We also are contemplating power requirements for our hill. We currently have a 3 KVA Honda EU3000is generator, which is ridiculously too powerful as the observatory only uses about 360 watts peak power and less once it's up and running. That wastes a lot of fuel. A big battery (say 12v 650 AH) would be enough to run it for 12 hours of winter astronomy plus some darks, and still leave 60% charge in the battery, which will prevent sulfation and prolong battery life.
We've been studying smart-ish battery monitors. It seems that monitoring the voltage tells you almost nothing about the charge state, due to temperature, recent history, and age and condition of the battery. A smart battery monitor comes for about $300 to $400. The idea is you charge the battery until it is absolutely chokkers, then press a button saying "this is full, mate." It then uses a bank balance approach to monitor amp-hours withdrawn until you get to say 60% charge remaining, and then closes a relay which starts the generator via a standard 2-wire interface. (We've put one of those on the generator as an after-market kit. It pulls the choke and everything.) The generator is now both running the observatory and also recharging the battery. The controller continues to keep a tab on total AH deposited back into the battery until it thinks it is 100% recharged, and kills the generator.
We think that should save so much petrol it will pay for the battery in a couple years.
One could use a much smaller battery but it would mean many more charge-discharge cycles and many more generator cold starts, which would probably wear everything out much faster and not really save money.
The big battery could also accept solar panels. A timer could disable the generator until before 4pm, so as to make use of the sun if there was any. But a huge bank of solar panels would not really beautify the hillside so very much.
