There have been a number of UPS issues in this area. The scenario goes like this: There is a power failure and the UPS takes over, while the genset gets up to speed. The UPS doesn't "like" the power it is seeing from the genset (too much distortion, not the right frequency, etc) and won't allow the genset to provide power. The UPS stays on until the batteries are dead and the communications equipment "goes dark." From talking to several service technicians and generator companies, the problem appears to be the UPS sensitivity settings (if the genset is operating correctly!). A couple of UPS companies have very tight tolerances built into their units and are not easily adjustable. I would recommend that you talk to other users and find out what UPS systems are working with particular gensets, before you buy!