Link provided above will give you all the information you need. We just had an emergency generator installed a few months ago. I was hoping we would lose power so we could test it out! Went with a Generac 18kw, which does the entire house and kicks in automatically when power goes out. A few weeks ago we lost power for a minute and it kicked in after about 5 seconds of the power being out and shut off immediately when power came back on.
Only issue is if you have natural gas or propane piped to your house then they can hook the generator up to that. If not then you need to get a propane tank installed, which adds to the total cost. And you need to decide if you want the tank buried or above ground (the tanks are much bigger than the generator itself). Buried costs more. We ended up getting a 500 gallon tank that we put in the woods behind our house so you can't see it at all. They used to let you rent the tanks with a stipulation that you buy the propane from the same company that provided the tank, but now they said you need to buy the tank so you can fill up with whoever you want.
Generator installed was about $9500. Then another couple thousand for the tank and to dig the trench from the tank to the generator. Then obviously the cost of 500 gallons of propane.
One of the generator installers we talked to was Chamberlin Electric, from Hudson, NH. Pick out 2 or 3 places and have them come to your house and they'll assess how big of a generator you need and give you a proposal. Good luck.
ChamberlinElectric.com