I've been inspired by this thread to think seriously about setting up a similar system for my own home. I have two machines - my firewall and internal web server - that use Via EPIA motherboards and cases which take 12VDC and have external 12V-4.5A 'laptop'-style power supplies to supply it. These would be easy to hook up to a DC UPS - I'm pretty sure that the internal power supply boards in the cases take care of minor voltage and amperage fluctuations that you might see from a SLA battery.

My main thought was to add a set of solar panels to the mixture and to try and wean the system off drawing home power altogether. Pulling numbers out of the air, I want a decent three days' worth of supply from the battery with no mains and no solar power, and ideally the system should run completely from batteries drawing no power from the mains at all. This means installing a sizeable quantity of batteries and solar panels - the charging input total has to be over twice the draw per day, which is (off the top of my head and adding a bit of fudge factor, 12V*10A*24Hr = 2880WHr). The grand plan is for the system to also power the media centre PC, which would have its own modified square-wave inverter. This would easily add 4800WHr and therefore will be quite expensive, so it's in the distant future. But the roof has little shade, and here in semi-outback Australia we get plenty of sun...

My plan is to start small - just get a battery or two and arrange them so that the machines have a small battery back-up. Include a circuit that detects whether battery power is going to fail (lack of mains and voltage under e.g. 10V) and notifies the machines so that they can shut down gracefully. Continue to add batteries to extend the battery-uptime life. Gradually add solar panels as money permits (through a solar panel controller) to supplant the mains backup until the entire system is solar powered.

A couple of questions to the electrical boffins here:

• Can anyone point me in the direction of a simple circuit or instructions on how to make a detector that will send a signal to shut down the machines should the battery pack be too low on charge to keep going?
  
• Would there be any problem with simply plugging the battery directly into the machines? Can you see any problem with battery chargers or other devices that might play with the charge going into the batteries? Do I need some sort of output isolation or voltage regulation circuit?
  
• Jaycar has an integrated device that will take the input from a wide variety of solar panels and charge a wide variety of batteries. Can you see any problems with feeding this device 12V from a mains converter (e.g. the two I have already powering the machines) to simplify the system?
  
• Any other tips, tricks and traps? Anyone else built a hybrid system like this?
  

Thanks in advance,

Paul