The S-Curve of Surplus Charging: Why Renewable Surplus Becomes Relevant for Electric Vehicles

The expansion of variable renewable generation increasingly creates periods in which renewable electricity could exceed national demand. Battery-electric vehicles can absorb part of this energy if charging is shifted to these periods. Existing studies commonly focus on individual target years, specific charging strategies, or local grid conditions, while the longitudinal development of national renewable-surplus charging potential across historical backcasts and prospective scenario years remains less established. This paper introduces the descriptive S-curve of surplus charging for Germany. A quarter-hourly model scales the 2023 generation profiles of solar, onshore wind, and offshore wind according to policy-based capacity pathways from 2019 to 2035, compares renewable generation with electricity demand, and maps the resulting surplus to the modeled daily energy requirement and aggregate storage capacity of the electric-vehicle fleet. The modeled share of annual EV charging demand that can be supplied by renewable surplus rises from 11% in 2023 to 89% in 2028 and then approaches saturation at 97% in 2035. In 2035, however, the fleet absorbs only 27% of total modeled surplus, indicating that EV charging and stationary storage can serve as complementary rather than competing flexibility options. Within the modeled sequence, two complementary relevance criteria are applied: 2023 marks the beginning of the steep transition phase, while the illustrative 50% annual-coverage threshold is crossed in the 2026 scenario. In addition, the model indicates that sufficient surplus to cover the fleet’s daily charging demand is available on more than half of all days from the 2027 scenario onward. The results indicate that surplus charging changes from a niche opportunity into a system-relevant flexibility option during the 2020s. The result is scenario-dependent and should be interpreted as a potential estimate rather than a forecast of realized charging behavior. Although the numerical transition years are specific to Germany, the framework can be applied to other power systems to identify country-specific surplus-charging transitions as variable renewable generation expands. Index Terms — s-curve of surplus charging, surplus charging, electric vehicles, renewable energy, smart charging, surplus electricity, demand-side flexibility, sector coupling.