Substitution potential of surplus electricity-based e-methane production for LNG: The case of Kyushu and Hokkaido in Japan

• Storage and demand constraints push systems toward self-sufficiency • E-methane production in photovoltaic-rich regions is limited by low full-load hours. • Offshore wind curtailment creates favourable conditions for e-methane. • Curtailment may be reduced by 2–54% in Hokkaido through e-methane production. • Regional gas self-sufficiency reaches 1–23% at base LNG market price in Hokkaido. This paper investigates domestic e-methane production by utilizing surplus electricity in Japan as a complementary strategy to imports. Kyushu and Hokkaido are selected as case study regions due to the challenges of decarbonization and increasing curtailment in these areas. Two scenarios are defined, which represent differences in renewable deployment rates and electrification across different sectors. A regional energy system model is applied to quantify future electricity curtailment under different LNG price scenarios for 2030 and 2040 and to assess the role of e-methane as a flexibility option alongside batteries and hydrogen storage. The results indicate that renewable electricity curtailment becomes a structurally relevant challenge in both regions, driven by high solar penetration in Kyushu and offshore wind expansion in Hokkaido, combined with limited transmission capacities. Curtailment ranges from 1.5 to 6.4 TWh in Kyushu and 0.4–3.5 TWh in Hokkaido, enabling the installation of up to 542 MW and 1260 MW of e-methane production capacity, respectively. In Kyushu, e-methane can act primarily as a short-term transitional flexibility option, complementing storage technologies during phases of rapid PV expansion but losing relevance as transmission capacities are increased and short-term storage systems being deployed. In Hokkaido, e-methane may contribute to the energy system by supporting seasonal balancing, increasing regional self-sufficiency and reducing exposure to LNG imports under conditions of limited interconnection and frequent offshore wind curtailment. At the base LNG price, the impact of e-methane on curtailment reduction is generally modest. Nevertheless, reductions of approximately 41% in Kyushu in 2030 and up to 54% in Hokkaido in 2040 under low additional demand are observed, reflecting the strong dependence of e-methane deployment on the availability of renewable surplus electricity.