Planning reliable wind- and solar-based electricity systems

•Year-to-year weather variability substantially impacts least-cost electricity system planning.•Performance of designed systems is influenced by the length and variability of weather data used.•Planning highly reliable systems with wind, solar, and energy storage necessitates about 40 years of weather data.•Incorporating traditional dispatchable generation can achieve similar reliability with 15 years of weather data. Resource adequacy, or ensuring that electricity supply reliably meets demand, is more challenging for wind- and solar-based electricity systems than fossil-fuel-based ones. Here, we investigate how the number of years of past weather data used in designing least-cost systems relying on wind, solar, and energy storage affects resource adequacy. We find that nearly 40 years of weather data are required to plan highly reliable systems (e.g., zero lost load over a decade). In comparison, this same adequacy could be attained with 15 years of weather data when additionally allowing traditional dispatchable generation to supply 5 % of electricity demand. We further observe that the marginal cost of improving resource adequacy increased as more years, and thus more weather variability, were considered for planning. Our results suggest that ensuring the reliability of wind- and solar-based systems will require using considerably more weather data in system planning than is the current practice. However, when considering the potential costs associated with unmet electricity demand, fewer planning years may suffice to balance costs against operational reliability. [Display omitted]