I’ve set enough generators on concrete pads to tell you the number one thing people get wrong before the truck even shows up: they guess. They see a shiny 26 kW unit at the home store, figure bigger is safer, and write a check for two grand more than they needed. Or they go the other way, grab a 10 kW unit, and wonder why the lights dim every time the well pump kicks on.
Sizing isn’t magic. It’s arithmetic plus a little judgment about how you actually live. And the good news is that a typical single-family home lands in a fairly narrow range once you stop trying to power every single circuit at once.
Let me walk you through how I do it on a real job, so you can walk into that showroom knowing your number.
Key takeaways
- Most average homes (2,000-3,000 sq ft) are well served by an 18-24 kW air-cooled standby generator.
- You don’t have to power everything at once. A load-managed 20 kW unit runs more of a home than people expect.
- Your biggest single load is usually central air or a well pump, not your fridge or lights.
- Overbuying costs more upfront, burns more fuel at idle, and rarely buys you anything useful.
Getting the standby generator size for house right starts with a number
Every sizing job comes down to one figure: your peak running load, measured in watts. That’s the most electricity your house draws at any single moment when the things you care about are running together.
Here’s the part people miss. Your home almost never runs everything simultaneously. The dryer, the oven, the AC, and the well pump all firing at the exact same second is rare. So you’re not adding up every nameplate in the house. You’re estimating a realistic worst-case moment on a hot afternoon or a cold night.
A quick way to ballpark: an average 2,000 to 2,500 square foot home with a single central AC unit pulls somewhere between 18,000 and 25,000 running watts during that peak. That’s why the 18-24 kW class exists. It’s built around exactly this kind of house.
Running watts versus starting watts
Anything with a motor, your AC compressor, well pump, or fridge, needs a jolt of extra power to start spinning. That surge, the starting watt draw, can be two to three times the running number for a second or two.
A 4-ton central AC might run at 5,000 watts but demand 15,000 for that startup instant. Your generator has to swallow that spike without stalling. This is the single most common reason an undersized unit “works fine” for months, then browns out the first time two big motors start close together.
A load list you can actually build in ten minutes
Grab a notepad. Walk the house. Write down the big stuff and rough watt figures. You don’t need lab precision here, just honest estimates.
| Appliance | Running watts | Starting watts |
|---|---|---|
| Central AC (3-4 ton) | 4,000-6,000 | 12,000-18,000 |
| Well pump (1 HP) | 2,000 | 6,000 |
| Electric water heater | 4,500 | 4,500 |
| Refrigerator | 700 | 2,200 |
| Sump pump | 1,000 | 2,500 |
| Furnace blower | 800 | 2,300 |
| Lights, TV, outlets (whole home) | 2,000-3,000 | 2,000-3,000 |
| Range / oven | 3,000-5,000 | 3,000-5,000 |
Add up the things you truly want running during an outage, then account for the biggest motor’s starting surge on top. On a job last winter in a 2,400 square foot house, we landed at about 19,000 running watts with the AC swapped for a heat pump on backup heat. A 22 kW unit covered it with headroom to spare.
If your electric range and dryer are the only things pushing you into a bigger, pricier generator, ask your installer about load management. Shedding those two loads for a few minutes at a time can drop you down a whole generator size and save real money.
Why load management changed the math
Ten years ago, sizing meant adding up everything and buying a monster. Modern automatic transfer switches with load management flipped that.
These systems watch your generator’s output in real time. When you fire up the oven while the AC’s running, the controller briefly pauses a low-priority load, say the water heater, so the generator never gets overwhelmed. You don’t notice, because a water heater holds temperature for a long stretch.
The practical result: a well-configured 20 kW unit can serve a home that would’ve needed 30 kW under the old add-everything approach. That’s a smaller pad, a smaller gas line, and often a couple thousand dollars saved. If you want the full picture on picking a unit, our guide to sizing a generator for your home goes deeper on the load-shedding setup.
Air-cooled or liquid-cooled for an average house?
For the homes we’re talking about, this one’s usually easy.
Air-cooled: the standard choice
Air-cooled units run from roughly 10 kW up to about 26 kW and cover the vast majority of single-family homes. They’re cheaper, quieter than they used to be, and a normal install runs most homeowners $5,000 to $12,000 all-in, generator plus transfer switch plus labor and permits.
If your house is under 3,500 square feet with one AC system, you’re almost certainly in air-cooled territory.
Liquid-cooled: when you genuinely outgrow the class
Once you’ve got two central AC units, a 5-ton system, electric heat, a pool, and a shop, you may push past 26 kW into liquid-cooled territory (typically 27 kW and up). These are built for continuous heavy duty and cost noticeably more. Most average homes never need one, and I’ll gently talk people out of them when the load list doesn’t justify it.
Sizing and fuel calculations should be confirmed by a licensed installer who can verify your gas meter and service can support the unit. An undersized gas line will starve the engine no matter how big the generator is. The U.S. Department of Energy has a solid primer on home electric systems and backup power worth reading first.
The fuel question nobody thinks about until day three
Size affects fuel burn, and fuel burn matters more than the sticker price during a long outage. A 22 kW generator on natural gas at half load burns roughly 200 cubic feet per hour. On propane, expect around 2 to 3 gallons per hour at that load.
Do the math on a multi-day outage. At $3 a gallon of propane, running heavy for three days can quietly cost $150-$200. That’s another reason not to oversize, since a bigger engine idling to power a small load still drinks more than a right-sized one.
- Total your realistic peak running watts, not every nameplate in the house.
- Add your largest motor’s starting surge on top of that number.
- Match it to the next standby size up (usually 18-24 kW for an average home).
- Ask whether load management can drop you a size.
- Confirm your gas or propane supply can feed the unit you chose.
Your next move before you buy
Spend twenty minutes with that notepad and build your honest load list this week. Once you have a running-watt number and know your biggest motor, you’ll walk into any quote conversation as the informed one in the room, and you’ll spot an oversell instantly.
From there, it’s worth reading up on how the install actually goes, what a standby generator installation looks like day of, and the common mistakes people make buying a standby generator so you don’t repeat them. If you’re still deciding between a permanent unit and something you roll out of the garage, our beginner guide to portable home backup lays out the tradeoffs, and the full whole-house standby generator guide ties it all together.
Get the number first. The rest gets a whole lot simpler once you have it.
