Sizing guide
Will a generator run my air conditioner?
Yes, if it is sized for the compressor's startup surge. Cummins rates its 20kW air-cooled standby units to start and run a 5-ton air conditioner with the rest of the house already running; two systems on one 20kW unit need load management. A portable is not a whole-house answer for central air.
Why the compressor decides everything
An air conditioning compressor draws several times its running current for a fraction of a second when it starts. That spike is called locked rotor amps, and it is far larger than anything else in a house.
A generator has to survive that instant, not just the steady load afterwards. If it cannot, the voltage sags, the compressor stalls, and repeated stalling eventually destroys it. This is why a house with two 4-ton systems can need a bigger generator than a much larger house with one.
It is also why square footage is a poor guide. Two houses of identical size, one with a single system and one with three zones, are completely different sizing problems.
What the ratings actually mean
Manufacturers publish a motor starting figure in kVA, measured at a 30% voltage dip. For the Cummins air-cooled range it is 26 kVA on the 13kW, 34 kVA on the 17kW, and 40 kVA on the 20kW units.
In plain terms, the 20kW units are rated to start and run a 5-ton air conditioner under full household pre-load — meaning the rest of the house is already running when the compressor kicks in. That is the realistic scenario, not a compressor starting on an empty generator.
One system, two systems, three
A single 3 to 4 ton system in a smaller home is comfortably inside a 13kW to 17kW unit.
Two systems, common on two-story homes, is the usual 20kW case, but only with load management. Without it the generator has to be sized for both compressors starting at once, which pushes the choice toward a liquid-cooled unit.
Three or more systems, or a 5-ton plus a pool and a well, is where liquid-cooled units at 25kW and up start to make sense.
Fuel changes the number: on natural gas the 20kW air-cooled units produce 18kW; on propane, 20kW. The 13kW and 17kW units are rated the same on both fuels. Fuel comparison.
Load management is the cheap answer
Intelligent Load Management monitors what each large load needs and restores only what the generator has capacity to run. On the Cummins air-cooled units it handles up to four loads.
In practice the second air conditioner is held back for a short time while the first one starts. That is how a 20kW unit covers a house with two systems.
A soft-start module on the largest compressor works from the other direction, reducing the startup surge itself. Price both against the next generator size before accepting a larger unit.
Why a portable will not do it
A portable has to be started and connected by hand, and it feeds only the circuits you choose through a manual transfer switch or an interlock. Its output is sized for refrigeration, lighting, fans, pumps and electronics. That is useful in a short outage, but it is not a way to keep central air running.
Never connect a portable by back-feeding a dryer or range outlet. Without a transfer switch or interlock it can energize the utility line a lineworker is repairing.
Questions
What size generator do I need to run a 5-ton air conditioner?
A 20kW air-cooled unit is rated to start and run a 5-ton compressor under full household pre-load. If that 5-ton is the only air conditioning and the rest of the house is modest, a 17kW may cover it. If there is a second system, a pool pump or a well, you are into 20kW with load management or a liquid-cooled unit above it.
Can one generator run two air conditioners?
Yes, with load management. The generator holds the second compressor off until the first has started, so it only has to cover one startup surge at a time. Without load management the unit has to be sized for both compressors starting together, which on most two-system homes means a liquid-cooled unit rather than a 20kW air-cooled one.
Will a portable generator run central air?
Not as a whole-house answer. A portable has to be started and connected by hand, feeds only the circuits you select through a manual transfer switch or interlock, and is sized for essentials such as refrigeration, lights, fans, pumps and networking. Central air needs a standby unit sized for the compressor's startup surge, with an automatic transfer switch.
Does a soft start let me buy a smaller generator?
It can. A soft-start module reduces the compressor's startup current, and that surge is usually what forces the generator size up. Price one against the next generator size before accepting a larger unit; we check whether it applies to your compressor at the site survey.
How do I find my air conditioner tonnage?
It is normally encoded in the model number on the outdoor condenser data plate — 024 is 2 ton, 036 is 3 ton, 048 is 4 ton, 060 is 5 ton. Divide by 12 if you find a BTU figure instead.
Get a real number for your property
Equipment pricing is published on every model. The installation number takes about 45 minutes at your address, and it costs nothing.
Related across UTS
- Air conditioning is part of the load calculation for any home generator.Generator sizing for a 200-amp home, AC included (Clements Electric).