I have the following:
8kw
5kw
1.6kw inverter
Bought in that order.
I had a 4kw and it would not start my well so I sold it. Of course the 1.6 kw will not either.
The 8kw is the main one for heavy load conditions. I bought the 5kw after realizing that the septic tank pump was even more important than the well. After all, who wants nasty brown fluid backing up into their house. This happened during hurricane Isabel, but just into the garage sink. Rain water was leaking into the septic tank.
The 5kw uses about 1/2 gallons of gas an hour and has a 6 gallon tank. The 8kw uses about 2/3 of a gallon an hour and has a 4.5 gallon tank. I tended to use the 5kw most and it will start the well if there are few other loads. It got run a lot.
Since gas can be difficult to get when the roads are blocked with trees wrapped in power lines and the gas stations are without power, I tend to try to conserve gasoline. Hence the recent purchase of the 1.6kw inverter generator. It uses a gallon in 9 hours at 25% load. I have found that the freezers and refrigators need to be run at least 12 hours a day and it is better to run them more. My plan is to use the 1.6 kw all night (it is very quiet) just for the frig and freezer. Also I have a solar hot water heater. It needs 40 watts from about 11am to 5 pm. I usually did not run the generators in the afternoon, but with the solar heater I need to. It seems a shame to use a big generator and 0.5 gallons an hour for 40 watts. If I am short of gasoline I would run the inverter most of the time and use the other generators to charge the well and drain the septic tank. I can probably get by on 2.5 gallons a day with a 20 hour run time. This sure beats 10 gallons per day.