A weather data evaluation from a climate station in Lyngby, Denmark, was carried out. Twenty years of measurements show that the increase in global radiation was almost 3.5 kWh/m2 per year, corresponding to a growth of the yearly global radiation of 7 % for the last 20 years. The global radiation variation between the least sunny year to the sunniest year was 20%. The increase in diffuse radiation was 1.9 kWh/m2 per year, corresponding to 20 years’ growth of up to 7 %. The annual diffuse radiation of nearly 19 % varied from the least cloudy year to the cloudiest year. A small increase was measured for the ambient air temperature. The measurements showed a yearly increase of 0.04 K per year. The average yearly ambient air temperature variation from the coldest to the warmest year was 3.1 K. According to the seasonal growth of the parameters measured, the ambient air temperature and diffuse radiation increased the most in the summer period, while the global radiation significantly increased in the spring months. The calculations of the solar collector's thermal performance in Lyngby showed that the energy output was mostly dependent on beam radiation. The ambient air temperature did not have a high influence on the thermal performance of the solar collectors compared to the influence of the total solar radiation.