ACFT LOST ELECTRIC PWR. RADIOS AND XPONDER LOST. ACFT MADE PASS BY TWR AND RECEIVED GREEN LIGHT TO LAND.

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RETURNING FROM NE PRACTICE AREA; RPTED 7 MI OUT. AT 5 MI; TWR LOST XPONDER AND ASKED US TO RESET. AFTER RESETTING XPONDER; WE SET UP FOR 45 DEG ENTRY TO L DOWNWIND RWY 4L. ABOUT 3 MI OUT; I REALIZED IT WAS VERY QUIET FOR A SATURDAY MORNING TRAINING ENVIRONMENT. AFTER TURNING DOWNWIND; I ELECTED TO LEAVE PATTERN BACK TO PRACTICE AREA TO TROUBLESHOOT RADIO PROB. ALL COM/NAVS LOST ALL AUDIO AND XMIT CAPABILITY. I ASSUMED THAT OUR XPONDER WAS DEAD ALSO BECAUSE OF THE LAST XMISSION WE HEARD FROM THE TWR. THE AMMETER WAS READING ZERO; EVEN WHEN SWITCHING TO BATTERY ONLY. I ASSUMED COMPLETE ELECTRICAL FAILURE AND ATTEMPTED TO RESET THE ALTERNATOR WITH NO SUCCESS. WE RE-ENTERED DOWNWIND FOR RWY 4L AND UNSURE IF THE XPONDER WAS SQUAWKING; I ELECTED TO SQUAWK 7700. BECAUSE OF THE BRIGHTNESS OF THE DESERT ENVIRONMENT AND REFLECTIONS OFF OF THE TWR CAB; WE COULD NOT VERIFY A GREEN LIGHT FROM THE TWR; SO I CIRCLED THE ARPT AND RE-ENTERED DOWNWIND. VERIFIED GREEN CLRED TO LAND LIGHT AT MIDFIELD AND MADE UNEVENTFUL LNDG. TWR ROLLED EMER EQUIP. THE TUG OPERATOR FELT WE SHOULD NOT HAVE SQUAWKED 7700 FOR A COM FAILURE SINCE EMER EQUIP WAS DISPATCHED AND TFC HELD UP UNNECESSARILY.