Sizing guide
Will a generator run my air conditioner?
A properly sized standby generator will. A portable will not. In Texas this is not a side question — air conditioning is the load that decides which generator you need, and almost every sizing mistake starts here.
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, which is most two-storey homes in Plano, Frisco and McKinney, is the classic 20kW case — but only with load management. Without it you are sizing for both compressors starting simultaneously, which pushes you toward a much larger and much more expensive machine.
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.
Remember the fuel derate: on natural gas the 20kW air-cooled units produce 18kW, not 20kW. That changes the sizing arithmetic and it is rarely mentioned.
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.
Practically, that means the second air conditioner is briefly held back when the first one starts. You will not notice it happening, and it routinely lets a 20kW unit cover a house that would otherwise need something far larger.
A soft-start module on the largest compressor does the same job from the other direction, reducing the startup surge itself. Both are dramatically cheaper than moving up a generator size, and both are worth pricing before you accept a larger unit.
Why a portable will not do it
Portables top out well below what a central air conditioner needs to start, and they have no transfer switch, so there is no safe way to feed a compressor circuit from one. A portable covers refrigeration, lighting, pumps and electronics — genuinely useful in a short outage, and not air conditioning.
In a July outage in North Texas, that distinction is the difference between an inconvenience and a house nobody can stay in.
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 module staggers the compressors so the generator only has to cover one startup surge at a time. Without load management you are sizing for both starting together, which typically doubles the requirement and moves you out of the air-cooled range entirely.
Will a portable generator run central air?
No. Portables cover essentials — refrigeration, lights, pumps, networking. Central air conditioning needs a permanently installed standby unit with a transfer switch.
Does a soft start let me buy a smaller generator?
Often, yes. A soft-start module reduces the compressor inrush, which is the constraint. It usually costs a fraction of the difference between generator sizes, so it is worth pricing before accepting a larger unit. We look at this 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.