Back-up battery life

[howard]

  I want a longer than a 20 hours battery
life on back-up, (20 hours - but only if the
alarm does not activate). Idea's ? Multiple
9 volt's in parralel?  Any idea how long a
300 watt UPS might last? How about a 12
volt  battery instead of 9 volt ---with a
diode maybe?  ALSO, if the alarm triggers
and say the thief leaves the back door open,
will the alarm reset and protect by itself?