What Size Generator for Whole House Backup? | Size By Load

Whole-house backup generators are typically sized 18–24 kW; larger homes with multiple air conditioners usually need 22–28 kW.

For the full breakdown, see our best Whole House Backup Generator guide.

Square footage makes a terrible ruler for generator sizing. You pick the size by the electrical loads that must stay on when the grid goes dark—not by your home’s area. List everything you want running, add the biggest motor surge, and that arithmetic is the actual answer.

A typical U.S. home ends up between 18 kW and 24 kW. Larger homes, or those with two or more air conditioners, usually need 22 kW to 28 kW. The table below gives a starting range; the five-step method in the next section gives the number behind it.

What Size Generator Do Most Homes Need?

Most U.S. homes sit in the 18–24 kW range. Generac puts smaller homes at 10–18 kW and larger homes with multiple air conditioners or high electrical demand at 22–28 kW.

Whole-house doesn’t mean every circuit at full power all at once. Generac rates its 18 kW Guardian for essential-circuit protection and says it reaches whole-house coverage only when paired with load-shedding switches. Its 22 kW Guardian is pitched as whole-house backup for many homes. Match not just the kilowatt number but the load-management setup that goes with it.

Size ranges like these are a starting point, not a substitute for measuring load. Generac’s whole-home size calculator walks through square footage, your household’s electrical usage, and the specific systems needing power during an outage—run it once your load list is finished.

Whole House Backup Generator Sizing: The Five-Step Method

The sizing method is five steps and works the same for every home. Each one changes the final number, so don’t skip ahead to the headroom.

1. Decide what has to run. During a long outage, do you need the well pump, sump pump, heat, and refrigeration, or the entire panel? That decision alone can move the size by several kilowatts.
2. Total the running watts of every load that will run at the same time. HVAC, water heater, refrigerator, pumps, and essential electronics all go on the list.
3. Add the largest starting surge. Electric motors draw far more current in the first seconds than while running, so the biggest air conditioner compressor or pump usually decides the final size.
4. Apply derating for site conditions. Generator output drops with high altitude and extreme heat, so the same home may need extra capacity in Denver that it wouldn’t need at sea level.
5. Add 20–25% headroom and round up to the next standard size. That cushion covers aging motors, added appliances, and voltage dips a tightly sized generator can’t absorb.

If you have a year of electric bills, cross-check your result against the highest monthly demand you actually hit. Some Generac sizing guidance calls for a unit 25% larger than that peak, which catches seasonal loads a mental list tends to forget. Before committing to a model, read our tested whole-house generator roundup to see how each candidate handles surge ratings and load shedding.

One more constraint: whole-house standby is not a portable-generator setup. It needs proper transfer equipment, a permanent fuel supply (usually natural gas or propane), and local code approval. Have a licensed installer confirm the model against your panel capacity and fuel-system sizing before you buy.

Sizing Mistakes That Push The Final Number Up

Most wrong-size purchases come from one of five predictable mistakes:

• Sizing from square footage alone instead of an actual load list.
• Ignoring locked-rotor current or starting watts from large motors.
• Forgetting that some generators need load-shedding switches before they can cover a whole panel.
• Skipping derating for altitude or heat.
• Choosing a generator before the transfer switch, fuel system, and local code requirements are figured out.

Don’t mistake load management for a limitation—it’s how modern whole-house systems keep a smaller unit affordable and still power everything that matters. A system that staggers the well pump, water heater, and HVAC loads can run on a fraction of the capacity a simultaneous-everything design would need.

The upshot: a typical U.S. home can plan on 18–24 kW, and a larger home with several big motors should plan on 22–28 kW. The final number comes from your load list plus the largest surge, not from square footage. Run the five steps, then have a licensed installer verify the result before you spend.