Until battery systems decrease in cost (if they ever do), I will use the grid as my battery. So it probably does make sense to use grid electricity when the sun is not sufficient to power the pump. You would have to run the numbers on how often the generator would run and how much it costs for fuel. I currently have a backup generator, but I have never used it as the power has never gone out.Good luck on your project. Do you know an electrician who can help you with the transfer switches?
My 24 255W panels have a combined output of 6120 watts. I regularly reach 6100 watts on a sunny but cold day. The voltage drop so far has been minimal. The larger the cable, the lower the drop. My 4 gauge aluminum wire is working great. I was worried once that I would break the cable when I was pulling it. But I got through that and have had no problems. If you use aluminum cable, just be careful working with it and be sure to use the antioxidant paste on it.