The battery backup unit automatically switches between AC house power and DC power from a marine deep cycle battery (similar to a car battery).

The key components are (1) deep cycle marine battery, (2) power inverter DC to AC, (3) relay switch that automates the power switching process, and (4) battery tender trickle charger

I only use mine to power pumps which draw less than half the wattage of the inverter.

 
(1) Battery
A deep cycle marine battery is better able to withstand discharge and recharging. They are available as boat batteries and are economical ($60 for 90 amp hour battery).

(2) Power inverter DC to AC
Pumps require maybe 200% of rated watt draw on start up so consider this for your inverter. My pumps are cheap Mag Drives so I use a cheap inverter that is 110 volt and commonly available. For expensive pumps, a better, more expensive inverter may help prevent damage to the pump. My inverter from Sam's Club Warehouse was $35. A key trick is to open the case of the inverter and wire the on switch to the relay. The result of this is the inverter is off when house power is on, but switches on when the house power is off.

(3) Relay
This is a switch that detects a loss of current and changes from one power source to another. I used a quad pole double throw relay from an electricians parts store in Knoxville (Carlton-Bates 584-3393, $20). The wiring of this is complicated, and you would probably be better off consulting an electrician. However, you can meet with me on how to do this and take a look at the diagram from the tripod web site. There is a labeling error in his diagrams (the brown wire is AC power and the white wire is inverter power in lower left close up).

(4) Charger
You want to use a charger that slowly charges and keeps the battery fully charged. These batteries release explosive hydrogen gas when recharged at the usual rapid rate of an ordinary battery charger. The Battery Tender (name brand, $49) that I use is a 750 milliAmp unit that automatically turns on and off. This recharges over 24 hours the amount of energy my unit uses in about 1 hour of battery time, and hopefully will not release dangerous levels of hydrogen.
 

I assume no responsibility for the use of this information. You can contact me on how I use my battery back-up, and the details on assembling the unit. I have an article from FAMA, March 2003 by Scott Mooney discussing systems on the market. Also see www.jehmco.com
 
Battery backup, DIY UPS-like, links below, but next link is a site for exact how to-
 
http://reefcentral.com/forums/showthread.php?s=&threadid=64918
 
How to site-
http://jean.yen.tripod.com/id51.htm
 
Ready made system-
http://www.jehmco.com