Battery energy storage systems (BESS) find increasing application in power grids to stabilise the grid frequency and time-shift renewable energy production. In this study, we analyse a 7.2 MW / 7.12 MWh utility-scale BESS operating in the German frequency regulation market and model the degradation processes in a semi-empirical way. Due to observing large temperature differences between the individual battery packs within a battery container, we include thermal effects in this model. In the analysed BESS the battery packs near the floor experience much lower temperatures in an annual average than the battery packs near the top (maximum average of 32 °C, minimum average of 23 °C). Operating in the Frequency Containment Reserve market, the annual capacity degradation differs up to 0.97% between the highest and lowest observed average temperatures. Hence, the BESS's lifetime could be extended by up to 11 years. Based on a detailed analysis of the BESS, we conclude that spatial temperature gradients within the battery containers are larger than expected and have a profound effect on lithium-ion battery ageing on system level. We extend this degradation model to study the technical potential of batteries in different energy market applications such as the day-ahead market with long periods of high charge and discharge rates (up to 1 h with a power to capacity ratio of 1 C) and the intraday market with volatile price spreads and therefore frequent and short periods (of up to 0.25 h) of high charge rates of up to 1 C. Our results suggest that the cooling system of energy storage systems needs to be carefully designed according to the intended application in order to control the temperature of the individual battery packs effectively. Slowing down ageing will be also beneficial for reusing 2nd life batteries stemming from a prior automotive application to extend the overall lifetime of such batteries.