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A few reasons why we can be a bit more confident than the starting probabilities here:

Ofgem RIIO-ET3 Final Determinations (December 2025):

  • Total price control revenue reduced 5.3% (from £48.8bn to £46.2bn over the ET3 period)

  • Revenue explicitly reduced in 2026/27 and 2027/28 (increases in later years to smooth the consumer bill impact)

Plus, NESO's tariffs for 2026/27 ended up substantially below the reference class-based forecasts (£7.61bn, 14.7% below the September 2025 forecast of £8.918bn). Was driven by lower Ofgem allowances, changes to how they're handling expenses from offshore wind, and updated forecasts on demand and embedded generation.

Got Claude Code to generate my forecasts via the CLI, using lognormal distributions and the anticipated 8% reductions in the coming years:

Year

NESO Sep-25

My Median

My Sigma

Rationale

2027/28

£10.278bn

£9.5bn

0.13

RIIO-ET3 reprofiling reduces early years; ~8% downward revision; NESO initial forecast due April 2026

2028/29

£11.657bn

£11.0bn

0.17

5.3% envelope cut partially offset by reprofiling towards later years

2029/30

£12.685bn

£12.5bn

0.20

Reprofiling adds revenue to later years; roughly in line with September forecast

2030/31

£13.629bn

£13.5bn

0.22

Strongest reprofiling benefit; widest uncertainty at 4-5 year lead

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