Hi,
Puny on the scale of things usually discussed here, but I was frustrated with power from my off-grid system going to waste (and no 12V-input grid-tie inverter to dump excess to grid in the obvious way).
So I put in a little code to control one of the digital outputs of the k8055 USB board from my off-grid power SheevaPlug that shifts my ADSL Internet modem from mains to off-grid when the battery is very full.
It's only about 8W of load (with a very inefficient ~1W of relay drive when off-grid) but it looks like it's taking about 0.1kWh/day load from the mains, including the evening peak FWIW, and leaving the battery state fairly stable.
http://www.earth.org.uk/off-grid-stats.htmlI only switch off-grid when the battery voltage hits 14V and stays high for more than an hour (to get a decent charge), and switches back to mains when battery voltage drops below 12.7V.
Rgds
Damon