I put 30 MWac in “Specific capacity” but only got 18 MWac in my design. Why?

There are different reasons as to why a specific capacity is not achieved. One could be because there isn’t enough space on your PV plant to fit that exact capacity. If that’s the case maybe try changing the number of inverters per power station or having the PS inside the DC field.  If you do have enough space, then please try using the “Maximum capacity” option and enabling the “Install the maximum peak power” button.