Power to heat technologies are a necessary solution for future development of renewable energy systems in small and large scale grids. Such technologies can be used in conjunction with heat engines as electrical energy storage, an application that is often referred to as Pumped Thermal Electricity Storage (PTES). In this paper a PTES system powered by both electrical and thermal energy was studied from a thermodynamic point of view. By means of simulating the steady state operation of the system, the optimal working fluid is identified, and the round-trip efficiency and the storage capacity are characterized. Focusing on waste heat recovery applications, the absorbed and returned amounts of electrical energy are calculated with respect to the thermal capacity and temperature glide of the heat source. The need for a trade-off between storage size and round-trip efficiency is highlighted, and the effects on the storage optimal size are discussed.