CalcCoverageTwo numbers size a generator — not one
Every appliance makes two demands on a generator. The running watts are what it draws once it is going. The starting watts are the brief surge a motor pulls in the first second or two — a fridge, pump or air conditioner can demand two to three times its running watts just to start. Size for running watts alone and the generator stalls the first time a big motor kicks in; size for every surge at once and you buy a machine far bigger than the house needs.
The sizing method used here is the one the manufacturers publish: add up the running watts of everything that will run together, then add the single largest starting surge — motors start one at a time, not in chorus — and buy the first standard size class that covers that peak with 20% headroom on top. The headroom is not padding for its own sake: a generator run flat-out at 100% of rating is loud, thirsty and short-lived, and the next appliance is always added eventually.
The wattages are typicals — the label is the law
The figures in the checklist are the standard published typicals the generator makers themselves size with. They are honest averages, and any individual appliance can sit a third above or below its line — an old fridge draws more than a new one, a 5-ton air conditioner varies by model. Before a purchase, read the data plate on the two or three biggest loads and enter them in the "your own appliance" row; the plate replaces the typical, not the other way round.
| Appliance | Running W | Starting W |
|---|---|---|
| Refrigerator | 180 | 900 |
| Well pump, 1/2 hp | 1,000 | 2,100 |
| Sump pump, 1/3 hp | 800 | 1,300 |
| Central AC, per ton (approx.) | 1,200 | 1,800 |
| Electric water heater | 4,500 | 4,500 (no surge) |
| Electric clothes dryer | 5,400 | 5,400 (no surge) |
Two loads deserve a second look. Electric resistance heat — water heaters, dryers, ranges, baseboard heaters — has no surge at all, but its running draw is enormous, and dropping even one of them from the outage list can move the answer down a whole size class. And central air conditioning is the single biggest swing item in most houses: if the AC can stay off during outages, say so by leaving it unticked, and let the smaller, quieter, cheaper-to-run machine win.