This paper compares the historical development of innovation systems related to biogas and improved cooking stove technologies in Rwanda and Kenya by applying the 'functions approach'. It argues that the accumulation of functions in these four renewable energy technological innovation systems (TISs) differed substantially. We find that the accumulation of TIS functions at early stages of development is determined more by national and international contextual factors than by specificities related to technologies or internal dynamics (interaction of functions). Further examination of the functional patterns of TISs suggests that differences in the accumulation rates of functions explain the differences in diffusion rates, with broader and more balanced TIS functional accumulation being related to higher diffusion rates. The paper concludes that systematic support, including well-directed international development assistance, would enable the development of mature and balanced TISs that nurture the diffusion of renewable energy technologies.