A micro scale electrically heated rotary kiln for slow pyrolysis of biomass and waste was designed and built at the University of Perugia. The reactor is connected to a scrubbing section, for tar removal, and to a monitored combustion chamber to evaluate the LHV of the syngas. The system allows the evaluation of gas, tar and char yields for different pyrolysis temperature and residence time. The feeding screw conveyor and the kiln are rigidly connected; therefore a modification of the flow rate implies a modification of the inside solid motion and of residence time. Part I of the paper describes the theoretical and experimental evaluation of the working envelope of the reactor, as a function of feedstock density and humidity content, to obtain pyrolysis conditions inside the kiln. This paper describes the development and resolution of an energy balance of the reactor under pyrolysis conditions. Once the rotational speed n is fixed, the aim of the balance is to obtain the composition of the yields of the pyrolysis of wood biomass, in terms of syngas, tar and char. Results can be used to choose the correct rotational speed before doing the real pyrolysis test.