To provide backup on the event of mains/backup AC supply failure, inverters or extended UPS battery bank is normally used to maintain the load just for the alternate power to be available. This traditional setup method is not really cost effective because the inverter energy is not utilized to its capacity.
A cost efficient way of doing this setup is to introduce automatic battery bank current (autonomy time left) monitoring and switching-on the alternate power, only when the inverter is utilized to a preset threshold. This would reduce excessive fuel wastage