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Sizing a generator wrong is one of the more common, and more expensive, mistakes homeowners make. Buy too small and it trips or can’t start your AC compressor. Buy too big and you’ve paid for capacity you’ll never use. Getting the number right starts with deciding what you actually need the generator to power, then running the math against your specific home.
Start With Your Panel and Your Priorities
Before any wattage math, decide what “backup power” means to you. Some homeowners want everything to keep running exactly as normal, including central air, the water heater, and every outlet in the house. Others just want the essentials: refrigerator, some lighting, a window AC unit or two, the garage door, and enough outlets to keep phones charged and a router running. That decision determines which sizing approach applies to you.
Two Ways to Size a Generator
Whole-House Sizing
This approach covers your entire electrical panel, including central HVAC. It requires a full load calculation based on your home’s square footage, your AC tonnage, and every major appliance on the panel. For most homes in the DFW area, this lands somewhere in the 20 to 26 kW range, but it varies enough by house that a generic number isn’t reliable. A licensed electrician runs this calculation against your actual panel, not a rule of thumb.
Essential-Circuits Sizing
This approach only backs up a defined set of circuits, chosen ahead of time and wired through a sub-panel or load-management device. It’s a smaller, less expensive generator, often in the 10 to 14 kW range, and it’s a reasonable choice if running the whole house isn’t the goal.
Rule-of-Thumb Wattage for Common Loads
These are starting points, not a substitute for a real load calculation, but they’re useful for understanding what you’re working with.
| Appliance | Typical Running Watts | Starting (Surge) Watts |
|---|---|---|
| Refrigerator | 600-800 | 1,200-2,200 |
| Central AC (3-ton) | 3,500-4,500 | 6,000-9,000 |
| Well pump | 1,000-2,000 | 2,000-4,000 |
| Electric water heater | 3,000-4,500 | 3,000-4,500 |
| Sump/garage pump | 800-1,200 | 1,500-2,500 |
| Lighting circuit | 600-1,000 | 600-1,000 |
Why AC Load Changes the Math in Texas
Central air conditioning is the single biggest variable in sizing a generator for a DFW home. AC compressors draw a heavy surge of power the moment they start, well above their running wattage, and a generator that’s undersized won’t have the headroom to start the compressor at all, even if it can handle the running load once it’s on. In a Texas summer, an undersized generator that can’t start the AC defeats a good part of the reason to have one. This is the main reason whole-house systems in this region tend to run larger than what homeowners in milder climates typically install.
Fuel Type and Sizing
Natural gas and propane generators are rated slightly differently, and propane units generally produce a bit less output than the same generator running on natural gas. If your sizing calculation is close to a threshold between two generator models, the fuel type you’re running can be the deciding factor.
Why a Load Calculation Beats Guessing
Online sizing calculators and appliance nameplate wattage give you a ballpark, but they don’t account for simultaneous loads, your panel’s actual amperage, or how your specific AC unit behaves on startup. An electrician’s load calculation looks at your real panel and your real appliances, not averages, which is the difference between a generator that works when you need it and one that trips offline during the first heat wave outage.
Common Sizes for DFW Homes
As a general pattern (not a substitute for your own calculation): smaller homes on essential-circuit backup often land around 10-14 kW, mid-size homes running most of the house typically fall in the 16-20 kW range, and larger homes with central air and multiple major appliances on whole-house backup usually need 22 kW or more.
For cost ranges tied to these sizes, see our whole-house generator cost guide.
Get an Actual Number for Your Home
None of the ranges above replace a site-specific load calculation. Neighbors Electric (TECL #41555) handles sizing as part of every whole-home generator installation in Fort Worth, walking your panel, tallying your major loads, and recommending a unit sized for what your home actually draws, not a generic estimate. Call (817) 533-0943 or get in touch to have someone look at your panel.
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Neighbor’s Plan
Protect Your Home with the Neighbors Electrical Plan
Keep your home’s electrical system safe, efficient, and running smoothly year-round. The Neighbors Plan provides routine inspections, preventative maintenance, and exclusive member savings, all designed to help you avoid costly electrical problems before they happen.
Annual Electrical Safety Inspection
A full system inspection to ensure your wiring, outlets, breakers, and electrical components are operating safely.
Priority Service
Skip the wait and receive faster scheduling when electrical issues arise.
Electrical Panel Tune-Up
Prevent problems before they start with routine panel maintenance and system checks.
No Service Call Fees
Members avoid standard service charges, saving money on every visit.
15% Off All Electrical Work
Members save on repairs, upgrades, and installations whenever electrical work is needed.
Extended Parts & Labor Warranty
Enjoy up to a 10-year warranty extension on qualifying work while you remain a member.
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Licensed electricians delivering safe, reliable electrical work for homes and businesses across the Dallas–Fort Worth area.