In this thesis, variations in efficiency between two different solar collectors have been measured. Solar collector efficiency has been measured at different levels of flow. In addition, there has been made calculations of heat gained by the solar system in the course of a day.The effect of reduced transmission at an increasing angle of incidence, has been discussed.Comparisons of the two models show a minor improvement in efficiency in the latest solar collector models. Comparing daily energy gain for the system show the similar result. The difference is seen most clearly when considering efficiency per .A solar heating system consists of a solar collector, a heat store and a heat distribution system. The system examined in this thesis, has water as a heat carrier medium, and a pump that circulates the water through the solar collectors. The essential elements of a solar collector are the collector plate which can transfer energy from the sun to the water, and a cover sheet. The latter’s function is to prevent heat loss from the collector plate, and as such it makes the solar collectors function as a green house.Efficiency is an indicator of how well the solar collector, eventually the entire system, manages to exploit solar energy.In the thesis, efficiency has been analysed using a calorimetric method. Here the large storage tank holding almost 500 litres is used as a calorimeter. An easy way of measuring efficiency is looking at the increase in temperature in the heat store, and total radiation in the course of one day. Studies of the new solar collectors show that as much as 50% of the energy received by the solar collectors in the course of a day is useful energy.The flow rate tells us how fast the water flows through the collector plates. The water absorbes energy from the collector plates, which are heated by the sun.In the experiments, three types of flow were chosen manually. The flow rates were between 0.85 and 2.0.Comparing data from both models show no significant difference in efficiency by changing the flow rate. The efficiency is apparently not affected by variations of flow rate in the solar collectors.