Load control of populations of thermostatically controlled loads (TCLs) is considered a promising approach to match generation and consumption in electricity grids from the demand side. However, when these loads become synchronised they present a decaying oscillatory aggregate demand, which results in undesired power peaks. In this paper we describe the nature of these oscillations and develop a list of factors that determine its shape. We perform a sensitivity analysis
which allows us to identify important relations between the physical parameters of the TCLs and their aggregate dynamics. Beyond describing fundamental behaviour, these relations can help develop and validate analytical expressions that facilitate control design, enabling the use of TCLs for demand response.