European green-hydrogen assessments often infer conversion opportunity from annual renewable quantities even though electrolyser output depends on both electricity chronology and technology-specific operating constraints. This study develops a harmonised physical screening framework for 40 European countries that separates annual renewable-energy scale from hourly chronology and propagates uncertainty through alkaline (AEL), proton-exchange-membrane (PEM) and anion-exchange-membrane (AEM) electrolysis. The renewable-performance gap is defined as the positive difference between reference generation and generation associated with an empirical upper-performance benchmark derived from country- and technology-specific historical capacity factors. It is a counterfactual screening quantity rather than observed curtailment or market-available surplus electricity. For the 2024 baseline, 2025 hourly series are used exclusively as normalised chronology donors and contain no 2025 annual generation or capacity information. All positive-gap hours are converted through 5,000 aligned Monte Carlo simulations including electricity capture, sizing, availability, minimum load, specific electricity consumption and delivery losses. The descriptive European aggregate P50 is 1.919, 3.104 and 6.094 Mt H2 yr-1 in 2024, 2030 and 2050. A joint alternative specification yields medians 1.5– 6.3% below the central estimates while preserving country rankings (𝜌 > 0.99; unchanged top ten). A controlled common-engine factorial sensitivity shows that replacing the 2025 chronology with a 2019–2024 multi-year profile changes P50 by only 0.4–0.8%, whereas broadening the upper- performance benchmark changes it by 5.9–8.2%. Benchmark characterisation is therefore the larger source of specification sensitivity, while the broad spatial ordering remains stable.