Process heat represents a major share of final energy consumption in the industrial sector and can partly be provided by solar thermal systems. To date, there has been little experience with solar heat plants for industrial processes operating at medium temperature levels (100–250 °C). This paper focuses on the analysis of reduced solar gains by heating-up processes (capacitive thermal losses) in a parabolic trough collector field with an aperture area of 627 m² providing solar heat for a Swiss dairy at 120 °C. Heating-up thermal masses is experimentally quantified by a new method using existing temperature sensors. The unused solar thermal gains of heating-up periods amount to 18% of possible useful solar gains in 2014. In winter months, this share can reach 50%. Preserving the hot fluid content in an ideally insulated storage in the evening could avoid heating-up in the morning and reduce capacitive thermal losses by 38%. With properly installed insulation thermal losses of the piping system during operation are theoretically proven to be below 3% of useful solar gains. The analyses are based on the evaluation of highly time-resolved measurements of one year.