BSUoS from 1 October 2026: NESO fixes winter at £12.49/MWh, and what to quote

The number, and how it compares

From 1 October 2026, Balancing Services Use of System (BSUoS) is charged at £12.49/MWh. That is Fixed Tariff 8, covering 1 October 2026 to 31 March 2027, published by NESO on 29 December 2025 alongside the tariff currently running.

The comparisons brokers need in front of them:

• Fixed Tariff 7 (April to September 2026): £13.74/MWh. The winter rate is £1.25/MWh lower than the summer half-year now ending.
• Fixed Tariff 6 (October 2025 to March 2026): £15.69/MWh. Winter 2026/27 is £3.20/MWh below last winter, about 20 per cent.
• Fixed Tariff 5 (April to September 2025): £10.74/MWh. Summer 2026 sat well above the previous summer, which is why some clients felt their non-commodity line rise in April even as headlines said BSUoS was falling.

The pattern matters more than any single figure. The seasonal gap has narrowed sharply. Last year the winter tariff was nearly £5/MWh above the preceding summer; this year winter sits below summer. Anyone quoting on the old assumption that winter BSUoS is always the expensive half will over-price an October start.

Full detail sits in NESO's BSUoS charges pages and the final 2026/27 tariff publication.

Why it is fixed ahead, and why demand pays all of it

Two code changes did the work. CMP308 removed BSUoS from generation with effect from 1 April 2023, so the charge falls wholly on demand. Chargeable volume roughly halved overnight, which is why the £/MWh rate roughly doubled without the underlying cost pool changing. CMP361 and CMP362 then replaced the old arrangement, where BSUoS was settled in arrears at a rate that moved every half hour, with a fixed tariff set in advance for a defined period.

Things changed again for this charging year. Ofgem approved CMP408 and CMP415 (WACM1) in October 2024, taking effect from 1 April 2025. The notice period dropped from nine months to three, and the fixed price period moved from six months to twelve, split into two seasonal six-month tariffs published together each December. Tariffs 7 and 8 were the first pair set this way. NESO now publishes an initial forecast each June, draft tariffs each September and final tariffs each December. Draft Tariffs 9 and 10, covering April 2027 to March 2028, were published on 21 September 2026, with finals due in December. Background on the modifications sits with the CUSC panel material.

Keep one caveat in your back pocket: fixed does not mean immovable. CUSC carries reset mechanisms allowing NESO to re-open a tariff in extreme circumstances. That power has not been used, but it explains why careful pass-through clauses still exist.

Where it lands on a half-hourly bill

BSUoS is a volumetric charge applied to half-hourly metered demand, settled through Elexon's BSC settlement runs and invoiced by NESO to suppliers. On a client's invoice it should appear as a single line: consumption for the period multiplied by the published £/MWh, usually grossed up for distribution and transmission losses because the chargeable volume is measured at the transmission boundary rather than at the meter.

That grossing-up is legitimate and typically adds around 6 to 9 per cent depending on voltage and distribution region. It is also the most common hiding place for margin.

Fixed, pass-through and flex: who carries the 1 April risk

On a fully fixed contract, BSUoS is baked into the unit rate. The supplier carries any movement, which since April 2023 is close to zero risk for periods already published. For a contract straddling 1 April 2027, the supplier is pricing an unknown tariff only if the quote is issued before December 2026. From January 2027 the April 2027 rate is known, and any "BSUoS risk premium" in a fixed price for 2027/28 is hard to justify.

With pass-through, the customer takes the tariff change on 1 April automatically. Budgets should be built as two half-years, not one annual average.

Under flex, it depends on whether the non-commodity basket is fixed for the term or passed through. Many flex products fix commodity and pass through everything else, so the 1 April step change lands with the client. Read the schedule rather than the sales summary.

The practical point for a straddling contract: whoever holds the risk, the change is now foreseeable. Anyone charging for uncertainty across 1 April 2027 after mid-December 2026 is charging for something that no longer exists.

Worked example: a 250 MWh SME site

Take a site on 250 MWh a year, with a typical 45 per cent summer and 55 per cent winter split, so 112.5 MWh in the April to September half and 137.5 MWh in the October to March half.

Contract year April 2026 to March 2027:

• Summer: 112.5 MWh x £13.74 = £1,545.75
• Winter: 137.5 MWh x £12.49 = £1,717.38
• Total: £3,263.13, a blended £13.05/MWh

Same site, April 2025 to March 2026:

• Summer: 112.5 MWh x £10.74 = £1,208.25
• Winter: 137.5 MWh x £15.69 = £2,157.38
• Total: £3,365.63, a blended £13.46/MWh

The year-on-year saving is about £102, roughly £0.41/MWh, before losses. Apply an 8 per cent loss factor and the billed figure for 2026/27 rises to about £3,524.

Where a contract starts on 1 October 2026, the first six months are locked at £12.49/MWh, giving £1,717 for the winter half. The summer 2027 half is not final until December 2026, so use NESO's draft Tariff 9 and add prudence.

What allowance to build into renewal quotes

For supply periods up to 31 March 2027, quote the published figures and nothing more: £12.49/MWh from October, £13.74/MWh for the summer just ending.

Beyond that date, look at how drafts have travelled. Tariff 7 went £14.55 initial, £14.21 draft, £13.74 final. Tariff 8 went £11.03, £11.93, £12.49. Movement between draft and final was in the order of £0.50 to £0.60/MWh in both directions. A prudence margin of around £0.75/MWh over the September draft is defensible for a 2027/28 quote issued now; anything above £1.50/MWh needs explaining.

Multi-year quotes running past March 2028 can use a working allowance in the £12 to £14/MWh range before losses, given the last four published tariffs have sat between £10.74 and £15.69. Flag it as an assumption in the quote rather than burying it.

Spotting over-recovery in a fixed price breakdown

Ask for the non-commodity breakdown in £/MWh and run these checks:

• Does the stated BSUoS rate match the published tariff for each half-year of the term? A single annual number for a term spanning 1 April is a red flag on its own.
• Back out the implied loss factor. If the quoted rate divided by the published tariff implies more than about 1.10, ask what the extra covers.
• Look for a separate "BSUoS risk" or "balancing risk" line sitting on top of a tariff that is already fixed and published.
• Check for reconciliation wording that allows retrospective BSUoS recovery. Since April 2023 the only legitimate trigger is a NESO tariff reset under CUSC, so the clause should say so explicitly.
• Confirm BSUoS is applied to half-hourly volume for the correct dates, not apportioned across a calendar year.

Checks to run before your next renewal

Pull the client's current contract and confirm three things: the BSUoS treatment, the loss factor applied, and whether the term crosses 1 April. Then take twelve months of half-hourly data and split it into summer and winter halves rather than using an annual average, because the seasonal split now drives the answer. Compare the last three invoices against £13.74/MWh grossed up, and query anything more than a few per cent adrift. Diarise mid-December 2026 for NESO's final Tariffs 9 and 10, then refresh any 2027/28 quotes issued before then. Clients on pass-through should get a two-line budget note showing the October step down to £12.49/MWh, so the credit on their winter bills is expected rather than queried.