The output of solar thermal systems is in general not measured but calculated by using the installed capacity. Only for very large systems the output is measured. The assessment of the installed capacity is in general done by a questionnaire to the industry about the collector area that is sold. In a similar way also the area of non-covered collectors is assessed. This method gives a reasonable accuracy. The main differences occur in the life-time that is assumed for the solar systems. Some countries have a methodology to assess the life time and so calculate to total collector area in use.

If this is not done by the countries themselves, it should be done before international comparisons are made. The IEA Solar Heating and Cooling programme assumes a life-time of 25 years [9]. There is little knowledge about the actual systems still in use, but in a fast growing market the errors of misjudging the life-time is very small. There is a need for a better estimation of the life­time of solar thermal systems. This will be country specific, because the types of solar systems and their quality vary widely.

Surveys of the installed collector area are given for the international market by:

• The IEA Solar heating and cooling programme: an overview of the world based on the best data that the experts from the programme can find. The report includes a division between different collector types and a calculation of the produced heat.

• Eurostat: the official statistics

• The IEA statistics office: They have the same data as Eurostat

• Estif: The Estif data focus on market development. Estif includes expected market development

• EurObserv’ER: They follow the official statistics but publish the available data quicker than Eurostat. The aim is to follow the targets set by the EU.

In general the figures have an acceptable accuracy. The data is not as good as for example the electricity produced by wind power, but more accurate than the energy produced by wood stoves in households.