Choosing a size
13kW, 17kW or 20kW?
Pick by air conditioning, not floor area. The RS13A (13kW, $4,419.04) starts one central AC plus essential circuits; the RS17A (17kW, $5,326.86) covers most single-AC homes; the RS20A ($5,925.25) is for two AC systems with load management, and is rated 18kW on natural gas and 20kW on LP.
The short answer
| Model | Output | What it covers | Equipment |
|---|---|---|---|
| RS13A | 13kW | Essential circuits plus one central AC in a home up to roughly 1,800 sq ft. | $4,419.04 |
| RS17A | 17kW | Most of a 2,000-2,500 sq ft home with load management on the larger AC. | $5,326.86 |
| RS20A | 20kW | Whole-house coverage for a typical 2,500-3,500 sq ft DFW home with two AC systems, using load management. | $5,925.25 |
Equipment only, at the Cummins suggested retail price. Installation is quoted separately after a free survey.
Square footage is the wrong unit
Most online sizing charts are indexed on floor area. In Texas the load that decides which of these three you need is air conditioning, specifically the moment a compressor starts. A compressor draws several times its running current for the first fraction of a second, and a generator has to absorb that surge without the voltage sagging far enough to stall it.
Two houses of identical size can land on different models: one with a single four-ton system and one with two systems and a pool pump are different loads. Read what actually happens when a generator starts an air conditioner before you settle on a number, and the sizing calculator will give you a range in a couple of minutes.
What is identical across all three
All three are the same Cummins QSJ999G engine in the same aluminum enclosure, published at the same 65 dB(A) at 23 feet, running on either natural gas or LP vapor, listed to UL 2200 and installable at 18 inches from the building under NFPA 37. All three carry the same 5 year / 2,000 hour limited warranty.
The one difference besides capacity is fuel: the RS13A and RS17A are rated the same on natural gas and LP, while the RS20A is 18kW on natural gas and 20kW on LP. Going up a size buys more simultaneous load, not a different machine.
What the steps cost
$907.82 of equipment separates the RS13A from the RS17A, and $598.39 separates the RS17A from the RS20A. Installation barely moves between them — all three sit on a similar pad with a similar electrical tie-in, and what actually drives an installed price is the gas line, the distance from your meter, the run to your panel and your city's permit.
If you expect to add load later, such as a pool pump or a second AC system, size for it now: the equipment step is cheaper than replacing the generator.
When a bigger unit is the wrong answer
If the reason you are being pushed up a size is two compressors starting together, the other fix is to stop them doing that. A load management module holds one system off for the seconds the other needs to start. We set out both paths, with what each costs, in load management or a bigger generator, including the cases where the bigger machine is the better buy.
Above 20kW
These three are the air-cooled range and it stops at 20kW. Above that the line becomes liquid-cooled, starting at 25kW: a different engine turning at 1800 rpm rather than 3600, with a different footprint, sound level and service interval. If your load calculation lands past 20kW, the full Cummins lineup, 13kW to 200kW is the place to look, and every liquid-cooled model, 25kW to 150kW is priced on the site.
Questions
Is 20kW enough for a 3,000 square foot house?
Often, with load management that stops two compressors starting at the same moment, but floor area is not what decides it. Note the RS20A is rated 18kW on natural gas, the fuel most DFW homes use. Square footage is the wrong unit to decide on — a 3,000 sq ft home with two 5-ton systems and a well pump is a different problem from a 3,000 sq ft home with one 4-ton system. The load calculation at the survey settles it.
What is the real difference between 13kW, 17kW and 20kW?
How much air conditioning you can start, and how much load you can run at once. As a rule of thumb before a load calculation, the 13kW suits essential circuits plus one central AC, the 17kW most single-AC homes, and the 20kW two AC systems with load management. On natural gas the RS20A is rated 18kW, against 20kW on LP. Mechanically they are the same machine — same Cummins QSJ999G engine, same 65 dB(A) published sound level, same enclosure and the same 5 year / 2,000 hour warranty.
Is it worth stepping up a size just in case?
Sometimes. The equipment steps are $907.82 of equipment from 13kW to 17kW and $598.39 from 17kW to 20kW. But a bigger generator is not always the right answer to a simultaneity problem — if two compressors starting together is what pushes you up a size, a load management module that keeps them from starting together may let the smaller unit do the job. We compare the two paths in a separate guide.
Do the bigger units cost more to install?
Not much, in this range. All three are air-cooled units on a similar pad with a similar electrical tie-in. What moves an installed price is the gas line, the distance from the meter, the run to your panel and your city's permit, not which of these three sits on the pad. Our published installed estimates are $11,000 to $14,000 for 13kW, $12,500 to $15,500 for 17kW and $13,500 to $17,000 for 20kW. Estimate only. Every installation is different; a site visit is required to determine whether your installation will cost less or more than this estimate.
Should I get the model with the transfer switch included?
If your service size suits it, it is usually the cleaner buy. The RS20AC ships with a 200A service-entrance rated, NEMA 3R automatic transfer switch at $6,868.48, against $5,925.25 for the RS20A without one. Service-entrance rated means the switch can serve as your main disconnect, which often simplifies the install. The switch is sized to your electrical service rather than to the generator, so we confirm your service at the survey before specifying it.
We will size it against your actual panel
The survey is free and it produces a load calculation, not a guess from floor area. If the answer is the smaller unit, that is what we will tell you.
Rather just talk to someone?
Leave a number and we will call you. No sales script, no obligation.
Got it. We will call you. If you would rather not wait, ring (214) 957-4307 and ask for whoever is on generators.
Related across UTS
- If your home has 200-amp service, this answers the size question directly.What size generator a 200-amp home needs (Clements Electric).