The Integrated Collector Storage Solar Water Heater (ICSSWH) suffers significant ambient heat loss, especially during night-time and non-collection periods, due to its combined collection and storage function. A novel ICSSWH a Double Vessel Integrated Collector Storage (DVICS) comprised of two vertical concentric metal cylinders, with the resulting annulus filled with a Phase Change Material (PCM) and partial vacuum pressure, was designed to reduce thermal storage losses without compromising energy collection. The DVICS evolved through the development of 6 fabricated prototypes, including novel pockets and cowl configurations in the annulus, a polytetrafluoroethylene (PTFE) thermal break between the inner and outer vessel, elongation of the pockets to increase heat transfer to the PCM, aluminium vessel construction, differing water return arrangements, solar collection improvements to the vessel geometry, optimised pocket locations and transparent covers with insulation. The different DVICS designs were studied experimentally using thermal simulation, solar simulation and outdoor conditions at two European locations. The highest instantaneous collection efficiency was 43% and the lowest thermal losses coefficient was 0.98 Wm-2K-1. The heat transfer mechanisms were modelled using thermal resistance networks and dimensionless analysis. In addition the practical, environmental and economic considerations are presented.