Policy impact in technology growth models of solar PV and onshore wind power

Policies are key to accelerate the uptake of new energy technologies, but current growth models of technology diffusion fail to represent policies in forward-looking assessments and decision-making. Here, we propose a method to integrate policies into probabilistic modeling with six growth models, validate this method with out-of-sample testing, and demonstrate it on solar photovoltaics and onshore wind power in 40 European countries. We show that parametric integration of policies can improve the out-of-sample accuracy of probabilistic projections for most countries and models, while policy in early years of technology diffusion is the most decisive. Looking forward until 2040, an increase or decrease in policy efforts compared to today has limited effect on the projections of solar photovoltaics and onshore wind power in Europe though. This supports the view that weaker policies at this stage do not harm much, but stronger policies can be insufficient for targets in several countries either.