The design outdoor temperature for district heating systems is set to the lowest 5 day average outdoor temperature appearing once during thirty years. This design temperature varies from - 14 " C in the south to - 36 "C in the inland of the north of Sweden.
The number of degree-days , related to an equivalent indoor temperature of 17 " C, is ranging from 3000 in the south to 7000 in the far north. The equivalent indoor temperature of l7° C has been chosen to take into account internal heat gains from people and electric appliances.
The heat load composition, the design outdoor temperatures, and the number of degree-days give load duration curves, which can be characterized by load utilizationion times between 2800 and 3600 hours, giving load factors between 0,31 and 0,42. The higher figures appear in the north of Sweden.