Inter-annual electricity balancing in 100% renewable energy systems assessed through a techno-economic comparison of data series length, e-hydrogen versus e-methane, and multi-objective solutions

Ensuring year-round reliability in isolated 100% renewable energy systems necessitates a strategic combination of generation overcapacity and inter-annual storage, but its optimal design is challenged by uncertainties in weather data and technology choice. This study performs a comprehensive global techno-economic analysis comparing electricity-based hydrogen and electricity-based methane as storage energy carriers across 22-year and 85-year weather datasets. Results reveal electricity-based methane as the superior economic choice for inter-annual electricity balancing, with a global demand-weighted annualised inter-annual balancing system cost approximately 74% lower than for electricity-based hydrogen, driven by significantly cheaper underground storage. Furthermore, this research quantifies the critical risk of under-planning. In the curtailment-optimised case, using 85 weather years increases the required variable renewable energy overcapacity by approximately 57% and storage volume by about 58%, leading to a cost markup increase of roughly 58-59% on a global demand-weighted average compared to the shorter dataset. The cost-optimised case requires the demand-weighted average overcapacity to increase from 5.2% to 8.1%, and the total annualised cost markup of the balancing system to increase from 2.9% to 4.5%. To translate these findings into actionable policy, this work introduces the ‘tau-analogy’, a novel heuristic that systematically identifies balanced and cost-effective solutions on the Pareto front of trade-offs between system cost and inter-annual storage sizing. This research provides strong evidence regarding the hydrogen versus methane debate for inter-annual storage and establishes a new benchmark for data robustness in energy system planning, offering a pragmatic framework for designing economically viable and physically resilient future energy systems.