Moving ceilings: Out-of-sample evidence on growth-curve forecasts of national wind and solar power diffusion
Hareketli tavanlar: Ulusal rüzgâr ve güneş enerjisi yayılımına ilişkin büyüme eğrisi tahminlerinin örneklem dışı kanıtı
This paper is under review at a peer-reviewed journal. The journal name and full text will be added after acceptance.
Abstract
Logistic and other S-shaped growth curves are widely used to project the diffusion of wind and solar power and to infer saturation levels from early data. This paper tests whether they forecast better than simple trend methods. Using annual generation in 47 national wind and 35 national solar markets, it produces 13,440 rolling-origin forecasts from seven methods at origins 2014–2019 and horizons of one to five years, and it tracks how fitted ceilings change as data accumulate. The logistic curve was no more accurate than exponential extrapolation in mean rank and was significantly less accurate than the Gompertz and Bass curves, a damped-trend model and an equal-weight combination; the damped trend was the most accurate single method for both technologies. All S-curves under-forecast: five years ahead, actual generation exceeded the logistic forecast in 73% (wind) and 77% (solar) of cases. Ceilings were unstable. The median ceiling estimated with data to 2024 was 2.4 times (wind) and 11.1 times (solar) the ceiling estimated with data to 2014, 83% and 67% of the 2014 ceilings had been exceeded by 2024, and up to a third of fits ended on the bounds of the parameter space. The findings indicate that the carrying capacity of renewable electricity diffusion is policy- and cost-dependent rather than a fixed property of national systems. Planners should treat S-curve ceilings as scenarios rather than forecasts, combine growth curves with damped trends and re-estimate ceilings every year.
Keywords
Technology diffusion; Logistic growth; Renewable electricity; Forecast evaluation; Damped trend; Carrying capacity; Energy transition
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