DA40 pilot reported an alternator failure that failed their Garmin 1000 resulted in the pilot's loss of radio and transponder ability to communicate.

Narrative

The training flight was 2.5 hours. I left ZZZ1 to return to ZZZ. I flew at 2000 feet because I planned to contact ZZZ to approach from the South below the ZZZ2 Class C Airspace. As I flew toward ZZZ I got a Low Voltage warning. I pressed the Engine button on the G1000 Multi Function Display to check the instruments. After reviewing them I realized the fuel quantity gauges indicated the same fuel quantity; 15 gallons and 15 gallons; since the beginning of the flight. The battery voltage reduced from 24 volts to 19.5 volts. I attempted to contact ZZZ Tower on XXX.X as the MFD (Multi-Function Display) showed data warnings and red X's on all the instruments while the map feature on the left screen went blank. My radios quickly became inoperative. I changed my heading to avoid the ZZZ Class D Airspace while I turned off the Master Switch to stop draining the battery in an attempt to get one last radio call out before entering the airspace. I followed Visual Navigation Aids to fly from the South of ZZZ to a visual checkpoint that is 10 miles Northeast of ZZZ. I began holding over the checkpoint as I sent text messages to my Chief Flight Instructor and the Flight School Owner to inform them I had no instruments or radios due to a battery or alternator failure. The DA 40 has a back-up Attitude Indicator; Airspeed Indicator; Altimeter; and Magnetic Compass available with an electrical failure. Fuel flow; fuel quantity; oil pressure; oil temperature; and CHT (Cylinder Head Temperature) readings were not available with the data warning message. I had a brief phone call with the flight school owner to let him know I was requesting priority handling and asked him to contact ZZZ Tower. Due to the loud feedback over the radio caused by the avionics failure; I could not hear his response. My position over the checkpoint made the closest airport ZZZ. I turned on the Master Switch and made a call to ZZZ Tower on XXX.X and requested priority handling. The G1000 showed data warnings over the map page and the MFD showed red X's over the instruments. I tried to input XXXX into the Transponder and hit Enter. The system failure made it difficult to determine if the Transponder input was accepted or if the radio call was received. I flew over Interstate X for the Visual Navigation Aid to enter a Right Base for Runway XX at 2000 feet to fly above the pattern altitude for ZZZ and to stay below the Class C Airspace of ZZZ2. I overflew the field wagging my wings while looking at ZZZ Tower for Light Signals. I did not see any other aircraft in the haze so I entered the pattern and executed a No Flap Landing.The cause was an alternator failure. The G1000 demands a significant amount of power and the battery drained quickly during my return flight. After the battery voltage went below 19.5 volts the system failed. I was unable to monitor any instruments for Manifold Pressure; Tachometer; Oil Pressure; Oil Temperature; Cylinder Head Temperature; Fuel Pressure; Fuel Quantity; Ammeter; or Voltmeter. I only had the Pitot Static Airspeed Indicator and Altimeter; the Magnetic Compass; and the back up Attitude Indicator.To prevent any such incident from occurring again I purchased a portable comm radio. The lack of communication limited my options. I calculated fuel consumption during an unusual situation with excessive radio feedback and aircraft performance uncertainty. The proper radio calls would have allowed for communication for proper separation and aircraft avoidance from the Tower of ZZZ.