In the framework of the REBUS project (Competitive Solar Heating Systems for Residential Buildings) a new concept of solar combisystem was developed. The system consisted of two units, the Technical Unit and the Solar Store Unit. The Solar Store Unit in principle is a buffer tank filled with space heating water, which needs 5 connections at the right heights for charging and discharging. For this Solar Store Unit in principle any simple available tank can be used because all operational devices are integrated in the Technical Unit. The proposed standard tank has a volume of 300lt and it is integrated in a 60 x 60 cm cabinet with a nice casing, which is perfect for aesthetical integration. The standard tank is insulated with PUR-foam, which is filled between the tank with a diameter of 500 mm and the 60 x 60 cm cabinet. In order to improve the performance of the solar combisystem, keeping the solar tank into the 60 x 60 cabinet a new combined insulation concept was used, which enabled to increase the diameter of the tank; using a 20mm thick vacuum panels, which are integrated into the PUR foam, allowed to increase the diameter of the tank by 10% (550 mm instead of 500 mm), which increased the volume by 20%, from 300lt to 360lt. In this paper are presented detailed investigations on the heat loss coefficients of the two tanks measured from laboratory tests and a comparative system analysis using the two different tanks.