Start at your outdoor HVAC unit, not your square footage. The compressor out there is the largest single electrical load in most North Alabama homes, and the current it draws in the instant it starts — the locked-rotor amps printed right on its nameplate — anchors the whole calculation. A home with a 4-ton unit usually lands in the 22kW-and-up class once you add whatever else is running at that moment — Generac rates its 22kW to start a compressor as large as five tons. Everything in sizing and fuel supply builds outward from that one number.
Here’s the method, in the order a real sizing visit runs it.
Why does sizing start at the air conditioner?
Because motors start hard. A resistive load like a toaster draws the same current from the first millisecond. A compressor motor doesn’t — it draws a surge of current before the rotor turns, then settles way down once it’s spinning. Your generator has to ride out that surge without dropping the rest of the house.
Finding your unit’s size takes two minutes. The data plate lists cooling capacity in BTU per hour; one ton equals 12,000 BTU per hour, so a 48,000 BTU/h unit is 4 tons. Bigger tonnage, bigger compressor, bigger starting surge. That’s the chain that makes tonnage — not square footage — the first sizing input.
What are locked-rotor amps, and why do they run the math?
Locked-rotor amps (LRA) is the current the compressor draws at the moment of start, while the rotor is still effectively locked in place. It’s on the same nameplate, usually a startlingly large number next to the modest running amps.
The arithmetic is plain: watts equal volts times amps, and your compressor starts across a 240V circuit. Multiply the LRA by 240 and you’re staring at the real reason generator sizing exists. The unit you buy has to produce that starting surge on top of whatever the house is already drawing — refrigerator, lights, furnace blower, the well pump if you’re out in the county.
Two honest caveats. Generators handle brief motor-start surges differently than continuous load, and manufacturers publish their own start ratings — Generac, for one, rates its 22kW and 24kW units to start a five-ton compressor. And soft-start kits can shrink the spike enough to matter. Neither caveat changes the method — read the nameplate first.
Essential circuits, whole home, or the middle path?
Once you know your starting load, you’re choosing what the generator carries:
- Essential circuits. Refrigeration, lights, furnace blower, one or two more. Smallest unit, real trade-offs, and you pick your compromises in advance.
- Whole home, no management. Everything runs whenever it wants. Simple — and the option most likely to collide with your gas meter’s capacity in-city.
- The middle path. A mid-size unit with a load-managed setup that pauses the water heater or dryer for a few minutes when demand peaks. Whole-house coverage without whole-house peak capacity.
Most homes that walk in asking for the biggest unit walk out with option three. Not all — if someone in your house runs powered medical equipment, buy the headroom and skip the management.
What should a sizing visit actually measure?
If a salesperson quotes you a size without stepping outside, that’s not a sizing visit. A real one reads:
- The HVAC nameplate — tonnage and LRA, both units if you have two systems
- The panel — what’s on it, what’s electric vs. gas, any 240V surprises (well pump, shop equipment, EV charger)
- Your simultaneous-use reality — what actually runs together on a July afternoon or a January night
- The fuel supply — meter size in the city, tank size in the county, or which fuel fits your address if you’re on the edge of both
Then the result gets checked against the kW-by-tonnage chart — and if it keeps landing near the top of the range, read whether a 22kW actually covers a whole house before anyone talks you up a tier.