lsayre wrote:Was the relay idea workable though?
Oh, sure! The idea is sound. I did not take the time to wrap my brain around the diagram that you drew, but 009's link shows a proper relay. I myself have built several 300 watt pure sine wave back up's. They cost about $120 for the part not including the batteries or charger. I also built a 1,000 watt. All are working and working well. Here's a photo of a relay I have that's "ready to go". Just follow the directions on the green tags & voila!
And, yes, the inverter is OFF until the power is lost. No need to keep the inverter "on" all the time. So, when they are on line & doing their job, when the power goes out, the relay drops closed, the inverter starts, the power now comes from the batteries instead of from the outlet.