A B737-300 CREW DID NOT COMPLETE THE AFTER START CHKLIST BEFORE TKOF WHEN THE APU WAS TURNED OFF. ELECTRICAL POWER WAS LOST BECAUSE THE ENG GENS WERE OFF.

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START UP AND TAXI WERE NORMAL UNTIL WE REACHED THE DEP END OF RWY XL. I CALLED FOR THE BEFORE TKOF CHKLIST IN ANTICIPATION OF BEING CLRED FOR TKOF; AND AT ABOUT THAT SAME TIME; WE WERE IN FACT CLRED FOR TKOF. BEFORE THE FO RESPONDED TO ATC AND ACCEPTED THE CLRNC; I TURNED THE APU OFF; RESULTING IN A LOSS OF ELECTRICAL PWR. I IMMEDIATELY TURNED ON BOTH ENG GENERATORS; RESTORING PWR. THE FO TOLD TWR WE NEEDED TO HOLD SHORT FOR JUST A MOMENT AND TWR CANCELED OUR TKOF CLRNC. WE HAD NOT MOVED ACROSS THE HOLD SHORT LINE. WE CHKED ALL SYS AND RE-RAN THE BEFORE TAXI AND BEFORE TKOF CHKLISTS; DISCUSSED THE SITUATION; AND WERE BOTH COMFORTABLE THAT WE COULD CONTINUE; HAVING RE-RUN THE APPROPRIATE CHKLISTS. WE CONTACTED TWR READY TO DEPART AND DEP AND REMAINDER OF FLT WAS NORMAL. THE FO AND I DISCUSSED WHETHER OR NOT THIS WAS AN ABANDONED TKOF; AND SINCE WE HAD NOT ACCEPTED THE CLRNC; WE BOTH FEEL IT WAS NOT. HOWEVER; WE CONCUR THAT FILING AN ASAP IS WARRANTED; IN THE EVENT THAT TWR RPTS IT AS AN ABANDONED TKOF; AND MORE IMPORTANTLY; TO PROVIDE THE DATA ON A FLOW/CHKLIST XCHK ERROR. I ATTRIBUTE THIS EVENT TO MY NOT PLACING THE ENG GENERATORS ON LINE AFTER THE GND CREW WAS CLR; PER NORMAL SOP AND FLOWS. I THEN DID NOT CONFIRM THAT THE GENERATORS WERE ON LINE DURING THE BEFORE TAXI CHKLIST (WHICH WE DID RUN PER SOP); EVEN THOUGH THE BLUE LIGHTS WERE STARING ME IN THE FACE. CLOSER ADHERENCE TO SOP; FLOWS; AND CONTINUED VIGILANCE IN XCHKS DURING CHKLISTS. OMISSION OF A FLOW STEP; FOLLOWED BY MISSING IT ON THE CHKLIST.