Robot batteries and the hurricane

[linuxboy]

Hi all,
I was wondering what you think of the following idea (and whether or not anyone knows the math to calculate how long the batteries will last).

Due to hurricane Irene my house will _probably_ lose power. In order to deal with that and keep cell phones / laptops charged I had an idea. I have a couple batteries from the KOP at my house and I am planning on getting a 12V socket and hooking it up to the battery. I have a power inverter that plugs into a 12V socket and outputs two AC sockets and a USB socket. Does this seem like a feasible plan? If I am running an 85Watt power adapter off it my calculations say I should get ~24 hours but I didn't know how to factor in the AC/DC conversion so I'm hoping you guys might be able to help me with that. Thanks!!
Oliver