Running watts, starting watts, and how sizing actually works
Every generator carries two ratings, and the difference between them is the whole trick of sizing. Running (rated) watts is what the unit can supply continuously. Starting (surge) watts is what it can supply for the few seconds a motor needs to spin up - and motors are greedy at start-up: a refrigerator that runs on about 700 watts can briefly demand around 2,200, and compressors and pumps in general can pull roughly three times their running draw for a moment. Size to running watts alone and the first fridge cycle stalls the generator; size to the sum of every surge at once and you buy twice the machine you need.
The standard method, which this calculator applies, is in between: add up the running watts of everything you want powered at the same time, then add the single largest starting surge among your motor-driven appliances, because with a little discipline you stagger start-ups rather than flipping everything on at once. A 10% headroom factor on top, rounded up to the next 500 watts, gives you a rating to shop with rather than a number to squint at. Space heaters and microwaves are resistive loads - no motor, no surge - which is why adding one raises the running total but not the surge premium.
The wattage figures behind the checklist are conservative planning estimates from common generator sizing charts, not your appliances' nameplates. Before you buy, check the label or manual on your actual refrigerator, sump pump, and furnace: a modern high-efficiency fridge can run under 200 watts while an old garage unit exceeds 800. If your own numbers differ, trust them - the method stays the same, only the inputs change.
Two safety facts belong next to any sizing answer. First, placement: the CPSC says to run a portable generator outdoors only, at least 20 feet from the house, exhaust pointed away - generator exhaust carbon monoxide kills far more people than power outages do, and a CO-shutoff model is worth seeking out. Second, fuel: a generator without stored, stabilized, safely kept fuel is a lawn ornament, and the fuel storage guide covers the shelf-life and quantity rules. For the wider plan the generator slots into, start with power-outage planning.