What Size Whole House Generator Do I Need?
Most homes need roughly 18 to 26 kilowatts to run the whole house, or 10 to 14 kilowatts to cover the essentials. Your air conditioning is the deciding load, so the reliable way to size a unit is a proper load calculation rather than a rough guess from square footage.
The right answer to what size whole house generator you need is given in kilowatts, and it hinges on whether you want the entire home running or only the essentials. Running everything, most homes come in around 18 to 26 kilowatts. Covering just the must haves, 10 to 14 kilowatts often suffices. Along the Gulf coast the load that settles it is nearly always the air conditioning, so below is how to size a unit the right way rather than guessing.
It Comes Down to Kilowatts
A generator’s capacity is rated in kilowatts, and matching that to your home is the decision that makes or breaks the purchase. Undersize it and the unit strains or shuts down under load. Oversize it and you have paid too much for the machine and its install. The target is a unit that carries your true peak demand with a little to spare.
How Much of the House?
| Coverage | Typical size |
|---|---|
| Essentials only | 10 to 14 kilowatts |
| Most of the home | 16 to 22 kilowatts |
| Everything at once | 22 to 26 kilowatts or liquid cooled |
Settling how much of the house you want on backup comes before any number, because running the whole home at once demands far more capacity than keeping the fridge, some lights, and one air conditioner going.
AC and Humidity Set Gulf Sizing
Along the Bay, air conditioning is the load that fixes the size. A large system pulls a heavy surge the instant its compressor kicks in, and in the humidity a home cannot go long without it in a summer outage. A generator that cannot swallow that surge falls short exactly when it counts, so size for the air conditioning first.
If you run two air conditioners or one oversized system, say so when the unit is sized. Two compressors starting can lift you out of an air cooled unit and into a larger one.
Estimating Your Number
List everything you want running during an outage.
Note each item’s running and startup watts, the AC especially.
Total the running loads, then add the largest startup surge.
Match that to a unit with headroom, not the bare minimum.
Let an Installer Calculate It
A wattage list gives a ballpark, but the dependable way to size a whole house generator is a load calculation by someone who fits them. They weigh your panel, your AC surge, and code, and the calculation costs a fraction of buying a unit that turns out too small.
Undersizing to shave the price is the costliest mistake here. A generator that cannot start your air conditioning leaves you without cooling in a Gulf coast outage, which defeats the reason you bought it.
Frequently Asked Questions
What size whole house generator do I need?
About 18 to 26 kilowatts to run a whole home, or 10 to 14 kilowatts for the essentials. Your air conditioning drives the exact size, so a load calculation is the reliable way to decide.
How many kilowatts to run a whole house?
Most homes need roughly 18 to 26 kilowatts to run everything at once, with large homes or heavy air conditioning reaching the top of that or moving to a liquid cooled unit.
What size generator do I need for my air conditioner?
The AC is usually the biggest load and surges hard on startup, so the generator must handle that surge. It often sets the whole size, so have it calculated for your specific system.
Can a whole house generator be too big?
Yes. Too large a unit costs more upfront and to install and runs inefficiently at light load. Aim for one that comfortably covers your peak demand with a little headroom, not far beyond it.
How do I calculate the generator size I need?
List what you want to run, add the running watts, allow for the biggest startup surge, and match it to a unit with headroom. For accuracy, have an installer run a load calculation.
