Rethinking S-curves 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 S-curve with near-exponential expansion followed by slowdown. Using national deployment data, we show that renewables’ growth departs from ideal S-curves. Instead, it proceeds through a formative phase of erratic growth, followed by takeoff and brief acceleration that ends at low levels of market penetration (≈3% of electricity generation). Renewables then enter a prolonged steady growth phase punctuated by pulses of acceleration and deceleration, with an overall cruising speed that is slower than the first growth peak (≈0.7 percentage points [p.p.] year⁻¹; interquartile range [IQR]: 0.4–1.5 for wind and 0.3–1.9 for solar). We develop diagnostic tools and metrics for these phases. Hindcasting shows that these outperform year-on-year trends and fitted S-curve parameters. Peak growth in front-runner countries provides a reliable upper bound for global expansion.