The National Median Energy Cost ($) is the cost of the energy to fuel your property if it used the amount of energy equal to the National Median Site EUI.
National Median Energy Cost for your property =
Your Energy Cost * (Your National Median Site Energy / Your Site Energy)
This FAQ explains how your National Median Site Energy is calculated.
If your property cannot calculate a Site Energy or Energy Cost, then Portfolio Manager will not be able to calculate National Median Energy Cost.