The demand for local heat storage to help manage energy demand in dwellings is likely to increase as the electrification of heat through heat pumps becomes more widespread. Sizing thermal energy storage systems has been an important topic in contemporary literature, but the effect of the electrical load shifting tariff and the service the householder receives in terms of space-heating and hot water delivered, however, has not and this is particularly important when households transition from conventional gas fired to low carbon technologies. This paper takes a whole system modelling approach to understand the impact of user demand patterns and load shifting scenarios on the volume of energy storage required for a heat-pump installation. The work uses monitoring data from several family homes to drive the simulation and finds that the level of service the householder receives is sensitive to their patterns of consumption, thermal energy storage volume and the electricity tariff, with some households being far more sensitive to tariff choice than others. The paper introduces a novel, quantifiable measure of service for space-heating and hot water systems that can be incorporated into thermal energy storage sizing procedures.