Generator Sizing: The Math Nobody Shows You

Walk into a generator conversation with most companies and the sizing happens fast — suspiciously fast. A glance at your house from the driveway, a question about square footage, and a number appears: "You'll want the 24kW." Maybe you will. But square footage doesn't run air conditioners, and driveways don't reveal well pumps. Loads do the running — and loads can be counted. Here's the math a proper installer does, shown the way we'd show it at your kitchen table.

Watts, and the two kinds that matter

Every electrical device draws power measured in watts. Generators are rated in kilowatts — a 20kW unit produces 20,000 watts. So far, simple. The wrinkle that ruins lazy sizing is that motors — air conditioners, well pumps, pool pumps — draw two different amounts: their running watts, and a much larger surge for the second or two when they start. An A/C compressor that hums along at 4,000 watts may demand two to three times that at startup. A generator sized only for running loads stalls the first hot afternoon your A/C and well pump happen to start together.

The counting, step by step

A load calculation lists what the generator must carry and adds honestly:

• The non-negotiables: refrigerator and freezer (500–800W), lighting and plugs for the lived-in rooms, internet equipment, security system. These are modest — a few thousand watts covers them.
• The big steady draws: electric water heater (4,000–4,500W), electric range if you cook during outages (3,000–5,000W in use), dryer if you insist. Gas versions of these? Nearly zero — which is why two identical-looking houses can need very different generators.
• The motors, with their surges: each central A/C unit (3,500–6,000+ W running, more at start), well pump (1,000–2,500W running, high surge), pool equipment (1,500–2,500W). The calculation doesn't just add these — it asks which could start simultaneously and reserves surge headroom for that moment.
• The modern additions: an EV charger is a 7,000–11,500W appliance. Often the right answer is load management — the charger pauses automatically when the generator is near capacity — rather than thousands of watts of extra generator. A good installer offers that option; a lazy one just sells the bigger unit.

Add the running loads you'd use at once, add surge headroom for the worst realistic starting moment, and you have your number — traceable, on paper, specific to your home. These wattage ranges are planning figures; the real calculation uses your equipment's actual nameplate data, which is exactly what a site assessment collects.

Why both sizing mistakes cost you

Undersized is the obvious failure: breakers trip, the unit sheds load or stalls, and the August afternoon it was bought for becomes the afternoon it disappoints. Oversized fails more quietly: you paid thousands extra up front, the unit burns more fuel at every exercise cycle and every outage, and — counterintuitively — a generator loafing far below its intended load doesn't run healthier. Right-sized isn't a compromise between the two mistakes; it's the actual engineering answer.

What this means for your next step

You can rough-count your own loads from the lists above and get within shouting distance of your size — it's a worthwhile exercise, and our free Generator Guide includes the worksheet. The final number should come from a professional assessment: nameplate data, starting characteristics, gas supply sizing (an undersized gas line starves a correctly sized generator), and the transfer-switch design that ties it together.

That assessment is free from us, and it ends with the math on the table — whichever brand and size it points to. Our generator services cover Kohler (as a certified dealer), Generac (as a certified installer), and other major brands, serving Cypress, Katy, Fulshear, Tomball, and The Woodlands.