Capped wind power density as a turbine-agnostic metric for interannual wind variability

Interannual variability (IAV) of wind resources is a key source of uncertainty in wind energy planning and financial risk assessment. However, IAV is commonly quantified either from annual mean wind speed, which is not directly proportional to energy yield, or from annual energy production, which is turbine-specific and thus difficult to compare across fleets and regions. To address this limitation, capped wind power density ( WPD c ) is introduced as a turbine-agnostic, production-oriented metric that explicitly accounts for cut-in, rated, and cut-out wind speeds. This study assesses IAV across Germany for the period 1991–2024 using high-resolution modeled wind speed time series at 29,807 wind turbine locations. Compared to wind speed, WPD c provides a more representative measure of production-relevant energy availability. IAV is quantified using the coefficient of variation. Onshore wind speed exhibits an IAV of 3.5%, while offshore values are slightly lower at 3.2%. For WPD c , IAV increases to 8.5% onshore and 5.1% offshore, highlighting the nonlinear amplification of variability in energy-relevant metrics. A convergence analysis indicates that at least 14 years of data are required for robust IAV estimation. Overall, the WPD c framework provides a physically consistent and production-relevant basis for improved wind resource risk assessment.