Hi guys, still a newbie on the forum, but have read quite a few posts that have helped me installing my RCT Axpert inverter as a backup system currently, and eventually expand the system with Solar as well. For this I would like thank the Forum. Thus my post, I'm looking for another RCT Axpert 5Va 4000W, but their prices are basically the same as the newer upgraded 5Kw system, why? Just a shame the 5Kw system is not compatible with 4Kw unit. Anybody maybe have a second hand 4Kw unit lying around?