The collector area is a useful figure for the solar thermal experts, but it cannot be compared with the installed capacity in other fields. Therefore the IEA Solar Heating and Cooling programme, Estif and other trade associations have adopted a value of 0,7 kw/m2 as average capacity [10]. This conversion factor has been adopted by the IEA statistics department. Eurostat is considering using the same factor for their statistics. The installed collector capacity can now be compared with other technologies [9].

1.2. Monitoring of the solar thermal production

The total thermal production is in general calculated from the installed collector area. Most countries use a simple figure per square meter of collector. The IEA Solar Heating and Cooling programme has a more sophisticated method that includes the simulation of a typical solar system for each country [9]. Eurostat takes over the figures from the statistical offices in the EU-countries. They ask for the collector input as in their definition in the input-method. This is the energy falling on the collector minus the collector losses. Most countries seem to use a figure that is available in their country. In table 1 it can be seen that there is significant difference in the production per

square meter of collector. It varies from 64 to 903 kwh/m2 [8]. This difference cannot be explained by the difference in insolation or quality of the solar collector systems.

The ThERRA project is proposing a fixed method for calculating the collector production, based on measured data. If no measured data are available a default value can be used.

In the benchmark report of the methodology the difference with the current methods is found [11].