This paper describes the performance testing of a 2000-litre phase change material used in a Thermal Energy Storage (PCM-TES) demonstrator unit invented in DIT’s Dublin Energy Lab and installed in an office building in Cork in 2014. The PCM-TES is connected to a Micro-CHP unit and stores waste heat from the CHP at evening peak tariff periods when the building heating loads are lowest. The CHP is also connected to a 2000-litre water tank allowing direct comparison of the energy storage capacities and performances of both storage systems. Charging results presented show that the PCM-TES holds 6.5 times more heat for the same plant room footprint, allowing the CHP to run continuously during peak periods and producing a better overall electrical/ thermal efficiency. Discharging results show how the PCM-TES stored energy can be used to pre-heat the building heating system early in the morning, shifting CHP thermal demand to align better with the day rate electrical tariff period. The PCM-TES eliminates the need for back-up gas boilers to be used for the early morning heat demand peak. A discussion of PCM-TES benefits over a water-based TES is supported and presented here.