How to choose battery for desktop computer typical

I am trying to find a battery to be sued with the inverter in order to get the desktop PC on even during power outages and also during instances in remote places where power is not at all available. I am searching for batteries. The main specs are 12 V and current. To guess the current capacity of the Battery I have to find out how much does a typical Desktop PC consumes. Is there is any benchmark kind of data available? How to measure it at least if possible? I need the battery to support at least 8 hours of continuous desktop usage. Looking forward for your support.
