I build a box put a car radio and speakers in it and a PC PSU to power it, ID like to rebuilt it and put 2 PC PSUs in it to run a car stereo and amp but I would also like to attach a pair battery cables so I can run it off a battery when I'm outside(farm) or powers out. Im not sure how to switch the power input, I don't want to backfeed and the PSUs won't be hooked together so I'd need to switch both inputs to the stereo and amp. Is this a switch or something I can get? Any ideas help

This many be long and rambling, but humor the new kid on the block, please. We have a 12x20 building at our place mid-way between our home and barn that my wife and I are going to turn into an office for her, and also as a guest "cabin" for when we have company. Our house and barn have 200 amp service on both, and my question is if I add a tankless water heater, air conditioner, heater, lights, etc in the cabin, at peak operation this will likely pull more than 100 amps so what is the best way to proceed with wiring it without putting strain on either the house or the barn? I don't want to be welding in my shop or have the water heater running there when the one in the cabin kicks in and burn something up, or down. Any advice will be most appreciated... I'm a farmer and a schoolteacher trying to find a safe way forward. Thanks in advance.