Compact thermal storage using a hybrid phase change material (PCM) store for domestic heating applications is investigated. The primary focus is on thermal demand during the electrical grid-stress period (16:00 h–20:00 h on winter weekdays) when the primary heat source, a heat pump, is turned off. Though this phenomenon may be evident in other countries, the main focus of this work is on electrical grid-stress in UK conditions. In this work, PCM encapsulated in pipe coils surrounded by water in a hot water storage tank is considered. Two alternative samples of salt hydrate are evaluated experimentally and the results are used to inform system modelling. A new model is proposed for relating the enthalpy and temperature of the PCM during melting and solidification. A compact hybrid store design is proposed and a detailed thermal model of the hybrid store with an air-source heat pump is constructed and applied to an example house. Seasonal energy results compared with a conventional water tank are broadly similar but the hybrid store offers better comfort tracking during grid stress periods – average house temperatures falling below 19 °C for 22.7% of the time with a conventional store but only 5.8% of the time using the hybrid store.