While it's been discussed here before, I'll try to make a summary that will help you out.

First of all, you need to know what kinds of loads you're going to need to power. This will be your primary concern. If you want to run your whole house, with air conditioning and all the lights and all that, then you'll need a fairly large "backup" system. These are big, permanently installed generators that often use propane or natual gas as a power source. These are expensive, costing anywhere from $3,000 for a smaller system up to $20,000 for a large system. They have automatic "cutover" switches that fire up the generator a few seconds after a power failure. They have a decent run time, meaning that if you have natural gas, it will run as long as the gas is flowing, and if you have propane, you can get a few days out of a large tank.

On the other hand if you want to run only a few "critical" loads, you'll probably do fine with a portable generator that you can connect to your house as needed. for example, in my house, I have a well, a septic pump, a fridge and a freezer as well as an oil furnace. These, plus a light or two, are my "critical" loads, so what I did was go to all of the devices and figure out how much power they needed to run.

This isn't too hard. You can do it the easy way, which is to count up the amps on all the breakers you want to power and multiply by 120, then multiply by 0.60 to get the approximate size generator you'll need, as rated in watts. This assumes that all your motors won't be starting up at once - which they probobly won't.

For example, my well pump is on two twenty amp breakers.
My septic pump is on a 15 amp breaker
My fridge is on a 15 amp breaker
My freezer is on a 15 amp breaker
The lighting I want to use is on a 15 amp breaker.

So I add the amps - 20+20+15+15+15+15 = 100
100 * 120 = 12,000
12,000 * 0.60 = 7,200

So, it looks like I'll need a 7,200 watt generator or so. There are 7,000 watt generators, but it's not a good idea to go LOWER than your estimated load - always go a bit HIGHER if you can, or manage your loads so that you don't have the well turned on when the fridge is on. Actually, this is what I do, because our generator is rated for a constant 3,500 watts total, but I use a transfer switch and manage what's on at any given time. Works fine.

If you hook up a generator to your electric system in the house you MUST install a transfer switch.