So I have a cabin up in the hills that has a solar system. When we bought the place there were no batteries. The cost to use IREA electricity is only around $26/month. I would like to install a hot tub outside and would like to run it off solar. First question is how do I determine the output of the panels and approximately how many batteries would be needed to accomplish this? Do solar systems switch back and forth between solar and line electricity when the batteries are running low?

Thanks
Lurch