What size standby generator do you actually need?

What size means in practice

Standby generators come in standard sizes. Here’s what each tier actually handles in a real home.

What sizing actually depends on

Starting wattage, not running wattage

AC compressors, well pumps, and big motors pull 2–3× their running wattage at startup. Undersized generators trip on the spike — even if they could run the load steady-state. That’s why this tool adds a 25% safety margin and counts well + AC at their startup figures, not their nameplate.

Cold-weather margin

Northern winters demand 15–25% more from a generator than the spec sheet says. We size for the night the storm rolls through and the temp drops to -10°F — that’s when a marginally-sized generator dies. Climates set to “cold + snowy” or “mountain” get an automatic 20% bump.

Load management changes the math

Modern Generac & Kohler units shed non-critical loads automatically, so you can run a 22 kW where you’d otherwise need 26 kW. Ask your installer what load-management features come standard — it can save you the cost of stepping up one tier.

Fuel type matters

Natural gas runs roughly 10–12% lower kW than propane on the same physical generator. If you’re on LP, this tool factors that in. If you’re switching, get a quote for both.

Generator sizing FAQ

Can I just go by square footage?

No. A 2,000 sq ft all-electric home with a heat pump and EV charger needs more than double the generator of a 2,000 sq ft gas-heated home with no big loads. Square footage is a sanity check, not a calculation. The toggles above matter more than the slider.

What happens if my generator is too small?

Two things, neither good. Either it trips during a high-load moment (AC startup on a 95°F day, well pump kicking on with the dishwasher running) and you lose power mid-outage. Or it runs near 100% capacity continuously, burns through fuel fast, and dies young — generators are designed to run at 50–75% load.

Is bigger always better?

No. An oversized generator costs more upfront, more to install (bigger gas line, bigger transfer switch), and runs less efficiently — generators don’t like running at 20% load any more than they like 100%. Size to your actual loads plus a 25% safety margin, not “biggest available.”

How much does a 22 kW Generac cost installed?

Typical 22 kW whole-home installs run $11,000–$16,000 in 2026 depending on gas line work, transfer switch placement, electrical service distance, and concrete pad. Heavy-load installs (30–36 kW with new gas line + 200A automatic transfer switch) can hit $20,000–$25,000. Always get a written quote that breaks down the load calculation — if your installer can’t show you which appliances they counted, walk.

Generac or Kohler?

Both are solid. Generac dominates the residential market — easier parts and service nationwide. Kohler runs quieter and tends to last longer in heavy-cycle use. If you have or plan to add solar, also weigh a battery-based backup (Tesla Powerwall, Enphase IQ Battery, Sol-Ark) against a generator — quieter, no fuel runs, but typically more $/kWh of backup.

Whole-home or essentials-only?

Depends on what you want. Whole-home (every circuit transfers) is the most popular pick. Essentials-only (sub-panel with the circuits you pick) is cheaper to install but you live with whatever you didn’t backup. Managed whole-home (everything works, but smart load management staggers AC and other big loads) is the modern compromise — that’s the 22–26 kW tier most installers default to.

How long does the generator run during an outage?

As long as the fuel lasts. On natural gas, that’s effectively unlimited — the gas company keeps running through outages. On propane, it depends on tank size: a 500-gallon tank at 50% load runs a 22 kW generator for roughly 4–5 days. Bigger tanks (1,000 gal) double that. Confirm your tank size and fill schedule before deciding between NG and LP.

Continue learning

Generators vs. Batteries · Battery Sizing Tool · Heavy Loads & System Capacity · Electrification Planning