With all due respect to all the suggestions so far, when it comes to electrical installations if you have to ask how to do it you shouldn't be doing it. If you are going to tie a generator into your grid supplied system you MUST use a transfer switch or you can backfeed the utility line and kill someone!!!! There is also a company that makes an interlock kit for your main panel that will prevent your utility and generator breakers from being on at the same time. It is quite a bit cheaper than a auto or manual transfer switch, but does not work for all panels. Take a look at the amperage on the breaker that goes to your well- probably a 20 or 30 amp 240 volt. If it is a 20 amp you will want a 7500 watt minimum and 10,000 watt if it is a 30 for general rule of thumb. What are you going to run the pto from? A tractor? If so I think you can buy a portable generator in the 7-10K range for a lot less than a pto unit that is more efficient and convenient than running your tractor.