Lack of access to modern energy services is a daunting development challenge in sub-Saharan Africa. The promotion of renewable energy technologies (RETs) by development partners and government organizations is one way of meeting this challenge. Despite substantial investment of effort, the diffusion of RETs in Africa has been disappointing, leading to a search for more effective policies and approaches. In this paper, we discuss the role of technological innovation system (TIS) in fostering technology diffusion. We argue that the functional build up of TIS shapes technology diffusion levels in least-developed country settings. We report a comparative analysis of the biogas TIS in Kenya and Rwanda by applying the so-called the 'functions approach to innovation systems'. On this preliminary evidence, we argue that patterns of accumulation of TIS functions may determine rates of technology diffusion. We argue that to ensure accelerated diffusion of RETs, policy attention should be paid to improving the functional performance of TIS.