Cheaper When the Sun Shines and Costly When It Sets

South Africa spent years rationing electricity after dark. It is now curtailing solar at noon. According to News24’s report on SAPVIA and NTCSA tariff talks, curtailment instructions rose from roughly 100 a month to more than 1,000, and the compensation backlog peaked near R2 billion before being cut toward R1.5 billion. The proposed fix is intuitive: make electricity cheaper when the sun shines, shift demand into midday, and stop paying generators not to generate.

The intuition is half right. The missing half is what the hourly system data actually show.

The mechanism is straightforward. When PV floods the midday grid, residual demand falls, but coal does not switch off. Units are backed down toward minimum stable level and held online so they can recover the evening peak. Author calculations from Eskom’s Data Portal Station Build-Up series (2017 to early 2025) show roughly 100% of midday thermal hours running near that floor. Solar displaces energy. It does not displace commitment. Midday “surplus” therefore coexists with must-run coal, which is why giving noon power away cheaply does not erase its system cost.

The numbers are stark. On 2024-08-07, the deepest full-year duck day in the sample, residual demand swung from a midday minimum of 22,145 MW to an evening peak of 30,576 MW: an 8,431 MW recovery in a few hours. Mean coal evening ramp collapsed from about 2,234 MW in 2017 to about 562 MW in 2025, while demand-side evening ramps stayed in the 2-3 GW range. Pumped storage and OCGT now carry the neck of the duck. That ramp, not the midday belly, is the binding scarcity.

That is the policy event behind the sunny-day tariff discussion. SAPVIA and NTCSA are right to chase daytime demand and storage. They are wrong if the reform stops at “cheaper when the sun shines.” A one-sided discount prices the belly and leaves the neck unpriced.

The relief is welcome, but it also raises substantive questions that require clear, evidence-based answers.

First, cross-shifted costs. If midday is discounted and 17:00-20:00 is not premium-priced, flexible customers and batteries get a one-way arbitrage: charge or consume in the cheap window, then reappear as load, or discharge for margin, on the evening ramp. The curtailment backlog may clear on paper while the physical ramp bill moves onto peaking fuel, storage wear, and whoever still pays the peak. Do not socialise the neck while privatising the belly.

Second, storage scoring. Batteries are not automatically the cure. If they are sized and contracted against midday curtailment hours alone, they can deepen evening scarcity when they recharge for the next sunny day or chase retail spreads that ignore ramp value. Storage procurement should be scored against the measured evening residual ramp visible in Eskom’s supply-side Station Build-Up data, not only against the number of curtailment instructions.

Third, durability of the tariff signal. Reclassifying sunny hours as off-peak, the pathway flagged by Chris Yelland in the News24 piece - is a useful incremental step only if the evening window rises with measured flexibility scarcity. A temporary accounting fix that clears a R1.5 billion backlog without restructuring TOU around the duck will invite the same crisis once the next wave of PV connects

Cheaper midday power is not generosity. It is a necessary correction to a system that now produces more solar than the midday customer base can absorb. But it is only half a flexibility signal. The other half is an evening premium that tells households, industry, and storage owners the truth the duck already tells operators: the expensive hours are when the sun sets and coal can no longer swing.

The choice now is strategic. South Africa can discount the belly and hope demand follows, while Pumped Storage Hydro and Open Cycle Gas Turbine silently finance the neck. Or it can price the whole duck: cheap when solar is abundant, costly when the ramp begins, and build storage against the constraint that actually binds. Whether sunny-day cheapness becomes industrial recovery or another deferred bill will turn on that second half of the story.