Buy too small and your generator stalls the moment the air conditioner kicks on. Buy too big and you pay thousands extra for capacity you will never use, burn more fuel every hour it runs, and live with a louder machine for the next twenty years. Sizing is the single most consequential decision in a whole house generator purchase, and it is also the step homeowners rush. Here is the method installers actually use, in five steps you can work through tonight.
Step one: decide what “whole house” means to you
The phrase means three different things, and they lead to very different sizes. Essentials-only coverage keeps refrigeration, heating, well or sump pumps, lights, internet, and a few outlets alive — typically a 7 to 12 kilowatt unit. Managed whole house coverage runs your main panel but uses load management to shed big appliances when demand spikes — usually 14 to 20 kilowatts. True everything-on coverage, including central air and electric appliances running simultaneously without compromise, generally starts around 22 to 26 kilowatts and climbs from there.
Most families land in the middle category once they see the price difference. Be honest about what you need during a real outage: in January you need heat and refrigeration; in July you need cooling. Electric water heaters, dryers, and ovens are luxury loads during an emergency, and designing around them can double your budget without buying much real security.
Step two: list your real loads in watts
Every appliance has two numbers that matter: running watts, what it draws continuously, and starting watts, the brief surge when a motor kicks on. Refrigerators draw roughly 600 to 800 running watts but need double that for a second at startup. A furnace blower runs 600 to 900 watts. A sump pump runs 800 to 1,200 and surges past 2,000. A half-horsepower well pump can surge near 3,000. Lights, routers, and televisions are almost negligible by comparison — a whole house of LED lighting plus electronics might total 500 watts.
The big animal is central air. A three-ton AC unit draws around 3,500 running watts, and its compressor motor can demand 8,000 or more starting watts on a hot afternoon. That single appliance is why undersized generators fail: everything runs beautifully until the thermostat clicks the compressor on and the generator buckles. A soft-start kit on the compressor can cut that surge dramatically, and it is one of the cheapest ways to shrink the generator you need.
Step three: add running watts, then respect the surge
Total the running watts of everything on your keep-on list, then identify the single largest motor start that could coincide with your normal running load. Your generator's rated running capacity must exceed the running total, and its surge capacity must cover the running total plus that worst-case motor start happening at the same time. Then add twenty to twenty-five percent headroom — generators lose capacity as they age, at altitude, and in extreme heat, and a unit loafing at 80 percent load lasts years longer than one pinned at maximum every outage.
Step four: run the numbers through a calculator, then check your panel
Once your worksheet lists real numbers instead of guesses, a generator size calculator turns them into a recommended kilowatt range in seconds, which is a useful sanity check on your arithmetic. Then walk outside and look at your main panel. A 100-amp service points toward the essentials or managed-power range; a 200-amp service opens the full-house options. If you are still unsure exactly how big of a generator you need for your house after running the numbers, any licensed installer will perform a proper load calculation during a quote — but arriving with your own worksheet keeps that conversation honest.
Step five: match the fuel to the size
Fuel choice changes the math. Natural gas from the municipal line is the default where available: it never needs refueling, though gas units produce slightly less power than their propane ratings. Propane requires tank space and a refill plan — a 20-kilowatt unit at half load can burn two to three gallons per hour, so outage duration belongs in your planning. Diesel portable units are efficient but noisy and entirely manual. Size the generator first, then confirm the fuel supply can actually feed it; an oversized generator starved by an undersized gas meter or propane regulator is a classic and expensive installation failure.
Quick reference by home type
As rough starting points that your load list should confirm: a small home or condo with gas heat usually fits 7 to 10 kilowatts; a mid-size home with a well pump and one AC unit typically lands at 14 to 20; a large home with multiple AC units, electric appliances, and everything-on ambitions starts at 22 to 26. Treat square-footage rules of thumb as marketing, not math — two identical floor plans can have wildly different electrical lives depending on heating fuel, well water, and air conditioning.
The cost of guessing wrong
Undersizing shows up at the worst possible moment: the generator that runs everything nicely until the AC compressor starts and the whole system drops offline mid-outage. Repeated overloading shortens engine life and can damage the alternator. Oversizing costs you at purchase, every month in exercise fuel, and in the larger pad, gas meter, and permit that big units require. Twenty minutes with a load worksheet and a calculator eliminates both failure modes — and turns you into the rare customer who walks into an installer's office knowing exactly what they need and roughly what it should cost.