I've got this one:
Outback Power gfx1312And it does do a blending of power. It is really nice to plug into a 15amp outlet and not worry about melting the outlet down..
You tell it how many amps to pull from the line in, and when you reach that point, it will start to use the batteries to boost. If the power quality falls outside of certain parameters, it will drop the incoming line and then run off batteries until the quality is restored or the batteries become too depleted.
Nice thing is it's cheap, under $1k; bad thing is it is not UL listed in the US, and you must do a separate bonding relay for neutral to ground.