What Size Generator Do I Need for My House?

How standby generator sizing works — running watts, starting surge and headroom — with typical sizes by home and the one appliance that changes everything.

Generator sizing comes down to one calculation, and it’s simpler than the sales literature makes it look. Once you understand it, you can sanity-check any installer’s recommendation.

The calculation

1. Add the running watts of everything you want on at the same time during an outage.
2. Add the starting surge of the single largest motor. Air conditioners, well pumps and furnace blowers draw two to three times their running wattage for a moment on startup. You only need to cover the biggest one, because they don’t all start at the same instant.
3. Add 20–25% headroom so the generator isn’t running flat out.
4. Round up to the next available size.

The sizing calculator does exactly this with typical wattages for common appliances.

Typical results

The A/C problem

Central air conditioning is the appliance that decides your generator size. A 3-ton unit runs around 3,500 watts but can surge past 10,000 on startup. That single surge is often larger than every other load in the house combined.

Two ways to handle it:

• Size for it. Simplest, most expensive. Push to 22 kW or more.
• Manage it. A load-management transfer switch or a smart A/C module can lock out the A/C until other loads settle, or run only one zone, letting a 14–18 kW generator cover a house that would otherwise need 24 kW. Ask installers about this — it’s frequently the difference between two price tiers.

If you’re honest with yourself about whether you need A/C during a typical outage in your area, you may save several thousand dollars.

Well pumps

The other size-driver, especially in rural homes. A 1 hp well pump runs at about 2,000 W but can surge to 6,000. If you have one, it’s non-negotiable — no pump, no water — so it goes in the calculation as a fixed cost.

Don’t oversize either

A generator running at a small fraction of its rating for long periods is inefficient and, on some units, can cause carbon buildup. The goal is a unit that idles comfortably at your normal outage load with margin for surges, not the biggest one on the truck.

What the installer will do

Before any reputable installer sizes your unit they will do a load calculation on site, per NEC Article 702. Your own calculation is for understanding the quote and asking good questions — not a substitute. If an installer recommends a size far off what the calculator gave you, ask them to walk you through their numbers. Either you’ve missed something, or they’re upselling. What the installed price looks like at each size →

Not sure what size you need? Use the sizing calculator — it takes about a minute.