Here's my $0.02 for anybody that wants to read it...
First of all, you need to map out the circuits in your house. Determine which circuit runs what light, plug, appliance, etc.
Second, determine what lights, plugs, appliances, etc are critical if you lost shore power. Consider both summer and winter power needs. Probably want HVAC, water heater, fridges, freezers, some lights, and some plugs. Probably don't need to run the washer, dryer, dishwasher, etc. You'll probably learn during this step that your house wasn't wired with much consideration for running on generator power.
Third, determine the power consumption for each of those circuits. The easiest way to do that is multiply the amperage rating of the breaker on the circuit by the voltage. A single pole breaker with a 15 amp rating is generally capable of 1,800 watts (15 amps x 120 volts = 1,800 watts). A double pole breaker with a 50 amp rating is general capable of 12,000 watts (50 amps x 240 volts = 12,000 watts).
Fourth, add up all the watts you calculated in step 3. That's the size of generator you'll need to be able to run ALL of those circuits completely loaded ALL at the same time.
***You probably don't NEED to run all of the circuits all at the same time on your generator.
Once you figure out your needs, you'll need to decide how you want to hook it up. Some permanent gensets are capable of sensing a power outage and starting up on their own. Some must be started manually. Some, such as portable generators, must be positioned and plugged in through a power inlet. Some have a transfer switch that only feeds a fixed number of circuits. Some can be hooked up with a mousetrap style interlock that backfeeds the panel.
No matter what you do, you ABSOLUTELY MUST make sure you don't backfeed power onto the grid. You run the risk of injuring someone who might be working on a down line. If you're powering your house from a generator, you must disconnect the house from shore power.
My personal setup at my house is a portable generator with a power inlet. I have an interlock that keeps the generator from feeding power to the panel unless the main disconnect is off. I mapped my whole house and determined which circuits are critical. It takes some knowledge of how the system works, but I can run basically everything in my house EXCEPT the emergency heat strips for my heat pump, the oven, and the dryer. My whole setup was $850.
