The hourly time step weather data are very common used in the field of evaluations and predictions of the performance of solar heating systems, especially in the computer simulation area. However, studies in [3] states that using the hourly weather data in simulation of solar systems may ignore the variations of solar radiation during one hour, it will cause a inaccurate simulation results. Using the smaller time step weather data instead can resolve this problem and make the simulation results very close to the fact. In this paper, the variations of solar radiation during hourly will be found through comparison of the same weather data based on different time steps (hourly and 6min). A whole weather data reference year based on 6min, 12min, 18min, 30min and 60min in Borlänge, Sweden will be built. Then the total radiation on a sloped surface, clearness index and solar utilizability based on these different time steps weather data will be calculated and compared with each other respectively. The results show that same weather data based on different time steps do not affect the calculation results of total radiation on a sloped surface (collector surface) but they may affect the beam and diffuse fractions and estimate of collector performance. After that all the weather based on different time steps will be used in the simulations of three different solar combisystems. As the result shows, using the hourly weather data leads to relative conservative estimate of solar heating systems compare with that based on shorter time step weather data, although the difference is very limited.