Solar thermal energy is often said to have an efficiency of 80 percent. However, this is only a snapshot taken at the collector test bench (without any heat release), that means: the value has no practical relevance! It is much more objective to compare the annual energy yields of both technologies. Well-functioning solar thermal systems with flat-plate collectors supply about 350 kWh of heat per square meter per year. A photovoltaic system with the same area earns about 200 kWh per square meter. In between there is a factor of 1.7. That means, for a typical hot water system you need 6 square meters of thermal collectors, or 10 square meters of photovoltaic modules. But this only matters if there is not enough space on the roof, because the sun is shining for free and there is no direct correlation between cost and efficiency. Incidentally, the area factor for solar thermal energy was still 2 in 2015, another indication of how fast the technology is evolving.