Consider a Schmitt trigger circuit instead of a timer for your auto circuit. Turn off a little above 12 volts and on just below your regulated voltage. A timer wouldn't make up for different loads or environmental conditions.
I was looking at the relays used to turn audio amps on/off... One of those would probably work, although for the type of activity I have in mind (grid computing), I would need a safe shutdown. The simplest solution would probably be amplifier relay > inverter > UPS > computer, using a UPS that reports power status to the computer--shutting it down when inverter power is switched off. I would have to buy a new UPS, as the one I have under my desk has problems with its status reporting feature. This is one of those projects I've been thinking on for a while, and have never actually done anything about.
