The correction method for temperature is based on data from Eurostat, the statistical office of the European Union. The correction is based on the number of heating degrees and the share of natural gas used for heating. The number of heating degrees in a day is defined the reference value of 18º Celsius minus the average outside temperature for that day. The assumption is that heating is required, if the outside temperature drops below 18º Celsius.
The daily differences are added up for each year. In 2008, the number of heating degree days was 2,785 and in 2007, there were 2,525 heating degree days. These numbers are corrected for 3,075 heating degree days, the average over the period 1971–2000. The share of natural gas used for heating houses is set at 78 percent. The remaining 22 percent is used for cooking and showering, etc.
Suppose, for example, that the volume of natural gas used in 2007 was 1,000 m3. The volume used for heating was 0.78 * 1,000 m3 = 780 m3, for cooking, etc. 220 m3. In that case, the corrected volume is: 3,075 / 2,525 * 780 m3 = 950 m3 + 220 m3 = 1,170 m3.