CALC-139
Great Britain solar output by month and by day in 2024, from NESO's half-hourly data
What this is
This is the project's own arithmetic, published in full so it can be checked rather than taken on trust. It is not a source. Every input it uses is cited to a record, and the conclusion it reaches only carries as far as those inputs do.
REGISTERED 2026-10-14. Drafted for lesson C0 (lessons records draft G, commit d24881a4), checked by an independent reader (FIX, all applied, round 2 PASS), landed at SBSP-coord's request. Script: lessons-solar-month.py. verified: VER-2387 CONFIRMED 2026-10-14.
Basis: MW of the dataset's SOLAR column, averaged over half-hours; daily MWh = sum of half-hourly MW times 0.5 h; Great Britain; timestamps as printed (UTC).
Script. lessons-solar-month.py, run with python -I on the filed CSV.
Why it was needed. Lesson C0 beat 1 says ground solar makes little in winter. No record gave winter output against summer output for Great Britain.
Inputs.
| Quantity | Value | Source | Stated or assumed |
|---|---|---|---|
| Half-hourly MW by source, 2009 to 2026-10-13 | columns SOLAR, GENERATION | SRC-788 | STATED (data) |
| Solar | SOLAR only (transmission-connected solar is under OTHER per NESO's dataset page) | SRC-788 row | STATED |
| Month mean | mean of SOLAR over every half-hour in the month, night included | this record | CHOSEN |
| Day energy | sum of the 48 half-hourly MW values times 0.5 h; only days with 48 rows | this record | CHOSEN |
| Years | 2024 for the claim; 2023 and 2025 as checks | this record | CHOSEN |
Working, transcribed (script output, run 2026-10-14).
sha256 aba35d17363348305819200a8fda707868fc40dfec21c3eddc58dd4e9f5604f3 rows 311745 | floor 300000: True 2024: month mean SOLAR MW solar share of GENERATION % half-hours 1 608 1.6 1488 7 2614 9.1 1488 2 770 2.2 1392 8 2460 8.5 1488 3 1413 4.2 1488 9 1684 5.7 1440 4 2098 6.5 1440 10 1184 3.8 1488 5 2479 8.6 1488 11 573 1.6 1440 6 2973 10.3 1440 12 332 0.9 1488 Dec/Jun mean ratio 0.112 (Jun 2973 MW, Dec 332 MW); full days 366 highest day 2024-06-02 98,177 MWh; lowest day 2024-11-23 1,369 MWh; ratio 0.014 December: lowest 2024-12-11 2,408.5 MWh, highest 2024-12-19 19,724 MWh; low/high 0.122 2023: Dec/Jun 0.102 (Jun 2899, Dec 296); highest day 2023-06-14 93,474.5 MWh; lowest 2023-12-27 958 MWh; ratio 0.010 2025: Dec/Jun 0.155 (Jun 3598, Dec 558); highest day 2025-07-08 117,894 MWh; lowest 2025-12-18 1,414 MWh; ratio 0.012 control: calendar 2024 SOLAR share of GENERATION 5.0 per cent (CALC-134: 5.0)
The 2024 month table is set out in two columns; the 2023 and 2025 lines are abridged.
The named days (SOLAR column, 48 rows each):
2024-11-23: n=48 peak=347 MW zero half-hours=33 sum=1,369 MWh 2024-12-11: n=48 peak=608 MW zero half-hours=34 sum=2,408.5 MWh 2024-12-19: n=48 peak=4,651 MW zero half-hours=32 sum=19,724 MWh 2024-06-02: n=48 peak=11,561 MW zero half-hours=15 sum=98,177 MWh 2023-12-27: n=48 peak=253 MW zero half-hours=33 sum=958 MWh
Result: December 2024 mean solar output was 0.112 of June 2024's (332 against 2,973 MW). The worst day of 2024 gave 0.014 of the best (1,369 against 98,177 MWh). Within December 2024 the worst day gave 0.122 of the best (2,408.5 against 19,724 MWh).
Where this fails. - Coverage. SOLAR leaves out transmission-connected solar farms (NESO counts them under OTHER). NESO says updates to show transmission solar will follow later in 2026, so a later download will differ. - Modelled values. NESO says seasonal decomposition corrects missing or irregular points. How the SOLAR estimate is made was not checked. - Capacity. The figures are not adjusted for installed capacity, and no held record gives it by month. If the fleet grew between June and December, that raises December against June, and the ratio for a fixed fleet would be lower, not higher. - Weather is inferred, not recorded. The file has no cloud data. The December pair is 8 days apart and both are within 10 days of 21 December, so day length changes little between them. - Year to year. 2025 monthly means differ from 2024 by -3 to +68 per cent (March 2,380 against 1,413 MW). The cause (capacity, method or weather) was not checked. - Within December. The 2024 low/high ratio (0.122) is the mildest of the three years; 2023 gives 0.047 (958 against 20,305 MWh) and 2025 gives 0.050 (1,414 against 28,130.5 MWh). - An independent figure disagrees on size. SRC-781 Figure 5 (Met Office, 1979 to 2013) gives a December median of daily output near 0.055 of June's; that is a ratio of medians, not of monthly means. Both show the same direction; the size differs by about two, and the reason was not checked.
Cite this
energyaperture.com/calc/calc-139
CALC-139. The working above, not a source document.