Scope and limits

NESO counts transmission-connected solar farms under OTHER, not SOLAR, so the absolute values understate Britain's solar output. The ratios are probably less affected, if transmission-connected farms follow the same seasons, but neither this nor how NESO derives SOLAR was checked. NESO says some values are modelled by seasonal decomposition and that the data are subject to change by a cleansing process, so the figures are for the file fetched 2026-10-13 (as for CLM-1431). The figures are not adjusted for installed capacity, and no held record gives Britain's solar capacity by month. If capacity grew between June and December 2024, the ratio for a fixed set of panels would be lower, not higher. The data do not record cloud. 11 and 19 December both fall within ten days of the shortest day of the year, so the gap between them is most likely weather, but that is an inference. Other years give similar results: December over June 0.102 in 2023 and 0.155 in 2025, and worst day over best day 0.010 in 2023 and 0.012 in 2025 (CALC-139). Within December the spread is wider in those years: the worst day gave 0.047 of the best in 2023 and 0.050 in 2025, against 0.122 in 2024. The 2025 monthly means differ from 2024's by -3 to +68 per cent, and the cause was not checked. A figure in a Royal Society report on Great Britain's electricity storage, built on Met Office weather data for 1979 to 2013 (SRC-781, Figure 5, read from the graph to about plus or minus 0.03), shows a larger swing: the median daily solar output is about 0.11 of the all-year average in December and about 1.98 in June, near one eighteenth. These are medians of daily output, not monthly means, and the reason for the difference from the NESO ratio was not checked. These are national totals, and one site can differ.