The Duck Curve Is Now South Africa's Reality: How C&I BESS Owners Must Reprice Arbitrage Contracts as Midday Solar Surplus Reshapes the Grid

The Duck Curve Is Now South Africa's Reality: How C&I BESS Owners Must Reprice Arbitrage Contracts as Midday Solar Surplus Reshapes the Grid

By SolarXgen Editorial | Field Intelligence Series | 7 May 2026

For years, the "duck curve" was California's problem — a graph on a slide deck at an energy conference, a cautionary tale from the other side of the world. As of 2026, it is South Africa's daily operational reality. The rapid, largely private-sector-led build-out of solar PV across the country's commercial and industrial (C&I) heartland has reshaped the Eskom grid's net-load profile in ways that are now forcing BESS asset owners to fundamentally rethink how they price and structure energy arbitrage contracts.

From Loadshedding Crisis to Solar Surplus: A Faster Transition Than Anyone Predicted

South Africa's solar ramp-up has been extraordinary in its pace. Private procurement has been leading the development of South Africa's utility-scale pipeline since early 2023, with a total of 2,738 MW across 384 projects registered with the national energy regulator in 2023, followed by 2,880 MW across 454 projects in 2024. These are not speculative numbers — a large portion of those projects registered over the last two years are scheduled to become operational in 2025 or 2026.

The aggregate effect on the grid is now visible. SAPVIA is forecasting approximately 3.5 GW to 4 GW of new solar capacity additions in 2026, through a mixture of private and public procurement. Meanwhile, South Africa's IRP 2025 envisages the procurement of 25 GW of solar PV and 8.5 GW of battery storage by 2039 — a policy ambition that is already being outpaced by private capital on the generation side.

The consequence is precisely what grid planners in California and South Australia observed before us: as more rooftop and utility-scale solar systems come online, the midday belly of the duck sinks ever lower. Increased use of air conditioning, EV charging, and other loads in the evening hours only makes the ramp steeper. The more solar we build, the more pronounced this curve becomes.

The Tariff Landscape Has Shifted — But Not in the Direction Arbitrage Models Expected

The standard C&I BESS arbitrage model in South Africa was built on a simple premise: charge during cheap off-peak hours, discharge during expensive peak hours, and pocket the spread. That model is being disrupted from two directions simultaneously.

First, the spread itself is changing. Eskom has stated that the difference between the most expensive electricity at peak times and the cheapest electricity at off-peak times is getting smaller. In the past, peak power could be eight times more expensive than the cheapest off-peak power — now it is closer to six times. But here's the crucial nuance: the gap is shrinking largely because the cost of electricity during the traditionally cheapest off-peak periods is going up significantly.

Second, the structural definition of "peak" has changed. With effect from 1 April 2025, Eskom revised the times of peak, standard and off-peak periods across all time-of-use (TOU) tariffs. The morning peak period has been reduced from three hours to two hours, while the evening peak period has been increased from two hours to three hours. This is not a minor administrative adjustment — it directly reprices the window during which BESS discharge is most valuable and compresses the morning charge window that many systems relied on.

The 2026/2027 tariff cycle adds further complexity. Eskom's standard prices for direct customers are adjusted with an annual increase of 8.76% effective 1 April 2026, while prices for municipal bulk purchases are adjusted by 9.01% effective 1 July 2026. Key adjustments have been made to the recovery of fixed costs through the Generation Capacity Charge (GCC), with the GCC rate increased to 30% of the originally proposed 2025/2026 Retail Tariff Plan rand-value, up from the previous 20%. For C&I customers on Megaflex or Miniflex tariffs, these structural shifts alter the fundamental economics of every arbitrage cycle.

The Duck's Belly Is Deepening: What It Means for Midday Charge Value

Here is the uncomfortable truth that many BESS owners signed into three- and five-year arbitrage contracts in 2023 and 2024 are now confronting: the midday period — when their batteries are supposed to be cheaply charging — is no longer reliably cheap.

With C&I rooftop solar now embedded across virtually every industrial park and commercial precinct in Gauteng, the Western Cape, and KwaZulu-Natal, the midday net load on municipal distribution networks has collapsed. Behind-the-meter solar is self-consuming during the hours when BESS owners expected to buy cheap grid energy. The "standard" tariff band, which covers most of the productive solar generation window, is simultaneously being priced upward under the new NERSA-approved structure.

Globally, this dynamic is well-documented. An oversupply of energy during low demand coupled with a lack of supply during high demand explains the large disparity between midday and evening energy prices — a disparity that, paradoxically, can erode simple buy-low/sell-high arbitrage as more storage participants chase the same spread. When solar generation dominates around midday, the grid is flooded with excess power, supply outstrips demand, and prices collapse to zero or even turn negative. South Africa's wholesale market is not yet fully liberalised, but the directional pressure is identical.

The C&I BESS Market Is Booming — Just as Arbitrage Models Must Evolve

None of this diminishes the scale of the C&I BESS opportunity — if anything, it amplifies the urgency of getting the commercial model right. In March 2026, Sungrow signed a landmark agreement with Herholdt's Group, its official distribution partner in South Africa, to deploy a total of 1,155 MWh of commercial and industrial (C&I) battery energy storage systems.

At the utility scale, the pipeline is equally compelling. The Naos-1 project will comprise 300 MW of solar PV capacity combined with 660 MWh of battery storage, located in the Free State province. The Mulilo Oasis cluster — 257 MW/1,028 MWh — is expected to reach commercial operation by late 2026, representing one of the largest grid-scale storage deployments on the continent.

The cost economics are also decisively in storage's favour. Utility-scale BESS capital costs outside China reached approximately US$125/kWh by late 2025. Global battery prices have fallen dramatically, translating to a levelised cost of storage around $65/MWh for large, contracted projects. At these price points, storage is no longer a premium add-on. As battery costs continue to fall, the arbitrage model will become viable for a more diverse spectrum of customer segments. SAPVIA sees 2026 as the year this transition moves from early adoption to mainstream practice.

How SolarXgen Advises C&I BESS Owners to Reprice and Restructure

At SolarXgen, our project teams are already working through contract renegotiations and new-build structuring with C&I clients across the country. Here is our field-derived framework for repricing arbitrage in the duck-curve era:

• Move from static to dynamic dispatch optimisation.

Contracts priced on a fixed peak/off-peak spread are obsolete. Modern BESS management systems must integrate Eskom TOU signals, municipal tariff structures, on-site solar generation forecasts, and weather data to optimise dispatch in real time. The charge window is no longer defined by the clock — it is defined by the net-load curve on any given day.

• Price the evening peak window, not the midday trough.

With Eskom's extended evening peak period now running three hours (up from two), the discharge window has lengthened. The duck curve calls for energy storage capable of shifting large amounts of energy from midday solar peaks to evening demand hours. C&I contract pricing should anchor on the value of that evening discharge, not assume a cheap midday charge will always be available from the grid.

• Stack revenue streams beyond pure arbitrage.

The BESIPPPP programme's primary goal is to procure standalone BESS that can provide ancillary services — frequency regulation, voltage support — alongside energy arbitrage and peak capacity. C&I BESS owners should be exploring demand charge reduction, network tariff avoidance, and, as the South African Wholesale Energy Market matures, ancillary service revenue. As BESIPPPP projects come online and the ancillary services market develops under the South African Wholesale Energy Market, hybrids will increasingly compete to provide auxiliary services such as frequency regulation, reserves and congestion management.

• Model curtailment risk explicitly.

Grid congestion in renewable-rich corridors — particularly in the Northern and Western Cape — means transmission infrastructure cannot evacuate all generated power, leading to curtailment. For C&I systems behind-the-meter, the equivalent risk is solar self-consumption saturating the charge window before the BESS can fill. Contracts must account for this with volume floors and force majeure solar provisions.

• Factor in the IRP 2025 trajectory.

South Africa has secured 1.7 GW/11 GWh of grid-scale BESS capacity through the BESIPPPP, with Bid Window 3 alone awarding 616 MW of BESS capacity. As this capacity comes online, the arbitrage spread will compress further at a market level. Contracts written today must include repricing provisions tied to Eskom TOU schedule revisions and market spread benchmarks.

The Policy Signal Is Loud and Clear

South Africa's IRP 2025 envisages the procurement of 34 GW wind, 25 GW solar PV, 8.5 GW battery storage, and 16 GW distributed generation by 2039, signalling a strong policy commitment to clean energy and creating a predictable investment environment. The duck curve is not an aberration — it is the intended consequence of that policy. Falling battery costs, escalating electricity tariffs, and mounting grid constraints are reshaping both public procurement and private investment strategies, pushing energy storage from the margins to the centre of the country's power transition.

The grid South Africa is building is, by design, one where midday solar surplus is the norm and evening ramps are the challenge to be managed. BESS owners who price their arbitrage contracts around the old loadshedding-era spread — where any grid power was precious at any hour — will find their returns eroding quarter by quarter. Those who reprice around the duck curve's actual shape, and build in the flexibility to capture multiple value streams, will find that storage economics in South Africa are more compelling than ever.

The duck has landed. The question is whether your contracts are priced for it.