Degree of efficiency: defines the output/input ratio for operation at nominal loading (i.e. effective output of a heating system)

Degree of utilization: In practice, systems are often not operated at nominal load, but rather only a minimal output is called for. When operating at partial load, other degrees of efficiency can be achieved than when operating at nominal load. The degree of utilization is understood as therefore an average output/input ratio during a particular time period. When this time period is a year, it is known as annual usage efficiency.