A C172 pilot shut down his radios to conserve battery enroute to his home base following an alternator failure.

Narrative

The aircraft that I was flying suffered an apparent alternator failure while approximately 20-30 NM north of FDK. The G1000 avionics system automatically switched to the standby battery and the flight resumed without additional problems. A short time thereafter; however; the PFD and MFD screens began to dim. In an effort to preserve battery power while I returned to FDK; I switched off the avionics bus that controls the MFD; thereby also switching off the transponder. As I approached FDK; I turned the avionics bus back on; but the radio began to crackle and I began to lose radio communication. I decided at that point to turn off the avionics bus again so that I could preserve as much battery power as possible so that I could communicate on the FDK CTAF. As a result; I unfortunately did not have a transponder operating within 30 NM of a class B airport. I made this decision as a safety of flight issue by weighing the benefits of having effective radio communication against the failure to use a transponder in class E airspace at low altitude within 30 NM of a class B airport.