, because it can be very large.
7. Operating Air Conditioning on Generator Power
Having the capacity is not quite the same as running the system well. A few practical points make the difference between air conditioning that works on generator power and air conditioning that trips the machine.
Start the generator and let it stabilise
Bring it up to speed with no load, then transfer circuits. Starting under load is hard on the machine and can cause voltage sag.
Bring the air conditioner on last
Let the smaller loads settle first, then allow the compressor to start into a stable supply with the full remaining capacity available.
Avoid other motor starts at the same moment
Do not run the microwave or start a pump in the seconds the compressor is coming up. Staggering start events is free capacity.
Raise the thermostat setpoint
A higher setpoint means fewer compressor cycles, and every cycle is another surge the generator has to absorb.
Watch for voltage sag or dimming
Lights dipping hard each time the compressor starts means you are close to the limit. Shed a load rather than living with it.
Consider running cooling in blocks
Cooling the house down for a few hours then letting the generator rest at low load is easier on the machine than continuous cycling.
Repeated voltage sag is worth taking seriously rather than tolerating. It stresses the generator, and brownout conditions are hard on motor windings and on sensitive electronics elsewhere in the house. If the lights dim heavily every time the compressor kicks in, the machine is undersized for that load and something needs to change — a soft starter, fewer simultaneous loads, or more capacity.
8. Common Mistakes to Avoid
Sizing on Running Watts Alone
A three-ton air conditioner runs on 3,000 to 3,800 watts and starts on 9,000 to 12,000. A generator sized to the running figure will not start the compressor at all — voltage sags, the breaker trips, and repeated failed start attempts can damage the compressor itself.
Using a Generic Table Instead of the Data Plate
Air conditioners vary enormously between models and ages, and this is the load that determines your entire generator purchase. The condenser data plate lists LRA, or locked rotor amps; multiply by voltage for an honest worst-case starting figure rather than trusting a typical range.
Not Pricing a Soft Starter First
A soft starter can cut a three-ton unit’s surge from 9,000-12,000 watts to often 3,000-5,000, which frequently moves you from a standby installation to a large portable. The device plus installation is typically a fraction of that difference, so it is worth quoting before buying any generator.
Forgetting Everything Else Running
The air conditioner is never the only load. A refrigerator, freezer, lights and internet add roughly 2,000 running watts before the compressor starts, and the generator has to carry that while absorbing the surge. Size for the total, not for the air conditioner in isolation.
Assuming You Need Whole-House Cooling
A window unit at 2,200-3,000 starting watts cools a bedroom for a quarter of the surge capacity a central system demands. For most families a comfortable night is what they actually wanted, and that is achievable without moving into standby generator territory at all.
9. Frequently Asked Questions
What size generator do I need to run central air conditioning?
For a three-ton unit alongside household essentials, 14,000 to 18,000 watts, which means a standby installation. The driver is the start-up surge of 9,000 to 12,000 watts rather than the 3,000 to 3,800 running draw. A soft starter can reduce that requirement substantially.
Can a portable generator run central air?
Usually not without help. Most portables top out around 9,500 to 12,000 watts, and a three-ton air conditioner can surge to 12,000 on its own before any other load. Fitting a soft starter to the condenser often brings the surge down enough that a large portable becomes viable.
What is a soft starter and does it help?
A device fitted to the air conditioner that ramps the compressor up gradually rather than applying full voltage instantly. It commonly cuts a three-ton unit’s starting draw from 9,000-12,000 watts to around 3,000-5,000, frequently moving you from needing a standby unit to a large portable.
Will a 10000 watt generator run central air conditioning?
A 10,000 watt generator may start a smaller two-ton unit with little else running, but it is marginal for a three-ton system, which can surge to 12,000 watts alone. With a soft starter fitted it becomes realistic, and with other household loads connected you would want more headroom.
How many watts does a 3 ton AC use?
Roughly 3,000 to 3,800 watts running, and 9,000 to 12,000 watts at start-up. The gap between those two figures is what drives generator sizing. Check the LRA figure on your condenser data plate and multiply by voltage for a conservative starting number specific to your unit.
Is it cheaper to fit a soft starter or buy a bigger generator?
Usually the soft starter, often by a wide margin. The device and its installation are typically a fraction of the difference between a large portable and an installed standby system, and it can move a household from needing 18,000 watts to needing around 9,500.
Do inverter mini-splits need less generator capacity than central AC?
Considerably less. An inverter mini-split ramps its compressor up electronically rather than starting it across the line, so it has very little start-up surge. A household on mini-splits can often cool one or two zones from a mid-size portable, which is not possible with a conventional condenser of similar capacity.
Why do my lights dim when the AC starts on generator power?
That is voltage sag, and it means the generator is close to its limit for that surge. It stresses the machine and brownout conditions are hard on motor windings and electronics. Shed other loads, fit a soft starter, or move up a size rather than tolerating it.
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The Bottom Line
For a three-ton central air conditioner alongside normal household loads, plan on 14,000 to 18,000 watts, which means a standby installation rather than a portable. The number is driven entirely by the start-up surge of 9,000 to 12,000 watts, not by the comparatively modest 3,000 to 3,800 running draw.
Before you buy that capacity, price a soft starter. Fitted to the existing condenser it commonly cuts the surge to 3,000-5,000 watts, which can move a household from needing an 18,000 watt standby unit to running comfortably on a 9,500 or 12,000 watt portable. The device and installation are usually a fraction of that price difference.
And check your own data plate rather than any table, including this one. The condenser lists LRA — locked rotor amps — and multiplying it by the supply voltage gives you an honest worst case for your specific unit. This is the one appliance where a generic figure being wrong costs you thousands.
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About the Author
Alex Rivers, Home Improvement Editor
Alex has spent over a decade working on residential and light commercial property maintenance, and now tests every tool, coating and machine that appears in these guides personally.