Beyond S-Curves: Phases, Models, and Metrics of Growth for Policy-Driven Energy Technologies

Debates about the future of wind and solar power are often framed around a false binary: either these technologies will continue accelerating or they are already losing momentum. Both views assume that renewables follow an ideal S-curve with near-exponential expansion followed by slowdown. Using three decades of national deployment data, we show that renewables do not follow ideal S-curves. Instead, they pass through a formative phase of erratic growth, followed by takeoff and a brief period of acceleration that ends at surprisingly low levels of market penetration. Renewables then enter a prolonged steady growth phase characterised by a cruising speed punctuated by pulses of acceleration and deceleration. We present a set of tools to diagnose these phases and measure growth at each phase. Hindcasting shows that these tools are more robust than year-on-year trends, fitted S-curve parameters, or historical analogies. We also demonstrate that peak growth rates in frontrunner countries provide a reliable upper bound for global expansion. We argue that the observed patterns arise from the strong dependence of renewables on government policies and that the proposed approach is relevant for data-driven projections of other policy-dependent energy technologies.