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 degrees C for 22.7% of the time with a conventional store but only 5.8% of the time using the hybrid store. (C) 2017 Elsevier B.V. All rights reserved.