Size a backup generator for your essential loads, including starting surge, and see roughly how much fuel it'll use.
Fuel estimate assumes running near the load factor above, not full rated output — lightly-loaded generators use disproportionately more fuel per kWh delivered.
Further reading: Off-Grid Solar 101: What It Actually Takes to Leave the Grid
Fuel rate assumptions: gasoline ≈ 0.4 L/kWh, diesel ≈ 0.3 L/kWh, propane ≈ 0.5 L/kWh (propane has lower energy density per liter). These are ballpark figures — always check your specific generator's fuel consumption curve, since efficiency varies notably by model and load level.
Buy for whichever is larger — same principle as inverter sizing. A generator that can't cover the surge from a motor starting will stall or trip even if its continuous rating looks adequate on paper.
For most homes, yes for typical outages — see the Off-Grid Calculator. Many people still keep a small generator as backup for the rare multi-day low-sun stretch beyond their battery's autonomy.
Backup generator sizing (or generator backup sizing — same thing) comes down to two numbers: your continuous running load with a safety margin, and your surge load accounting for the largest motor's startup spike. Whichever of the two is bigger sets the generator size you actually need to buy — that's exactly what the calculator above computes from your appliance list.