MCS has some good advice. That is a lot of generator for your typical home. A 15KW generator is going to have a 30HP engine and requires a lot of fuel to run it for any length of time. If it is a natural gas powered unit, that probably won't be as big an issue. You might have trouble feeding a large gasoline powered unit if you have an extended power outage in your area. Most gas stations don't have generator power and can't pump gas. I heard of people driving 60 miles to get fuel in the sattle area during the big storme 2 years ago.
In the case of backup power, IMO, Less is more, particularly if you are having to haul the fuel for it. Pare down what you absolutely need to power to remain safe and relatively comfortable, and use the online charts to decide what size you actually need.
As for generac, I have worked with some, and they are OK. I have had issues with support from a contract installer, but found excellent support from a larger generac dealer. Probably the biggest trouble with these small size home units occurs after about the 96 hour point during their first real power outtage. Being an air cooled engine, they burn a little more oil than a water cooled engine. After about 4-4 1/2 days, they run low on oil and shut down. You need to be able to check things like this, as during a power failure, the tech support is stretched pretty thin... Onan also markets similar sized units, mostly Honda powered, and they perform pretty well also.