Start with a question, not a number

Sizing isn't really about square footage first — it's about how much of your home you want running during an outage. A household that just wants the fridge, sump pump, and a few lights running needs a much smaller unit than one that wants central air conditioning and the whole kitchen available too.

Three common tiers

Essentials only (roughly 7–10 kW): refrigerator, sump pump, some lighting and outlets, and Wi-Fi.

Most of the home (roughly 14–20 kW): essentials plus most circuits, though possibly not central air at full load.

Whole home (22 kW and up): everything running simultaneously, including HVAC — the closest to not noticing an outage happened.

What actually drives the number

Running (continuous) wattage from everything you want powered adds up first. Then, because motors draw a brief surge when they start, sizing accounts for the single largest motor's starting wattage on top of everything else already running — this is why a home with central air or a well pump often needs more capacity than raw square footage alone would suggest.

Building in a safety margin

Rough sizing estimates typically add a margin — commonly somewhere in the 15–25% range — on top of the calculated starting load, to account for real-world variation in appliance draw, aging components pulling slightly more current than spec, and any future load changes like a hot tub or EV charger added down the road.

It's worth listing out any less-common loads specifically — a home office setup, medical equipment, an extra freezer in the garage — since these are easy to forget in a rough mental estimate but add up in a formal calculation.