Remember bigger is not always better, not only does a bigger generator tend to cost more, it will consume more fuel even when running at no load, and if it is a diesel will develop wet stacking problems if not regularly being loaded down to at least 50% capacity.
First off add up the amp load of the things you want to be able to run at once, most appliances will have a data plate stating running load, this will give you a good rough estimate. Most consumer and military surplus generators are labelled in watts, (industrial units tend to be rated in watts and KVA's both, but lets try to keep this simple), so you will need to convert those amps to watts, for 120V appliances multiple amps by 120 to get watts, for 240V appliances multiply amps by 240 to get watts (this is assuming household 120/240 single phase).
Any generator wired up for 120/240 single phase will have 2 legs of 120V each equal to half the total capacity. So will want to make sure they can be wired up so as not to overload either side, the 240V loads will draw equally from both legs, this is usually not a big problem to balance out on any generator over about 5KW, under about 3 KW and I think it is usually better to get one that is connected to output only 120V on a single leg if possible.
Next decide how much fuel you need to have on hand for an outage, or how much your willing to spend for fuel if you have a natural gas connection. A few years ago there was a guy on another generator message board that had a nice 30KW Onan EK he used it during a nearly two week long outage due to a Hurricane on the gulf coast (I forget which one), it provided power for his house, as well as the houses on each side, and was still running at only a fraction of its capacity. A few weeks later he received his natural gas bill, it was for over $3,000. Again there is no wrong answer here, for me I have little 3KW MEP-701a diesel (think 4-5KW+ for a contractor grade commercial unit, the military uses creative math and is ultra-conservative on their ratings) I can run it for nearly a week at full load off the 80 odd gallons I keep on hand (mostly in the form of a 55 gallon drum in my shed), this will let me run my refrigerator and freezer, plus a pair of window air conditioners (8,000 and 11,000 btu), plus some lights, tv, computer, etc. (I have gas stove, and gas heat, but electric water heater, and dryer so no doing laundry in an outage for me, unless I turn everything else off wash with cold water and maybe run the dryer on low heat) Compare that to the 30 KW generator I installed at my elderly mother's house, she lives in an all electric big house on a farm and wants to be able to live life as normal with no thoughts of load management during a power outage. It is connected to a 100 gallon diesel tank, which will run the generator for about 4 days at 1/4 load (8KW sustained) , or for about a day and a half at full load (which means all the air conditioners running, while stove and oven are on, while running hot water from all 4 electric water heaters, doing laundry and the well pump going). Of course she also has another 1600 gallon diesel tank in the barn for the tractors.
Now that you have made your decision on how much your willing to invest in fuel, go back and add up the loads for what you NEED during an outage, and add on a little more for safety. It is possible to survive somewhat comfortably in an emergency with a very small generator and very active load management, I have done it with a little 1,000 watt honda inverter generator, it will run 3 or 4, 13 watt CF bulbs on lamps plugged into extension cords around the house as well as a couple of small fans, and a small radio for news, plus cycling power between the refrigerator and chest freezer as well as a small 5000 btu window air conditioner every couple of hours, letting each run about 2 hours out of every 6. The nice thing about the little honda is it will run nearly 4 hours on half a gallon of gasoline at full load, and as much as 8 hours at 1/4 load, sure that 1/4 load is only about 250 watts, but with CF lights, and modern low power electronics, that is plenty to light a house, run a laptop computer, tv, etc.
Ike
p.s. when I mentioned refrigerators and chest freezers above, be aware modern ones draw only a fraction of the power of older units. A modern household refrigerator with the door closed may draw less than 200 watts on average (700-800 watts with compressor running, allowing for compressor to run 25% of the time), compare this to a 20 year old refrigerator that may draw closer to 800 watts on average with both a longer duty cycle and higher draw for its compressor, and don't even think about the draw on 30-40 year old refrigerators. The same is true of air conditioner, both window and central units.
