This article discusses the determinants of technology diffusion among the low-income adopters, i.e. those belonging to the bottom of the pyramid market. Our aim is to analyse the variables that determine the adoption rate of Solar Home Systems. The analysis is drawn from a qualitative case study in Bangladesh based on interviews, observation and documents. We re-conceptualize diffusion process into three basic components: source, innovation and adopter. In particular, we focus on the characteristics of the source that are specific to the low-income market. In our case, the source is a social entrepreneur who seeks social wealth rather than economic wealth. Our empirical study may help to illuminate diffusion processes conducted by social entrepreneurs at the bottom of the pyramid because it contributes to a more comprehensive list of variables that affect the innovation adoption rate.