So my question is: Shouldn't it be possible do build a UPS, that consumes a lot less, when in "normal" operational mode?
13W for 750W is only 1.73%.
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 .....that plug directly into existing motherboards.