Incase your power getting off for more hours than it is essential to oxygenate the water. In normal case, if you set the aquarium system properly than your aquarium get oxygenate for over four to six hour depending on the level of oxygenation.
Some people use the battery to use as an alternate to the electricity power. However the capacity is less but it is necessary to use the pump as alternative to aquarium.
It is essential to install the battery operated pumps to install the mini generator of power. It is necessary to install the battery operated generator. If your power change than you must have to change the water.