PA28 INST STUDENT AND INSTRUCTOR HAD TOTAL ELECTRICAL FAILURE IN NTD CLASS E AIRSPACE.

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I WAS ON AN IFR TRAINING FLT WITH MY INSTRUCTOR FROM OXNARD BACK TO MY HOME ARPT AT SANTA MONICA IN A CHEROKEE. THE TKOF AND CLBOUT PHASE WERE ROUTINE AND WE HAD LEVELED OUT AT OUR ASSIGNED ALT OF 5000 FT MSL; WITH A HDG OF DIRECT VNY. I NOTICED THAT THE OBS NEEDLES ON BOTH NAV RADIOS WERE OSCILLATING. THE LED FREQ DISPLAYS ON THE RADIOS THEN BLINKED OFF. THE CTLR ADVISED US HE WAS NOT RECEIVING OUR XPONDER CODE. I RECYCLED THE XPONDER BUT HE STILL WAS NOT ABLE TO RECEIVE IT. SHORTLY AFTER THIS; WE STOPPED RECEIVING XMISSIONS; AND IT WAS APPARENT THAT THE ELECTRICAL SYS HAD TOTALLY FAILED. RECYCLING THE ALTERNATOR/BATTERY ROCKER SWITCHES DID NOT DO ANYTHING USEFUL. I DSNDED IN VFR CONDITIONS TO 2500 FT MSL AND WAS ABLE TO RECOGNIZE MY POS AS BEING OVER SIMI VALLEY. I OVERFLEW THE 118 FREEWAY HEADED E; TURNED R AT THE E END OF THE SANTA SUSANNA PASS; FLEW S TO THE 101 FREEWAY AND THEN TURNED E SO AS TO AVOID THE BURBANK CLASS C AIRSPACE. AT THE SEPULVEDA PASS; I TURNED S AND AFTER RECEIVING A STEADY GREEN LIGHT-GUN SIGNAL FROM SMO TWR; I MADE A R BASE ENTRY TO RWY 21 AND MADE AN UNEVENTFUL LNDG. WE CALLED THE TWR AFTER LNDG TO SEE WHAT THEY WANTED US TO DO; BUT THEY SAID NO RPT WAS NECESSARY. THIS WAS POTENTIALLY A VERY SERIOUS SIT; AND WE WERE FORTUNATE TO HAVE BEEN FLYING IN VMC. BOTH THE INSTRUCTOR AND I HAD NOTED THE AMMETER SEEMED TO BE INDICATING A 'LOW-NORMAL' CHARGE CONDITION; BUT THE NEEDLE WAS CERTAINLY NOT FAR FROM THE MIDDLE OF THE GAUGE. THIS INCIDENT WAS INSIDIOUS IN NATURE BECAUSE THE ELECTRICAL FAILURE WAS GRADUAL AND NOT SUDDEN.