I would focus on the end power needed. In the case of a server, you're typically looking at 12VDC for the HDD's and 5VDC for the board. PicoUPS offers all sorts of pre-confectioned power supplies that are DC-DC solutions with very high efficiencies. Makes much more sense to go directly DC-DC with a switch mode than the AC intermediary steps. What's great about the PicoUPS products is that they're based on ITX and ATX power supplies that plug directly into existing motherboards.