What Size Generator Does Your House Actually Need? (2024 Guide)

TL;DR: A typical 2,000-square-foot home with a well pump, fridge, and furnace blower needs a 7,000 to 10,000-watt generator to cover essentials during an outage. If you want central air conditioning and every circuit running, plan on a 20,000 to 26,000-watt standby unit tied into your electrical panel. The right size depends on your starting (surge) watts, not just running watts, and an electrician can confirm the number with a load calculation for about $150 to $300.

_Last reviewed: July 2026 · 7 min read_

Guessing at generator size wastes money two ways. Buy too small and it trips or burns out the first time your AC compressor kicks on. Buy too big and you're paying for capacity, fuel, and maintenance you'll never use. Here's how to land on the right number for your specific house.

Okoniq Property Hub logs your appliance nameplates, panel amperage, and generator specs in one place, so sizing decisions and maintenance dates don't live in your head or a shoebox of receipts.

How do you calculate the wattage your house actually needs?

Start by adding up the running watts of everything you want powered during an outage, then add the single highest surge watt requirement on top. Running watts is what an appliance draws once it's operating steadily. Surge (or starting) watts is the short burst — often 2 to 3 times higher — that motors need to kick on, like a well pump, refrigerator compressor, or air conditioner.

A rough essentials list looks like this: refrigerator (600 running / 2,200 surge), sump pump (800 / 2,000), furnace blower (800 / 2,400), well pump (1,000 / 2,900), a handful of lights and a phone charger (400 running). Add the running watts together, then add the largest single surge value, not all the surges stacked. That usually lands between 5,000 and 8,000 watts for essentials-only coverage. If your panel is older or undersized, it's worth reviewing 100 vs 200 amp service before you commit to a generator that assumes headroom you don't have.

Should you size for essential circuits or the whole house?

Sizing for essentials only usually costs less upfront and covers what actually matters during a storm: heat, water, refrigeration, and lights. A 7,000 to 10,000-watt portable or small standby unit handles this for most 1,500 to 2,500 square foot homes, and you manually select which circuits run through a transfer switch.

Whole-house sizing means every circuit in your panel stays live, including central air, an electric range, and multiple bathrooms. That pushes you into 20,000 to 26,000-watt territory for a mid-size home, and central AC alone can add 3,500 to 5,000 starting watts depending on tonnage. The trade-off is cost: essential-circuit setups often run $3,000 to $6,000 installed, while whole-house standby systems run $10,000 to $18,000 installed as of 2024, per data from generator installers like Generac and Kohler dealer networks.

| | Essential-Circuit Sizing | Whole-House Sizing | |---|---|---| | Typical wattage | 7,000–10,000W | 20,000–26,000W | | Installed cost | $3,000–$6,000 | $10,000–$18,000 | | Covers central AC | Rarely | Yes | | Panel work needed | Transfer switch only | Full panel integration |

What size does a portable vs. standby generator need to be?

Portable generators typically top out around 10,000-12,000 watts, which is enough for essentials but not whole-house AC. Standby generators, permanently installed and running on natural gas or propane, scale up to 26,000 watts or more and start automatically within 10-30 seconds of an outage.

If your home still has older wiring, sizing the generator is only half the job. A standby unit ties into your panel through a transfer switch, and that connection depends on grounded, code-compliant circuits. If you're unsure what you're working with, signs you still have knob-and-tube wiring is worth a read before an electrician quotes the installation, since old wiring can add cost or delay the project.

How much does an oversized or undersized generator actually cost you?

An undersized generator costs you in nuisance trips, shortened equipment life, and the risk of running nothing when you need it most. Overloading a generator repeatedly can shave years off its lifespan and void the warranty on both the generator and whatever appliance it damaged.

An oversized generator costs you in fuel efficiency and upfront price. A 22,000-watt standby unit run at 30% load burns propane less efficiently than a correctly sized 10,000-watt unit run near 70-80% capacity, and the price gap between those two sizes is commonly $4,000 to $7,000 installed. The sweet spot is sizing for your actual peak load plus a 20% buffer, not the biggest unit a dealer offers.

Should you get a professional load calculation before buying?

Yes, if you're installing a standby generator wired into your panel, an electrician's load calculation is worth the $150 to $300 it typically costs. They'll check your panel's amperage, verify it can handle a transfer switch, and confirm your listed wattage against actual nameplate data rather than rough estimates. This also matters if any of your outlets or circuits are still on 2-prong wiring, since older setups sometimes need updating before a transfer switch can go in safely — see 2-prong vs 3-prong outlets for what that involves.

FAQ

What size generator do I need for a 2,000 square foot house?

Most 2,000-square-foot homes need 7,000 to 10,000 watts for essential circuits, or 20,000 to 22,000 watts if you want whole-house coverage including central AC.

Can a 7500 watt generator run a whole house?

A 7,500-watt generator can run essentials like a fridge, furnace blower, well pump, and lights, but it typically cannot run central air conditioning or an electric range at the same time.

How many watts does it take to run central air conditioning?

A typical 3-ton central AC unit needs about 3,500 to 5,000 starting watts and 1,500 to 3,000 running watts, which is why whole-house generators are usually sized at 20,000 watts or more.

Is it cheaper to buy a bigger generator than I need?

No, oversizing usually costs $4,000 to $7,000 more upfront and burns fuel less efficiently at partial load, so it's cheaper to size to your actual peak wattage plus a 20% buffer.

Do I need an electrician to size a generator?

For a portable generator you plug in manually, no. For any standby generator wired into your electrical panel through a transfer switch, yes, a licensed electrician should confirm the load calculation.

This is educational information, not electrical or safety advice. Call a licensed electrician to confirm your panel capacity and generator sizing before installation.