The issue here however is that the software calculates a result based on the whole building rather than on each individual housing unit. (passivehouseplus.ie)
But you can also choose to calculate the result as if the payments are made at the beginning of each period («in advance»). (wisemoneytips.com)
In the case of your 15 year mortgage, the program will only used those values up to 180 months when calculating your results. (treesfullofmoney.com)