I have an Onan 45KW GGFC on propane that I installed at my old house, and then brought with me when I moved into my new house 2 years ago. Random thoughts:
Be careful comparing the specs. The Onan I have is rated as being able to take the full load in one step, and the 45KW rating is a continuous rating at max temp which is over 110 F. Some of the Generacs have a rating that is a peak rating for some amount of time, and then they derate to a lower rating. That's sorta OK for house hold use, since you need some cushion for starting compressors on air conditioning and the like. But you need to know what the long term output of the generator is, and check for derating based on ambient temperature. You'd hate to get a 15KW, find that it's good for 12KW, but derates to 10.5 KW when it's 102 degrees outside. Generac is OK, just focus on what the continuous power output on a hot day is.
Aluminum housing sounds good, my housing has rust problems starting and I try to keep it clean.
In the Onan line, and many others, you may be able to choose what the control panel at the generator looks like. I have the one that has lots of lights on it that tell you why it's unhappy. These are often called the 12-light NFPA 110 systems.
After Hurricane Fran in NC in 1996, I decided to get the generator. It took a year for prices to come down. I installed it myself because the savings allowed me to by a
B21 TLB. Two toys for the price of one! The unit has been great. Have small outages each year, 15 min to 2 hours but then get "the big ones" in terms of wind damage or ice storm. I was in Florida once when an ice storm took out everything around here for 3 days. The value of calling home and having it all just work ("Hi Honey, it's 75 degrees and sunny here, how's the weather up there") was priceless.
As for sizing, best way is to get an amp meter and measure what your worst case load is for the house. Get a generator that's a bit bigger. At the 1st house, I took the resistive electric back-up for the air heat pump from one HVAC unit (a 10 KW strip) and moved it to breaker box I put in before the transfer switch. That let me "get by" with a 45 KW generator. It was an all electric house and it was cheaper to buy more generator than to start changing out appliances. If your house is predominately gas appliances, the generator gets much smaller (7KW for basics, 15-20 KW if you want air conditioning).
New house has geothermal heat, I only kept one 10KW strip on the generator for the master bedroom area. I can fire up the oven if I have an HVAC failure for the main part of the house, and my work area has a small oven I put in in case that system goes. The key focus here is look at your total installed cost. It may turn out that more generator is cheaper than lots of panel box re-wiring or new apliances. And consider installing a new panel box before the transfer switch to balance the generator size. This also lets you add stuff later (like welder circuits) without messing things up because your generator is undersized. Be sure to put the block heater for the generator on that circuit.
As crazy as it sounds, sometimes the cheapest part of putting in a generator system is the generator. All the work to get a pad put in, conduit run, transfer switch install, etc. may be the dominant cost. So just look at the total picture.
If you're not doing 100% automatic whole house backup (like George2615) you can pick and chose via breakers in your breaker box. Square-D makes a mechanical interlock system that makes the back feeding of the box safe. Don't count on being smart enough to throw the right breakers no matter how they are labeled. When it all goes dark, you're cranked and make mistakes. These will at a minimum pop your generator and at a maximum kill someone. The sizing process is the same- use a meter to measure the current of the stuff you want to power and then you're not guessing. Rule of thumb: Double the current draw of any motor (pump or compressor) and remember it. Add the single largest draw to the constant draw amount. That way, when the motor kicks in you're OK. This is where you also have to look a the Generac ratings carefully. The "Peak Power" vs. "average power" works for you in those cases. Note also that there are manual 200 amp transfer switches in the sub $500 range that work here to. Kick off the breakers for circuits you can't power, then turn on your generator. If you are not electrically inclined, work with an electrician.
Seems I can never make a short post to TBN...
