This thread started with regard to an aircraft on battery power.
My understanding (from TRIG) is that their TT21 transponder when pretty busy consumes about 320 milliamps from a 12 volt battery so a dedicated 7A/h 12V battery when new could, theoretically, keep it going for about 20 hours. In practice batteries don't stay new for long and have lower capacity when cold so you may only get 10 hours per charge. Enough for a day's flying.