nrcrash, thats the sytem I have, the xantrex pro watt 600. I've got it hooked up to two deep discharge batteries from walmart. (230 amp/hr total) I use them for my Engle 12 v refridgerator when camping. I find that they will run my Tarm and some circ pumps for 6-8 hours,depending on how many are running. You can throw a kill-a-watt meter on to find out your max draw and your draw over time.
I got a relay from Xantrex for the prowatt that will automatically switch over to the inverter when the power goes out. The only problem is that when on, the inverter does draw some power and over the course of a couple of weeks will deplete the batteries, and it has to be on for the relay to work.
I just leave it off and a float charger on the batteries until it looks like there might be a storm.
My thought is that this will keep the boiler running until morning so I don't have to get out of bed.