What Size Generator Do I Need for My Home?

Most homes between 1,500 and 2,500 square feet need a 16 to 22 kW standby generator, and homes from 2,500 to 4,000 square feet usually need 22 to 26 kW. That range gets a homeowner in Wilmette or Northbrook close when ComEd power drops, but it is not the final answer. Two houses the same size can need two different generators, because the real number depends on what is actually plugged into your panel, not just how many square feet you have.

This guide walks through the sizing chart, then shows you how to check it against your own appliances, your electrical service, and your Nicor or North Shore Gas meter, so you are not guessing when you call for a quote.

What Size Generator Do Most North Shore Homes Need?

Short answer: Start with 16 to 20 kW for a smaller home under 2,000 square feet, 20 to 24 kW for a mid size home up to 3,000 square feet, and 24 to 26 kW or higher for anything larger or all electric. This chart is a starting point. Your appliances decide the final size.

These ranges line up with the standard sizes that Briggs & Stratton and other major generator makers actually build, so the number you land on should match a real, purchasable unit instead of a custom guess.

Why Square Footage Alone Doesn't Tell the Whole Story

Short answer: Square footage estimates panel demand, but says nothing about your heat, cooking, or cooling. A gas heated 2,500 square foot home and an all electric 2,500 square foot home can need generators 8 to 10 kW apart.

Most North Shore homes run on natural gas heat from Nicor Gas or North Shore Gas, with electric central air conditioning. That combination keeps the generator size close to the chart above, since the furnace only needs power for its blower and controls, not to make heat.

An all electric home is a different story. Electric furnaces, water heaters, and ranges each add several thousand watts on their own. A newer all electric build in Northbrook or Glenview can need a 26 kW unit even at 2,500 square feet, the same size where a gas heated older Evanston home gets by on 18 kW.

Central air conditioning is usually the biggest swing factor. A 3 ton unit, common on a 2,000 to 2,500 square foot home, pulls 3,000 to 4,000 running watts and needs 6,000 to 8,000 watts to start. Leave it off the list, and a generator that looked right on paper will trip offline the first hot afternoon it is actually needed.

How to Calculate Your Home's Actual Power Load

Short answer: Add up the running watts of everything you want to power at once, then add the single highest starting watts on top of that total. That number, not your square footage, is what an installer sizes the generator to.

Start with a simple list. Running watts is what a device draws once it is on. Starting watts, also called surge watts, is the extra burst a motor needs for about a second when it first kicks on. Refrigerators, sump pumps, furnace blowers, and air conditioners all have motors, so each one has a starting spike your generator has to absorb, a distinction the Consumer Product Safety Commission's generator guide also walks through.

Add the running watts of everything on your list, then add the starting watts of only your single largest motor, usually the air conditioner. That total is a rough version of what a licensed electrician calculates using National Electrical Code Article 220, the standard reference for sizing electrical service and backup power. The code allows a simplified optional calculation for most single family homes: roughly 3 watts per square foot for lighting and outlets, plus the full nameplate rating of major fixed appliances, with a reduced demand factor above the first 10,000 watts.

One more limit to check: your natural gas meter. Every gas meter has a capacity rating in cubic feet per hour, usually printed right on the meter, and a large standby generator burns more gas per hour than a furnace and water heater combined. If your Nicor Gas or North Shore Gas meter is on the smaller end, a 22 or 26 kW generator may call for a meter upgrade before installation, which your installer coordinates directly with the gas utility.

You do not need to run this math yourself to get a quote. A licensed installer performs the exact version during a site visit, using your actual panel, meter, and appliances instead of estimates. The numbers above just help you sanity check the size an installer gives you.

Two North Shore Homes, Two Different Answers

Short answer: An older two-flat in Evanston and a new construction home in Northbrook can be nearly the same square footage and still need generators 6 to 8 kW apart, because their age, heating system, and electrical service are so different.

Picture an older brick two-flat in Evanston, built in the 1920s, around 2,400 square feet total. It likely has gas heat, window or through-wall air conditioning, and a 100 amp panel that has already been updated once. A generator in the 16 to 18 kW range usually covers a home like this comfortably, since there is no single large central air load competing with everything else.

Now picture a new construction home in Northbrook, also around 2,400 square feet. It likely has whole home central air, a 200 amp panel, a tankless or electric water heater, and more built in kitchen appliances running at once. That home often needs a 22 to 24 kW generator to cover the same square footage the Evanston two-flat covers at 16 kW.

Neither number is wrong. They are both correct answers to different questions, which is why a real site visit matters more than a square footage chart, and why Evanston and Northbrook homes can carry different price tags for what looks like the same size generator. Our Illinois generator permit guide covers the next step once you know your size.

Common Generator Sizes and What They Actually Cover

Short answer: Standby generators come in a handful of standard sizes: 13 to 14 kW, 18 kW, 22 kW, and 26 kW. Picking the standard size just above your calculated load beats chasing an exact custom number.

Both major brands we install, our Briggs & Stratton installs and Generac, build their lineups around these same size steps. A standard size is easier to service, and parts are easier to find years down the road.

Can a Smaller Generator Cover a Bigger Home?

Short answer: Yes, with a load management system. It is a control module that lets a smaller generator run a bigger home by briefly pausing lower priority circuits, like an electric water heater, whenever a bigger appliance like central air kicks on.

Briggs & Stratton's Symphony Choice system is one example on the units we install. It prioritizes up to eight managed circuits, so the generator only powers the ones that matter most at any given moment. That can let an 18 or 22 kW generator cover a home that would otherwise need a full 26 kW unit, for less money upfront.

Load management is not a way to undersize a generator that is genuinely too small for a home. It is a way to right size a generator for occasional heavy loads, like two air conditioning zones that rarely peak together. An installer can tell you during your site visit whether it makes sense for your home, or whether you should just size up.

Why Undersizing Is the Bigger Risk

Short answer: An oversized generator costs more upfront but works fine. An undersized generator can shut itself down mid outage, right when you need it most, or force you to choose which rooms lose power.

Standby generators are built with safety shutoffs that protect the engine from overload. If you ask one to run more than it is rated for, especially when a motor like a central air compressor tries to start, it can trip offline completely rather than run unsafely. That means every circuit loses power at once, not just the one that pushed it over the limit, right in the middle of a storm.

Bumping up to the next standard size, or adding a load management module, is almost always cheaper than living with an undersized system for the next 20 years. Our Illinois generator cost guide breaks down what each size actually costs installed, including the gas line and electrical work that scales with generator size. Once you know your target kW, our Generac vs. Briggs & Stratton comparison can help you pick a brand.

If your home leans toward the larger end of a range, whether from an all electric kitchen, a big sump pump, or a home office that cannot lose internet, it is worth sizing up rather than sizing to the middle of the chart.

Frequently Asked Questions

What size generator do I need for a 2,500 square foot house?

Most 2,500 square foot North Shore homes need a 22 to 24 kW standby generator, though gas heated homes with window air conditioning can sometimes get by on 18 kW, and all electric homes may need 26 kW or a load management system.

Is it better to oversize or undersize a generator?

Oversize. A generator sized slightly larger than your calculated load has headroom for future additions like a home office or an EV charger, while an undersized generator can shut itself down completely during the exact storm it is supposed to cover.

Does square footage alone tell me what size generator I need?

No. Square footage is a useful starting point, but your heating system, air conditioning size, and whether your home is gas or all electric can shift the right generator size by 6 to 10 kW even at the same square footage.

How much does a bigger generator cost compared to a smaller one?

Jumping from an 18 kW to a 22 kW generator typically adds a few thousand dollars installed, mostly in the unit itself, since the gas line and electrical work usually scale only slightly between standard sizes.

Do I need a bigger generator for an all electric home?

Yes, usually. An all electric home with an electric furnace, electric water heater, and electric range needs several thousand more watts than a similar sized gas heated home, often pushing the right size up to 26 kW or into load managed territory even under 2,500 square feet.

Will my gas meter limit what size generator I can install?

It can. Residential gas meters are typically rated between 175 and 275 cubic feet per hour, and a large standby generator burns more gas per hour than a furnace and water heater combined, so Nicor Gas or North Shore Gas may need to upgrade your meter before a 22 or 26 kW unit goes in.

The chart at the top of this guide gets you in the right neighborhood. The appliance list and the two home examples get you closer. The final number comes from a licensed installer looking at your actual panel, meter, and appliances in person. Fill out the form below and we will connect you with one local licensed and insured electrician who can size your system exactly.