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[en] Highlights: • Thermal inertia of a solar collector is investigated in various radiative regimes. • Thermal inertia effects depend on the solar radiative regime level and stability. • Averaged thermal capacitance is lowest during less stable days. • Overcast-sky days show the lowest values of the thermal inertia amplitude. - Abstract: The level and the stability are two main features of the solar radiative regime. The radiative regime level is connected with the relative sunshine value, which shows in relative terms how long the sun is visible on the sky, during a given period of time. The stability of the radiative regime may be quantified by the sunshine stability number, which provides information about the number of times when the sun changes its position from being visible to being covered by clouds. The thermal inertia of solar collectors affects their performance. The effect is expected to depend on the stability of the radiative regime. However, no specific study has been conducted on this topic. The objective of this paper is to estimate how much dependent are the thermal inertia effects on the two features of the radiative regime. An unsteady solar collector model is developed and tested against data provided by the manufacturer and independent measurements. Simulations were performed under the temperate continental climate of Timisoara (Romania, Eastern Europe). Four summer days and four winter days have been selected, covering different relative sunshine classes and different levels of the radiative regime stability. The steady state thermal capacitance of the collector is 5.37 kJ/(K m"2) in good agreement with results reported in literature for similar collectors. The daily amplitude of thermal inertia is the highest in less stable days with relative sunshine 0.4–0.7. The amplitude is lower for more stable days and for higher values of the relative sunshine. Overcast-sky days show the lowest values of the thermal inertia amplitude. The thermal capacitance of the collector increases when the mass flow rate increases. The daily average thermal capacitance has the lowest and highest values in less stable days with relative sunshine 0.4–0.7 and overcast-sky days, respectively. More stable days with relative sunshine 0.4–0.7 and clear sky days show intermediate values. The unsteady thermal capacitance of the collector averaged over all eight days is 21.19 kJ/(K m"2), which is close to the value 20.4 kJ/(K m"2) previously reported under the climate of Madrid (Spain) for a similar collector.