Sizing a standby generator means matching its output in kilowatts to two things at once: the electrical load your house actually draws, and the fuel supply that has to feed the engine. Most sizing advice stops at the first one. Around Huntsville, that’s a mistake — whether your gas meter can feed the unit kills more install plans than any load calculation, and out in the county, propane tank size sets your runtime before you’ve counted a single circuit.
Both halves get measured before we talk models. Here’s how each one works.
What does “sizing” a standby generator actually mean?
Every appliance in your house carries two numbers: what it draws while running, and what it draws in the instant it starts. Motors are the troublemakers. Your air conditioner’s compressor pulls its highest current at startup — for a moment, far above its running draw — and the generator has to survive that moment while everything else stays on.
So a correct size covers the biggest realistic starting event stacked on top of the load already running. That’s the whole game. Working it out from your HVAC nameplate takes about ten minutes and beats every square-footage rule of thumb ever printed.
A standby generator connects through an automatic transfer switch — the box that moves your house from grid power to generator power, usually in about ten seconds. The ATS doesn’t change the sizing math, but it’s where coverage decisions get physical: whichever circuits land in it are the ones the generator carries.
Why does fuel supply set the ceiling in North Alabama?
Because the fuel infrastructure here splits the market in two.
Inside Huntsville and Madison, homes run on utility natural gas. A residential meter delivers a fixed maximum flow, and your generator shares that supply with the furnace, the water heater, and the range — all of which may be running during the same winter outage. Many meters can’t feed a large generator plus the house without an upgrade {{TODO:verify-spec: Huntsville Utilities meter capacity and upgrade process}}. That’s why the 22kW question so often turns into a meter question.
Out in Harvest, New Market, and Hazel Green, there’s no gas main. You’re on propane, and the tank is the whole fuel conversation: a bigger generator burns through the same tank faster. Size and runtime trade off directly.
If you’re weighing the two fuels themselves — cost per hour of outage, storage, refill logistics — that comparison has its own page.
Should you cover essential circuits or the whole house?
There are three honest answers, not two.
- Essential circuits. A smaller unit carries refrigeration, some lights, the furnace blower, maybe one window unit. Cheapest path, real compromises.
- Whole home. Everything runs, nothing gets picked. Biggest unit, biggest fuel demand — and the option most likely to trip the meter problem above.
- The middle path. A mid-size unit plus load management, which pauses high-draw appliances for a few minutes when demand peaks. Most homes that think they need the biggest unit actually land here.
And a fourth answer nobody selling generators volunteers: none of the above. If your power goes out twice a year for an hour, a portable unit and an interlock kit cover you for a fraction of the cost.
What goes wrong when the size is wrong?
Undersized: the generator stalls or sheds everything the moment the AC tries to start against a loaded panel. You paid five figures to lose power slower.
Oversized: you paid for kilowatts you’ll never call on, you burn more fuel doing it, and in-city you may have paid for a meter upgrade the right-sized unit never needed. Bigger isn’t safer. Measured is safer.
The sizing chart shows how the tiers break out — including which numbers we’ve verified from manufacturer documents so far and which we haven’t. Where we haven’t, the chart says so instead of guessing.
Where do you start?
In the city: find last winter’s gas bill and check your meter before you fall in love with a unit size. In the county: walk out and read the size plate on your propane tank. Either way, the second step is the same — a sizing visit that reads your nameplates instead of estimating your square footage.