About 5 years ago, I bought a Generac whole house generator after I lost electric for 6 days in the middle of winter.
The model I purchased is rated at 16KW (propane) and 15KW (natural gas). The difference has to do with the volatility of the fuel source and its effect on engine power (note: this is my layman's explanation for the difference). Since my purchase, it has been needed 3 times including the most recent power outage many experienced in the mid-west due to the high wind damage. I live outside a metropolitan area and power was out from a few hours to 9 days within 5 miles of me. In my case, it ended up being only 3 hours, but the temperature was hot outside and it quickly became uncomfortable.
My unit came with 16 120 volt circuits. It was installed using propane. Later, when a natural gas line was installed in my area, I switched to natural gas. There is a small switch that you can turn to change the generator to run on natural gas.
The owner's manual for the unit I have is
HERE. The manual covers all similar generators as well as my particular model.
It is important to know what circuits you want to maintain in your house when the power goes off. You also need to put things in perspective, i.e. what do you want to maintain if there is a
SHORT power loss and what do you want to maintain if there is
LONG power loss. The two lists will be different. You will need to decide what is more important to you. For example, if the power loss is short term, you might not worry too much about power in your bedroom. If the power loss is long term, you might wish you had electric service in your bedroom if you are used to watching TV there, etc. before going to bed. I watch a lot a TV before going to sleep (broadcast and DVDs) so it was important for me to have power in my bedroom.
All of your 240 volt needs
will use 2 of your available circuits. At the time of my original installation, I need 240 volts for (1) heating - heat pump with propane backup system which also provides my air conditioning; (2) electric water heater; and (3) well pump for water. I chose
NOT to power the (1) clothes dryer and (2) electric stove and oven. I figured short or long term I could do without the clothes dryer. I have one of those small convection ovens so I figured I could do without the oven - short or long term.
If you have a wife and/or kids, she may feel differently about going long-term without a way to dry clothes or cook. My neighbor bought the same generator I did and during the recent outage, his wife complained she could not bake bread, etc.
THE REAL POINT IS YOU HAVE 16 CIRCUITS - CHOOSE WISELY !! And yes, what you want powered will vary slightly if a long term or short term outage is experienced.
OR EVEN TIME OF YEAR !! For example, I decided to add my garage door opener to my list of circuits. I did this because I did not want to have to get out of my car in a snow or ice storm to open the garage door. In the summer, that is not such a big issue with me since I could just park in the drive. But remember, my first extensive outage was in the winter and I was focused on what issues I had during winter weather.
A breakdown of the circuits I chose can be viewed
HERE.
I almost considered taking the heat pump out of my list of 240 circuits because I figured I could just use emergency gas backup function of my system to heat the house (
REMEMBER, MY ORIGINAL MAJOR OUTAGE WAS IN THE DEAD OF WINTER AND THE SYSTEM USES A BLOWER POWERED BY ITS OWN CIRCUIT). Now, that it is summer, I am glad I had A/C and had not taken the heat pump out of my list of covered circuits.
Besides the items I already mentioned - you will want power to your refrigerator and all of your freezers. I actually have 3 freezers (2 large and 1 small). In my area, we are susceptible to water so the
sump pump was important to me also.
Pictures of my entire setup can be viewed
HERE
pps: Sorry,but being an electrician and also having some personal experience with shocks and arc flash scars couldn't let it go without commenting. 
