What Size Generator Do I Need?

The short answer: most homes need a 16 to 22 kW generator. But that range is wide enough to mean a $3,000 difference in price, so getting it right matters.

The slightly longer answer depends on three things: what you want to power, where you live, and whether you care about running your AC during an outage. Spoiler: you probably do, especially in Florida or Texas.

This guide walks you through the exact math, gives you a cheat sheet by home size, and points out the one mistake that costs homeowners the most money. There's also a free calculator below if you just want the quick answer.

"Essential circuits" means the basics: fridge, lights, a few outlets, your internet, and maybe your heating system. "Whole house" means everything stays on: central AC, electric dryer, oven, the works. The price gap between these two options is usually $2,000–$4,000, and most of that difference is the generator itself, not the installation. Most popular sizes come from Generac, which dominates the residential market. For a deeper dive into pricing, see our installation cost guide.

If you live anywhere south of Virginia, you want whole-house coverage. Losing AC in July isn't just uncomfortable. It's a health risk, especially for kids and elderly family members. According to the U.S. Department of Energy, major power disruption events have increased by over 60% in the last decade. And if you're spending $8,000+ on a generator install anyway, the marginal cost to go from "some power" to "full power" is worth it.

Start with the non-negotiables: refrigerator, lights, phone chargers, Wi-Fi router. Then ask yourself — can I live without AC for 48 hours? If the answer is no (and for most people, it is), add your HVAC system to the list. That alone jumps you from 14 to 20+ kW. It's the single biggest factor in sizing.

Don't forget sump pumps if you have a basement. Medical equipment too. A CPAP machine only draws 400 watts, but you absolutely cannot lose it during a storm. And if you work from home, your office setup (computer, monitors, router) is probably non-negotiable too.

Here's a detail most guides skip: starting watts matter more than running watts for sizing. When your AC compressor kicks on, it draws almost double its running wattage for a few seconds. Your generator has to handle that spike without tripping. That's why step 3 exists.

Take your total running wattage, add the largest single starting wattage spike (usually your AC), then multiply the whole thing by 1.25. That 25% buffer isn't optional. It protects against simultaneous startup surges and gives your generator room to breathe. Generators that run consistently above 80% load get loud, burn through fuel faster, and don't last as long. Consumer Reports recommends this same 25% safety buffer in their standby generator buying guide, noting that undersized units are the number one source of homeowner dissatisfaction.

Most electricians will tell you to go one size up from your calculation. They're right, but not for the reason you think. It's not about safety margins. It's because generators are loudest and least efficient when running at 90%+ capacity. Drop to 70-80% load and the difference in noise is night and day.

Generators come in standard sizes: 7.5, 10, 14, 16, 20, 22, 24, and 26 kW for air-cooled units. Above 26 kW, you're looking at liquid-cooled models (Generac Protector series, Kohler 30-48 kW), which cost significantly more and require more space.

Always round UP, not down. A 22 kW generator running your 20 kW load at 90% capacity will be louder and burns through fuel faster. It won't last as long either. A 24 kW unit running the same load at 83% is a much better deal. You're looking at maybe $600 more for the bigger unit. On a ten grand project? Just do it.

The most common mistake isn't buying too big. It's buying too small.

Here's how it happens: you get three quotes. One installer recommends a 22 kW Generac for $12,000. Another says you can get away with a 14 kW unit for $8,000. You pick the cheaper one because, reasonably, $4,000 is real money.

Then hurricane season hits. The power goes out. Your 14 kW generator kicks in and powers your fridge, your lights, your Wi-Fi. Great. But it's 94°F outside and your AC won't start because the starting surge alone exceeds what your generator can handle.

Now you're sitting in a dark-ish house with an $8,000 generator humming outside and no air conditioning. And the cost to swap it out for a bigger unit? More than the $4,000 you saved.

Skip the 7.5 kW units too. They sound affordable, but you'll regret it the first time your AC kicks in and the generator struggles. If you're torn between two sizes, go bigger. The price difference between a 20 kW and a 22 kW Generac is roughly $500-$800 for the unit. On a $10,000+ project, that's a rounding error. Not sure which brand to pick? Read our Generac vs Kohler comparison.

Simple version: air-cooled generators cover the 7.5 to 26 kW range. They're cheaper and way easier to maintain. They're also smaller, which matters when your unit sits three feet from a bedroom window. Good enough for about 90% of homes. You can browse Generac's full air-cooled lineup on their official site.

Liquid-cooled generators start at 25 kW and go up to 150 kW. They run quieter, last longer under heavy use, and handle sustained loads better. But they cost 40-60% more and need more space for installation.

If your calculation came out under 26 kW, go air-cooled. Period. You don't need the extra cost and complexity. If you're above 26 kW, you have no choice because air-cooled units top out there. And if you're right at 24-26 kW and thinking about liquid-cooled "just in case," save your money. A properly sized air-cooled unit will serve you well for 10-15 years.

Not directly, but there's a nuance that catches people off guard.

Natural gas generators produce about 10% less power than their propane equivalents at the same engine size. So a 22 kW generator on propane might only deliver 20 kW on natural gas. Most manufacturers rate their units for both fuels. Check the spec sheet for the "natural gas" rating specifically. Don't just look at the headline number.

If you're on natural gas (most suburban homes), make sure your gas meter can handle the additional load. Your installer should verify this, but it's worth asking about. A generator pulling 200,000+ BTU/hour can overwhelm a standard residential gas meter. Upgrading the meter is usually free from the gas company, but it takes 2-4 weeks. Plan ahead. The National Fire Protection Association (NFPA 37) also requires standby generators to be installed at least 5 feet from any openable window or door, which is another factor your installer will account for.