What size standby generator you need depends first on whether you want whole-home backup or just the essential circuits — the AC, fridge, well or sump pump, and a few outlets — and then on a real load calculation that adds up the running load plus the startup surge of the motors. Sizing off square footage is the guess that either trips the unit when the AC kicks on or costs thousands more than the house needed. Load management can let a smaller unit cover a larger home.
The first decision: whole-home or essentials
Before any number, decide what you need to keep running. Whole-home backup powers the entire panel, central air included, and needs a larger unit. Essential-circuit backup powers only selected circuits — typically the refrigerator, a well or sump pump, some lights and outlets, and often one AC zone — through a smaller unit. For many Houma homes, essential-circuit is the more cost-effective answer; the question is which circuits an outage actually hurts. On a low-lying bayou lot, that's often the sump pump first.
The load calculation
Once you know what you're backing up, the size comes from a load calculation on the actual house: the air-conditioner, water heater, well or sump pump, refrigerator, lights, and outlets, added up with the startup surge of the motors. This is why two same-size homes can need different generators — AC tonnage, whether there's a pump, and what you want covered all change the total. A quote that hands you a generator size without ever asking those questions is guessing.
Motor startup surge
Here's the part square footage can't capture: motors draw several times their running wattage for a moment at startup. The AC compressor, the well pump, and the sump pump all spike when they kick on, then settle. A generator sized only for steady-state wattage will trip the instant one of them starts — which is exactly the failure mode of a unit guessed off square footage. Honest sizing builds in that surge so the generator carries the motor starting, not just running.
Load management: a smaller unit that covers more
You don't always have to oversize the generator to handle everything running at once. Load management — usually built into the transfer switch — temporarily sheds lower-priority loads when a big motor starts, so a smaller unit can carry the priority circuits and still start the AC. For a lot of homes this is the difference between a larger, pricier generator and a smaller one that does the job, which is why we check whether it fits during the load calc.
Getting it right in both directions
Sizing matters in both directions. Undersize it and the unit trips or can't carry what you need; oversize it and you've paid thousands for capacity you'll never use. The load calculation, with surge and load management factored in, is what lands the size where it should be. That's the whole reason we run it first instead of pricing a unit off the house's square footage.
| Approach | Result |
|---|---|
| Load calc + surge + load management | Right-sized; carries what you need without tripping |
| Square-footage rule of thumb | Trips on startup, or oversized and overpriced |
| Whole-home coverage | Larger unit, full panel, higher cost |
| Essential-circuit coverage | Smaller unit, priority circuits, lower cost |
Tell us the house — square footage, AC size, whether you have a well or sump pump, and what you can't lose in an outage — and we'll size it off a real load calc on the phone, then confirm on-site. Related: essential-circuit install and whole-home vs portable generator.