Running watts and starting watts are two different numbers
Every generator carries two ratings. Running watts (sometimes called rated or continuous watts) is what it can supply indefinitely. Starting watts (surge or peak) is a much larger figure it can hold for only a second or two. You need to satisfy both, because anything with an electric motor — a fridge compressor, a well pump, a sump pump, a furnace blower — draws a big inrush current the instant it kicks on, commonly two to three times its running draw.
The method is simple. Add the running watts of everything you want energised at the same time to get your continuous load. Then take the single largest motor's starting watts and add the difference between its surge and its running draw on top of that total, since only one motor realistically starts at any given instant. Finally add spare capacity, typically around 20%, so the generator is not running flat out for days: an engine held at full load burns more fuel per watt, runs hotter and wears faster. That final figure, divided by 1,000, is the kilowatt size to shop for.
Two practical notes. Nameplate watts are often printed on the appliance itself or in its manual, and using those beats guessing. And if you plan to back up a central air conditioner, a well pump or an electric range, size on those first — they dominate everything else on the list, and a soft starter on a large motor can meaningfully cut the surge you have to design around.
Estimates for planning only. Never back-feed a generator into a wall outlet: connecting a generator to house wiring requires a properly installed transfer switch or interlock, fitted by a licensed electrician to local code. Generators must be run outdoors, well away from windows and doors, because of carbon monoxide risk.