HEAT-INSYDE advances a ground-breaking closed-loop concept using thermochemical material to a compact domestic heat storage prototype at a Technology Readiness Level of 7 through a user-centric approach with real-life demonstrations in 3 different European climate zones. HEAT-INSYDE delivers an affordable, highly compact solution ( 25 years) performance. Its hybrid functionality combines compact storage with a highly efficient heat pump effect. The system balances central (grid) and decentral renewable energy supply, allowing configuration in both heat and electricity systems and creating new opportunities for grid flexibility. The proposal is based on two recent technological breakthroughs. The first concerns a multicyclic stable thermochemical material with a high energy density that can be produced on(semi-)industrial scale at low costs. The second concerns a reactor principle patented by two partners TNO and TU/e that uses the full potential of the first-generation thermochemical material. The effective energy density on a system level surpasses the state-of-the art water storage solutions by a factor of at least 10. The HEAT-INSYDE consortium is industry dominated and mobilizes all relevant stakeholders in the value chain: ranging from basic material production, material manipulation, reactor design and manufacturing, energy system integration to municipalities and real end-users. The consortium includes the City of Eindhoven, bringing a large-scale pilot opportunity and a national funding multiplier. The knowledge base consists of TNO, CEA and Eindhoven University of Technology (TU/e), all having an excellent track-record in thermal storage technologies and EU projects in this area.