An undersized generator will overload and shut down. An oversized generator wastes fuel and money. Here is exactly how DFW electricians size standby generators — from load calculations to real installation costs by kilowatt rating.
This guide is part of our Standby Generator Installation service in Dallas–Fort Worth.
The most expensive mistake you can make when buying a standby generator is guessing the size. An undersized generator will overload during the first summer heat wave, shut itself down, and potentially damage your air conditioner. An oversized generator burns more fuel than necessary, cycles on and off excessively, and costs thousands more upfront for capacity you will never use. At Clements Electric, we perform a formal load calculation on every home before recommending a generator size. Here is exactly how that process works and what size generator your Dallas–Fort Worth home actually needs.
Why Generator Size Matters More Than Brand
Homeowners love to debate Generac vs Kohler vs Cummins. But brand debates are irrelevant if your generator is the wrong size. A properly sized generator will outperform an undersized one from any brand, every single time. Sizing determines whether your generator runs at 50% load (ideal) or 95% load (stressful and short-lived). It also determines whether you need load-shedding modules, how large your gas line must be, and whether your existing electrical panel can handle the transfer switch integration without additional upgrades.
How to Calculate Your Home's Electrical Load
We use two methods to size generators: the full load method and the essential load method. Here is what each looks like in practice:
Full Load Method
The full load method assumes you want to power everything in your home simultaneously — every AC unit, every light, every appliance, every outlet. We add up the wattage of all your major appliances, add a 25% safety margin for motor starting surge, and select a generator that covers the total. For a typical 2,500 sq ft DFW home with two AC units, this usually lands at the top of the air-cooled range (20kW) with load management staggering the two compressors, or in the liquid-cooled range (25kW and up) without it.
Essential Load Method
The essential load method is more practical for most budgets. We identify the circuits you absolutely need during an outage: refrigerator, freezer, one AC unit, critical lighting, medical devices, security systems, and internet. We calculate the wattage for those circuits only and size the generator accordingly. This often drops the requirement to the 13kW – 17kW range, saving thousands on both the generator and the installation.
Common Generator Sizes for DFW Homes
Here are the sizes we install most often and the home profiles they match:
- 13kW (Cummins RS13A, air-cooled): Small homes, condos, townhomes, or essential-circuit setups. Powers refrigerator, lights, internet, and one smaller AC unit with load management.
- 17kW – 20kW (RS17A / RS20A, air-cooled): Whole-home coverage, central AC included, for most 2,000 – 3,500 sq ft DFW homes. This is the sweet spot for most DFW suburbs; load management lets it carry two AC units on staggered starts.
- 25kW and up (liquid-cooled RS range): Homes over about 4,000 sq ft, multiple HVAC zones, pools, guest houses, and workshops. Upgraded gas service is common at this size.
- 50kW+: Commercial properties, multi-family buildings, and estates with multiple structures. Requires site-specific engineering and gas line upgrades.
What Happens If Your Generator Is Undersized?
An undersized generator tries to supply more power than it can generate. The voltage drops. Motors on your AC compressor and refrigerator strain, overheat, and burn out. The generator's overload protection shuts it down repeatedly. In extreme cases, the generator itself is damaged. We see this most often when homeowners buy a 14kW unit for a 3,500 sq ft home with two AC units because the online calculator told them it was enough. Online calculators do not account for Texas heat, multi-stage compressors, or well pumps.
What Happens If Your Generator Is Oversized?
Oversizing is less dangerous but still wasteful. A generator running at 20% load builds carbon deposits in the engine, wastes fuel, and experiences wet stacking — a condition where unburned fuel accumulates in the exhaust. It also costs more upfront, requires a larger gas meter and regulator, and takes up more space in your yard. The ideal load range is 50-80% of the generator's rated capacity.
Installation Costs by Generator Size in Texas (2025)
These are complete installed prices including the generator, automatic transfer switch, gas line coordination, electrical connection, permitting, and startup testing:
- 20kW air-cooled (Cummins RS20A), typical complete whole-house installation: $13,500 – $17,000 all in
- 13kW air-cooled (RS13A): $11,000 – $14,000 all in
- 17kW air-cooled (RS17A): $12,500 – $15,500 all in
- Liquid-cooled 25kW and up: quoted in writing after an on-site survey
- All installed figures are estimates that vary with each home or business; equipment prices for every Cummins model are published at utselectricpros.com
The price variation depends on your home's distance from the gas meter, whether your electrical panel needs upgrading, and whether concrete pad preparation is needed. Homes in newer developments often have gas meters within 10 feet of the desired generator location, which keeps costs lower. Older homes in Dallas or Fort Worth may require 30+ feet of gas line trenching.
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Frequently Asked Questions
Yes, for most 2,000 – 3,500 sq ft DFW homes, central AC included. With a smart load management module it can prioritize circuits and run two AC units on a staggered basis — one starts, then the other. Homes over about 4,000 sq ft, or with multiple HVAC zones, pool pumps, electric water heaters, or workshops, usually move up to the liquid-cooled range, which starts at 25kW.
Yes. Larger generators consume more fuel per hour. A Cummins RS20A on natural gas uses about 268 cubic feet (roughly 2.6 therms) per hour at full load, per Cummins' published specifications, and liquid-cooled units use more. During a multi-day outage, that adds up — and an oversized generator running the same house means paying for unused capacity every hour it runs.
Not easily. The generator, transfer switch, and gas line are all sized together. Upgrading from a 20kW air-cooled unit to a liquid-cooled 25kW or larger usually requires replacing the transfer switch, upgrading the gas line, and sometimes upgrading the electrical panel. It is almost as expensive as a new installation. That is why we always recommend sizing for your 10-year needs, not just your current load.
