Test Reference Years are detailed annual climate data rows used primarily in calculation and simulation programmes. They are in most cases not simply measured climate information of a real year at a certain location, but an artificial annual climate based on the interpolation of measurements of several (partly up to 30) years that can include maximum and minimum values that haven’t been measured in the very same year. Test reference years usually contain hourly data of climate characteristics like outdoor temperature, solar radiation, relative outdoor air humidity, wind speed and direction, quantity of rainfall, etc. During the last years more and more countries have updated their Test Reference Years and worked on “Future” Test Reference Years that take into account predictions of the future climate development (climate change). Test Reference Years are often available at national weather services. Some computer simulation programmes offer weather data for free. One of these sources is the website of Energy Plus.