hi fully flapped,
as nut loosed says...what do you mean by complete power failure..??
if you have alternator failure ( and of course doing regular cruise checks you will have noticed this before you run out of battery power ), then you will still have probably at least ( depending on the condition of your battery ) 45 minutes and possibly a lot more ( especially if you pull circuit brakers ...or switch off all non essential equipment )...so no problem as you will still have power to run your stall warning..and make arrangements to land at the nearest suitable airfield..
in any event..i am sure you would pay close attention to your airspeed indicator on approach anyway..so why should it be such a problem even if you lost the electric stall warning...? in the average small plane getting close to the stall is so noticeable in attitude, wind noise, buffeting..( some may vary a little and may not have all three ) that it is difficult not to recognise the onset of the stall. this is why on flight test some of the stalls given ( not the fully developed ) you are asked to recover ' from the first indication of stall '....that is for the examiner to see that you can recognise the early symptoms ...and initiate the recovery from that point..
good you should wonder ' what if '...but i do'nt see it should be a problem for you.
fly safe..
the dean.