Purchased energy rate

You model it after determining the rate. The purchased heat may be allocated in the building on a cost/area but that is not the energy rate. In order to determine the cost/area, an energy rate was first used to derive the purchased heat annual cost. That annual cost would have been based on a certain level of consumption. Divide the cost by the assumed consumption and you should have a fixed rate you can use. You can also contact the purchased heat provider and see if they have a published rate tariff and model that. The rate must be in cost/unit of energy.