Neptune sells a 12V AC adapter for use with the Apex that can be attached to a standard UPS for battery backup (you should be able to find the details on Neptune's site). If you don't already have a UPS you want to use, a better alternative is to get one of the 12V battery backup units Belkin (I believe) makes. People sell them on eBay for around $20 shipped.
All these details and more can be found in the Unofficial User Guide stickied at the top of this forum... A must read if you have not already...