inverter would eat up the current in the battery,as was said,buy a diesel generator,it can be wired to cut in when the mains fail,this involves a few electrical mods,but a comp.electrician can do the job,what happens is this,when the mains fails,a relay trips,and starts the generator,this will power your house till either the power comes back on,(the relay de-energises,)or your diesel runs out,last one we did cost around £4.500,
but since the guy lived as you say,middle of nowhere,he was happy to pay it,you may get it cheaper,shop around.
mm
March 2011