On 20 May 2026, Britain’s National Energy System Operator (NESO) issued a market notice restricting its ability to reverse power flows across interconnectors with France, Denmark, the Netherlands, and Belgium [2]. The restriction, effective immediately and in place until year-end, is the latest consequence of a structural problem that NESO’s own statutory reporting has been documenting for several years. According to the 2025 Annual Balancing Costs Report — published under Condition C9 of the NESO Electricity System Operator Licence — it cost £2.7 billion to balance Great Britain’s electricity grid in the regulatory year April 2024 to March 2025 [1]. Thermal constraints were the dominant driver, accounting for £1.7 billion of that total, with constrained generation volumes rising 81 per cent year-on-year to 13.5 TWh. On current trajectories, NESO projects balancing costs will peak at approximately £8 billion per year in 2030 [1].
The mechanism is by now well documented. According to Scottish Government energy statistics, Scotland had 15.6 GW of operational renewable electricity generation capacity as of mid-2024, with wind representing the substantial majority [11]. The B6 boundary — the transmission corridor connecting Scottish generation to English demand centres — has a boundary capability of 6.7 GW, as identified in NESO’s Electricity Ten Year Statement [12]. Scotland’s installed generation capacity, therefore, substantially exceeds what its transmission corridor south can carry at any one time. When wind output is high, the surplus has nowhere to go, so it is curtailed. Gas is dispatched south of the constraint boundary to replace it. Consumers pay twice. Britain is, in short, generating clean electricity faster than its grid can absorb it — and paying a substantial and growing price for that mismatch.
For South Africa, this story is instructive in ways that extend beyond its immediate policy context. It provides, at scale and in real time, an independent empirical validation of a diagnostic framework whose application in South Africa I have argued for elsewhere [3,4]. The framework is the Cliff Intensity Index (CII): a dimensionless velocity ratio that measures whether the rate at which baseload capacity is retiring is matched by the rate at which replacement capacity can be integrated onto the grid. Britain’s constraint crisis is what the index looks like below 1.0 — when integration velocity runs ahead of retirement velocity, and the grid cannot absorb the generation it is being asked to carry. South Africa’s 2030 cliff is what the index looks like at 2.16 — when retirement velocity runs 116 per cent ahead of integration velocity, and the grid cannot replace the generation it is being asked to lose.
The Index as a Two-Sided Diagnostic
The CII was introduced as a diagnostic tool for coal-dependent emerging economies facing accelerated phase-out timelines [4]. Its formulation is straightforward: the numerator is the annualised rate at which baseload capacity exits the system; the denominator is the annualised rate at which total replacement capacity can be integrated, accounting for full transmission infrastructure lead times and scaled by an institutional friction multiplier. When the ratio exceeds 1.0, the system faces a risk of adequacy deficit. When it falls below 1.0, the system faces the inverse condition: surplus integration velocity generating curtailment pressure, transmission bottlenecks, and balancing cost escalation.
Britain’s constraint crisis is what the CII looks like when integration velocity outruns retirement velocity. Its offshore and onshore wind build programme has proceeded at a pace that the transmission network — designed for a different generation geography — cannot route to demand centres. The result is a system operator instructing generators to curtail clean output, dispatching gas as a replacement, and now restricting the cross-border trading instruments that had previously provided a pressure-relief valve [2]. NESO’s own projection that these costs will reach £8 billion per year by 2030 without accelerated transmission build [1] is, through the lens of the CII, the price of that uncorrected mismatch. Hirth et al. [8] identified the theoretical basis for this integration cost escalation a decade ago; Britain is now providing the empirical confirmation at the national-system scale
South Africa faces the opposite condition — and its consequences are categorically more severe. When integration outruns retirement, the cost is a balancing bill. When retirement outruns integration, the cost is darkness. As I established in a recent analysis of the Minister of Forestry, Fisheries and the Environment, Dr D.T. George’s determination of 31 March 2025 [3], the minimum emission standards (MES) compliance deadlines for coal stations are now legally binding to 1 April 2030, with no provision for further extension [7]. The retirement velocity that produces South Africa’s CII of 2.16 is not a modelling assumption. It is a ministerial determination.
What Britain’s Experience Adds to the South African Argument
The comparative value of the British case is not that it offers solutions transferable to South Africa — the structural conditions are sufficiently different to make direct policy translation hazardous. Britain’s transmission constraint is a routing problem: the generation exists, the demand exists, and the bottleneck is the corridor between them. South Africa’s cliff is an adequacy problem: the retiring generation will not be replaced on the relevant timeline regardless of how the wires are configured, because the transmission infrastructure required to integrate replacement capacity requires five to seven years of greenfield construction that no governance reform can compress [4,9]. At the 765 kV transmission voltages that South Africa’s projected generation mix requires for bulk power transfer across the distances involved, those lead times reflect environmental authorisation, materials procurement, and civil construction realities — not institutional pathology.
What Britain’s experience does provide is something more fundamental: it demonstrates, at a scale and in a system too large to dismiss as a special case, that the velocity of change in an electricity system is a binding physical constraint, not a planning variable subject to political adjustment. That this constraint manifests in a system operating with mature regulatory institutions, deep capital markets, and a technically capable independent system operator removes the most readily available explanatory exit: the problem is not institutional in origin. It is physical. NESO cannot wish away the B4 and B6 transmission boundaries. It cannot instruct Scottish wind farms to stop producing when the system is constrained, and simultaneously pretend the constraint does not carry a cost. The £1.7 billion thermal constraint bill in 2024/25 [1] is the market’s invoice for a velocity mismatch that planners did not price in when siting generation without commensurate transmission investment.
South Africa’s energy planning community faces an analogous reckoning, in the opposite direction. As the Medium-Term System Adequacy Outlook 2026–2030 establishes through probabilistic simulation [6], and as the CII establishes through velocity analysis [4], the convergent finding is 4,337 GWh of unserved energy in 2030 under the risk-adjusted scenario in which gas bridging is delayed. Two independent methodologies, neither designed to confirm the other, point to the same consequence of the same underlying condition: a retirement velocity that the grid cannot match. The sensitivity analysis in Koko (2025) [4] confirms that this finding is structural rather than institutional — even under perfect governance, the CII floor is 1.80, because the physics of greenfield transmission construction cannot be legislated away any more than NESO can legislate away the B6 boundary.
The Shared Lesson and Its Limit
Britain and South Africa share one lesson and diverge sharply on a second. The shared lesson is that energy transition planning, which models destination states without modelling transition velocity, is not conservative — it is physically incomplete. A national electricity grid is not a spreadsheet. Energy security is not a planning target that can be averaged across time or hedged with policy aspiration: it is a physical obligation discharged in every settlement period, continuously, against inertia, frequency, and voltage constraints within a physical network whose capacity to carry power between points is determined by decades-old infrastructure.
In his 2026 State of the Nation Address, President Ramaphosa declared that by 2030, more than 40 per cent of South Africa’s energy supply would come from renewable energy sources [10]. That is a destination statement. The CII asks the prior question: whether a physically realisable pathway to that destination exists at the velocity the timeline demands. Planners who project a destination without calculating the kinetics of the route are not being optimistic. They are omitting the physics of the problem entirely.
The divergence is in consequence. Britain’s incomplete planning has produced a constraint-cost problem: expensive, politically contentious, and now spilling across its European interconnectors, but ultimately a problem of economic efficiency rather than of system survival. South Africa’s incomplete planning, if left unaddressed, produces an adequacy problem: not a higher electricity bill, but an absent electricity supply. The trilemma I identified in the context of the George determination [3] — adequacy risk, climate finance forfeiture, and gas lock-in — has no analogue in the British case. Britain can afford to resolve its velocity mismatch over the remainder of this decade through network investment; it faces that challenge with the fiscal capacity to accelerate infrastructure spend when the political cost of constraint bills becomes visible to consumers, and against a demand trajectory that, while growing with electrification, does not exhibit the structural growth pressure that South Africa’s urbanisation and industrialisation imply. South Africa must resolve its velocity mismatch amid a legally fixed retirement cliff, an uncertain gas bridging programme, a £13.7 billion JETP financing architecture conditioned on demonstrable progress in the coal phase-out, rising electricity demand, and tighter fiscal constraints on the network investment needed to close the gap. To characterise this as primarily a governance problem susceptible to institutional remedy is to misidentify the binding constraint. A governance reform that halved transmission lead times from seven years to three would still leave the 2030 adequacy gap structurally intact, because the required build programme has not begun. The constraint is physical and temporal, not administrative.
The CII framework was designed to make this distinction measurable rather than qualitative [4]. Britain’s constraint crisis confirms that the framework’s predictions for this condition are empirically grounded — not theoretical constructs but observable system behaviours with quantifiable costs. That external validation directly matters to the credibility of its South African diagnosis. If the index correctly identifies the pathology of a velocity mismatch running in one direction, the burden of proof on those who would dismiss its reading running in the other direction is considerably higher.
South Africa’s planning community would do well to take note. The cliff is not a scenario. It is a determination [7]. And the velocity problem it represents does not resolve itself over time, as Britain is discovering, at £1.7 billion per year and rising, from the other side.
References
[1] National Energy System Operator (NESO). 2025 Annual Balancing Costs Report. Published June 2025 under Condition C9 of the NESO Electricity System Operator Licence. Retrieved from https://www.neso.energy/document/362561/download
[2] National Energy System Operator (NESO). Interconnector Trading Restriction — Market Notice. Effective 20 May 2026. Retrieved from https://www.neso.energy/industry-information/balancing-services/trading
[3] Koko, M. The Minister’s Determination and the Legislated Capacity Cliff: What South Africa’s Energy Planning Community Must Now Confront. African Times, 4 May 2026. Retrieved from https://www.atnews.co.za/the-ministers-determination-and-the-legislated-capacity-cliff-what-south-africas-energy-planning-community-must-now-confront/
[4] Koko, M. South Africa’s 2030 Electricity Capacity Cliff: Institutional Frictions, Sociotechnical Inertia, and the Political Economy of Accelerated Coal Phase-Out. Manuscript submitted to Energy Policy, 2025. Available at SSRN: https://doi.org/10.2139/ssrn.5794522
[5] Vinichenko, V.; Jewell, J.; Cherp, A.; Jakhmola, A. Historical precedents and feasibility of rapid coal and gas decline required for the 1.5°C target. One Earth 2021, 4(10), 1477–1490.
[6] National Transmission Company South Africa. Medium-Term System Adequacy Outlook 2026–2030. Johannesburg: NTCSA, 2025.
[7] George, D.T. Decision by the Minister of Forestry, Fisheries and the Environment in respect of the exemption applications submitted by Eskom SOC (Pty) Ltd in terms of Section 59 of the National Environmental Management: Air Quality Act, 2004 (Act No. 39 of 2004). Pretoria: Department of Forestry, Fisheries and the Environment, 31 March 2025.
[8] Hirth, L.; Ueckerdt, F.; Edenhofer, O. Integration costs revisited — An economic framework for wind and solar variability. Renewable Energy 2015, 74, 925–939.
[9] National Transmission Company South Africa. Transmission Development Plan 2024. Johannesburg: NTCSA, 2024.
[10] Ramaphosa, C. State of the Nation Address by President Cyril Ramaphosa. Parliament of South Africa, Cape Town, 12 February 2026. Published by The Presidency of the Republic of South Africa. Retrieved from https://www.thepresidency.gov.za/state-nation-address-president-cyril-ramaphosa-1
[11] Scottish Government. Energy Statistics for Scotland Q2 2024: Renewable Energy Capacity. Edinburgh: Scottish Government, September 2024. Retrieved from https://www.gov.scot/publications/energy-statistics-for-scotland-q2-2024/pages/renewable-energy-capacity/
[12] National Energy System Operator (NESO). Electricity Ten-Year Statement: Our Year-Round System Needs — Scottish Boundaries. Retrieved from https://www.neso.energy/publications/electricity-ten-year-statement-etys/our-year-round-system-needs
Matshela Koko is a Doctoral Candidate, Graduate School of Business Leadership, UNISA; Managing Director, Matshela Energy. Former Eskom Interim Group Chief Executive (2016–2017). He writes in his personal capacity.
