I am to use this entire system as a typical home security system and to set off an alarm if any abnormalities is detected. However, i do not want this system to depend entirely on battery as it might not be energy efficient, but if i am to use main power supply, it is susceptible to blackout and render the whole security system unusable.

Therefore, i have thought of the idea of using main power supply as the source and the battery as backup in case of power failure just to make sure the security system is not affected by the sudden power loss. In most cases power loss is only temporary so i do not think time of operation is really critical here nor power saving as far as it is concerned. The battery is there just so that it will keep the security system running until the main power supply is restored(which should not take more than 24 hours).