This Aprilaire programmable thermostat uses an internal disposable battery for its primary power. Alternatively, power can be supplied using a C wire (24 VAC). In this case, if a battery is used, the battery will serve as backup power. A "C" wire is not required for most installations (see website for details). It will work with a 2 wire system. There is no home/away feature on this model. A vacation feature provides the ability to override the regular schedule and setback the thermostat during a vacation period.