A TECTEM is a thermionic energy module. It will ultimately replace the central heating facilities annex domestic hot water systems in homes. It's merit stems from the fact that in conventional central heating facilities, high temperature heat is being used for low temperature purposes, In a TECTEM, some of this heat is used to generate electricity. The waste heat of the Thermionic Energy Converter (TEC) can still be used for producing domestic hot water and for heating a house. Because of this the marginal efficiency, with which electricity can be generated, can be as high as 80%. The specific electricity costs are rather low. Since the TECTEM saves energy as compared to electricity production by central station power plants, it also reduces CO2 emission. A brief overview is given of the design criteria leading to the design of the first TECTEM. The design of this prototype incorporating six TEC's is explained. It will serve as a testing vehicle for life and performance assessment of the TECTEM and several of it's components e.g. the solid state DC/AC invertor. The design of a burner/recuperator module is discussed. This TECTEM will be used for developing the first demonstration prototype in close cooperation with a Dutch manufacturer of central heating systems. The basic scheme, criteria and the performance of the solid state invertor, converting low voltage DC electricity into 220 V 50 Hz electricity is also discussed.