There are a lot of technological studies of learning curves for renewable energy sources, for instance Neij (1997, 1999), Mattsson (1997) and McDonald and Schrattenholzer (2001). These typically discuss how the costs have been reduced over time, whereas our question is why the costs have been reduced. There are also a lot of studies concerning technology policies, and how to promote renewable energy sources, like Grubb (1997) and Jacobsson and Johnson (2000). But analyses like this, where one tries to reveal how technology and other policies affect the learning curve, are not to our knowledge common. Cory et al. (1999) analyse the role of R&D spending for cost reductions in wind turbines, a topic that is closely related to our analysis.