Can AC and DC be converted and compared?
You are so confused it's a little hard to tell exactly how.  Power is EMF (electromotive force) times current.  When the EMF is expressed in Volts and the current in amps, then the product of the two is power in Watts.

Your server is drawing 1.5 A at 230 V, which means it is using 345 W.  When the main power goes out and the UPS supplies power itself, this doesn't change since the server never sees a difference (that is after all the job of a UPS).  The server is still using 345 W.  Instead of those 345 W coming from the power line, they are now ultimately coming from the battery in the UPS.  There is a circuit, called a inverter in the UPS that converts the battery DC voltage to the AC voltage the server expects.  We know the server is using 345 W coming out of this inverter.  These things of course aren't perfect, so there is some loss in the inverter.  Let's say it is 90% efficient (pretty good but not unrealistically so).  That means the power going from the battery into the inverter is 345/90% = 383 W.  The difference of 38 W is lost as heat in the inverter.  If the battery is 48 V, then it is supplying 383 W / 48 V = 8.0 A.