You can use these numbers along with the underground temperatures at your planned depth, to calculate how much additional heat will be required on the coldest days, which in turn will point to the minimum size of a supplemental heat pump, for example.
When using heat load to dictate the size of your supplemental heating system, the goal is to warm your home comfortably, even on the coldest days of winter. This measurement does not have to be exact, as its better to have a slightly more powerful system rather than one which is too weak to manage.
