[mike31154]

I have a battery operated air pump also as back up, that's about it at the moment. Earlier this year during an outage I used one of my truck batteries (dual batteries under the hood help start the diesel when it's cold outside) with an inverter to keep my tank running for about 5 hours or so. It's a smaller (400 watts max) inverter and can provide just over 3 amps of power. I was able to run my skimmer, 80 watts of T5HO lighting and some power heads. It was summertime so the heater was not needed.

In order for you to determine how long your deep cycle battery will run the return pump you have, you need to gather some numbers. The deep cycle battery will have an amp/hour rating stamped on it somewhere. Now all you need to know is the amperage or wattage your return pump uses. With these two known quantities, you'll be able to determine approximately how long your pump will run on the fully charged deep cycle battery.

The battery amp/hour rating is based on the total number of amperes the battery can supply in a 20 hour period of even withdrawal. Accordingly, a battery rated at 100 ampere/hours can supply 5 amperes per hour for 20 hours. If more than 5 amperes per hour is taken from the battery, it will not supply its 100 ampere/hour rating. On the other hand, if your return pump only uses 2 amperes, then the battery should be good for substantially longer than 20 hours. Keep in mind that this is under ideal conditions and realistically it's better to think in terms of there only being 75% of actual battery capacity available. For additional capacity you can always hook up a second battery in parallel.

Battery capacity rating is always related to an expected discharge duration. Here's the formula to make your calculation:

t=Q/I
where
Q is the battery capacity (typically given in AmpHours).
I is the current drawn from battery (Amps).
t is the amount of time (in hours) that a battery can sustain

If you need to calculate amperes for your pump from wattage, the formula is P=EI or I=P/E
where
I is the current (Amps)
P is the power (Watts)
E is the voltage (Volts), 115 AC in your case

HTH

BTW, what does your set up use to sense the power outage and switch over to the back up? Do you have some sort of relay hooked up??