Congrats, you bought the right type of AC. The operating costs will be much lower than a conventional AC. How you use the unit is now then next factor in determining the operating cost. Depending on your unit, you should have at least two useful modes: Cooling & Dry.
When first turning the unit on, use cooling mode to reduce the inside temp to your comfort level. Then switch the unit to Dry mode to keep the humidity down and the people feeling comfortable.
For the most part it is the humidity that causes us humans to feel hot. Cooling mode uses the most electricity as the inverter compressor revs up and down to maintain a somewhat more consistent real temperature. In dry mode, the compressor stays at a lower constant rate and ends up using less electricity while keeping the humidity level down. In dry mode, the room temperature can creep up a little over your set cooling temperature but you won't notice if you don't stare at a thermometer. Having the AC chase a specific temperature uses more juice than having the AC maintain a set humidity level within a temperature range of a few degrees.
The general consensus when primarily using dry mode to maintain comfort, 25 C is an efficient temperature to set. Without getting into BTU's other than to say, more equals faster temp/humidity control, Your 12,000 BTU unit will run longer at high revs than a larger unit to "cool" the same size space in cooling mode. So, once you get the room comfortable, switching to dry mode to maintain comfort and forcing the AC compressor to run at a lower speed, is where your electricity savings will come from over using cooling mode exclusively.