what I ended up doing....
I bought the Duracell back up battery. It has the same capacity as the Xantrex, is lighter and cheaper, and, get this - it's manufactured by Xantrex. After a lot of price shopping, Amazon had the cheapest price. I used it for the scheduled power outage (all went well), and it kicked in during a recent brief 1 1/2 hour outage we recently had. Good piece of mind. I also have a cheap $15 D battery-powered back up to use when the Duracell is being recharged (somewhere else).