$6-$10k+ depending on your budget and wattage requirements. I personally think that powering every breaker in your home is a waste of $$. Calculate the essentials (well pump, heating, fridges/freezers, sump pump, and main living areas) and be done with it. You don't have to light every room. Our previous home with about 2300sqft we got by just fine with a 7k standby Generac powering the main stuff. Our current home, smaller at 1800 sqft, has an 8k Generac but similarly wired.