The paper presents an analysis of thermodynamic losses in thermal reservoirs due to irreversible heat transfer and frictional effects. The focus is upon applications to large-scale electricity storage for which it is the loss in availability (or exergy) that is most relevant. Accordingly, results are presented as loss coefficients which are defined as the fractional loss of the entering availability. Only losses stemming from irreversibility are considered – heat losses to the surroundings are not included in the analysis. A number of simplifying assumptions have been adopted, but the results nonetheless clearly demonstrate the dependence of the losses on operating temperatures, reservoir geometry and mode of operation, and point the way towards methods of optimisation. Estimates for a typical installation suggest that the losses are not insignificant, particularly for one-off charge and discharge (i.e., for long-term storage), but remain acceptable for cyclic operation, so as to make the use of thermal reservoirs attractive for electricity storage schemes.