BESS Time-of-Use Arbitrage After the April Tariff Reset: A CFO's Quantification Guide

From 1 April 2026, every commercial and industrial electricity customer in South Africa faces a confirmed tariff reset. NERSA approved an 8.76% increase for Eskom direct customers, effective 1 April 2026, and a 9.01% increase for municipal customers from 1 July 2026. For CFOs who have been passive about energy costs, this is not a routine annual adjustment — it is the first tranche of a structured revenue recovery programme that NERSA itself acknowledged arose from calculation errors understating Eskom's allowable revenue by R54.7 billion. More hikes are locked in. The question is not whether to act, but which financial lever to pull first. Battery energy storage combined with time-of-use (TOU) arbitrage is the most immediately quantifiable answer on the table right now.

Why This Tariff Cycle Is Different

The 8.76% headline figure understates the real bill impact for many commercial users. NERSA's revised determination — triggered by a High Court ruling in December 2025 that rejected a prior NERSA-Eskom settlement — replaced the originally approved 5.36% increase with a significantly higher one. The approved increase for April 2026 is nearly 63% higher than what was originally set. For context, both the 8.76% and 9.01% municipal increases sit well above the prevailing consumer price inflation rate of approximately 3.5%, compounding the real cost burden on business balance sheets year after year.

Critically, Eskom is also restructuring how it recovers revenue — shifting a greater proportion into fixed network and capacity charges rather than pure per-unit energy consumption. For certain low-usage tariff categories, effective increases can reach up to 28% when fixed charge restructuring is included. This structural shift has a direct implication for BESS strategy: the per-kWh arbitrage window between peak and off-peak periods is widening, but the fixed-charge component means simply consuming less energy is no longer sufficient to protect your bill.

The TOU Arbitrage Mechanics: A CFO's Framework

Time-of-use tariff arbitrage is conceptually simple: charge your BESS during cheap off-peak periods, discharge it during expensive peak periods, and pocket the spread. In practice, the financial case must be built from verified current numbers. Here is how to frame the calculation for your site.

Step 1 — Identify Your TOU Tariff Structure

Most medium-to-large commercial and industrial properties in Eskom's licensed area are on Megaflex (large urban consumers) or Miniflex (medium urban consumers). Both use three TOU periods — peak, standard, and off-peak — with seasonal differentiation between a high-demand season (June to August) and a low-demand season. Municipal customers are increasingly migrating onto equivalent structured tariffs as municipalities adopt Eskom's new simplified bulk-purchase framework introduced with FY2026 tariffs. Your first task is to confirm which tariff applies at each point of delivery (POD) on your portfolio, because the spread between peak and off-peak rates — the arbitrage margin — differs materially between tariff classes and seasons.

Step 2 — Quantify the Peak-to-Off-Peak Spread

The actual c/kWh spread on Megaflex between high-demand-season peak and off-peak rates has historically been in the range of 3:1 to 4:1 on an energy charge basis. After the April 2026 reset — which applies the 8.76% increase across TOU periods — this absolute spread in rand terms widens proportionally. For a facility consuming 500 kWh during peak periods on a typical high-demand-season weekday, the cost differential between peak and off-peak rates represents a meaningful daily arbitrage opportunity. Multiply that by 65 high-demand weekdays per year and the annualised value per BESS discharge cycle becomes your primary return driver. Your energy consultant or BESS partner should be running this exact calculation from your actual interval meter data — not a generic estimate.

For operations directors: the Megaflex peak window on weekdays runs from 07:00–10:00 and 18:00–20:00 in the high-demand season (June–August). These are non-negotiable discharge windows. Off-peak charging windows (typically 22:00–06:00) define your charging cost. The tighter you can schedule your BESS dispatch to these windows, the higher your arbitrage capture rate.

Step 3 — Layer in Demand Charge Reduction

TOU energy arbitrage is one revenue stream. Demand charge (kVA) reduction is a second, often larger, stream that operates in parallel. Under Megaflex and Miniflex, network capacity charges are levied on a R/kVA/month basis against your annual utilised capacity during peak and standard periods. A correctly sized BESS that consistently clips your peak demand during these windows reduces your notified maximum demand (NMD) register — and the excess network capacity charge exposure that comes from NMD exceedances. CFOs should model these two value streams separately before combining them: arbitrage revenue and demand charge avoidance are calculated differently and have different certainty profiles.

Step 4 — Apply the BESS Sizing Formula

The sizing question cannot be answered without 12 months of interval meter data at 30-minute resolution. The financial model requires: (a) your peak-period consumption profile by season; (b) your peak kVA demand and how far above your NMD it spikes; and (c) whether you have an existing or planned solar PV system, because solar charges the BESS during the day and shifts your arbitrage calculation significantly.

As a rule of thumb for budgeting purposes, a commercial C&I BESS sized for 2-hour discharge at your peak demand level is typically the economically optimal configuration for pure TOU arbitrage. Longer-duration systems cost more but unlock additional value if your facility runs extended peak-period loads. On current global pricing, commercial LFP (lithium iron phosphate) BESS systems are tracking at around USD $180–$320 per kWh installed for containerised systems at scale, which at current exchange rates translates to a South African all-in project cost that has fallen materially compared to just two years ago. The cost trajectory, driven by Chinese LFP cell manufacturing overcapacity, favours buyers — but waiting for prices to fall further while tariffs continue to rise is a losing trade.

The Solar + BESS Combination: Compounding the Return

Standalone BESS arbitrage works. But the return profile strengthens materially when BESS is paired with rooftop or carport solar PV. Here is why: solar displaces your standard-period grid consumption during the day, while your BESS handles peak-period discharge. The combination means your BESS charges partly from free solar generation rather than from the grid — improving your effective arbitrage spread. Post-April 2026, with higher per-unit tariffs applying across all TOU periods, the solar self-consumption savings are also larger in absolute rand terms.

Eskom's tariff restructuring also introduces a phased Generation Capacity Charge (GCC) that increases from 20% in FY2025 to 30% in FY2026 and FY2027, partially affecting net-billing credits for grid-tied generators. This reinforces the financial case for maximising on-site self-consumption rather than relying on grid export credits — and BESS is the primary mechanism for shifting solar generation from midday into the peak window where it delivers maximum value.

Financing the Investment: The Zero-Capex Route

For CFOs protecting the balance sheet, the conversation does not have to start with capital expenditure. Power Purchase Agreements (PPAs) and solar lease structures allow commercial properties to access solar + BESS without upfront capital, with monthly payments structured against the energy cost savings generated. In a rising tariff environment, the PPA rate — fixed or lightly escalated — becomes a natural hedge against NERSA's annual reset cycle. The April 2026 increase and the confirmed 8.83% increase already approved for April 2027 mean that the tariff escalation assumption in any PPA model is now grounded in regulatory fact, not forecast. This makes financial modelling more defensible for board approval.

Property managers and facilities directors should note that the July 2026 municipal tariff reset — averaging 9.01% — creates a second trigger event within the same calendar year. Sites supplied through municipalities have a narrow window between now and June 2026 to complete a feasibility assessment, obtain SSEG approval from their municipality, and commit to a funded installation before the next municipal tariff hit lands.

What Decisions Do You Face Right Now?

The April 2026 tariff reset is days away. The decisions that need to happen before the July 2026 municipal reset are more pressing than they appear. Here is the practical priority sequence:

• Pull your interval meter data. If you do not have 12 months of 30-minute interval data for each of your commercial properties, request it from your utility or metering service provider immediately. No credible BESS sizing or TOU arbitrage model can be built without it.
• Identify your TOU tariff category. Confirm whether you are on Megaflex, Miniflex, Municflex, or an equivalent municipal TOU tariff at each site. The arbitrage spread and sizing logic differ by tariff class.
• Commission a multi-value-stream financial model. A credible BESS feasibility analysis should model TOU arbitrage savings, demand charge avoidance, NMD exceedance cost reduction, and (where applicable) solar self-consumption uplift as separate line items — not a single blended saving figure.
• Evaluate funded structures early. PPA and lease models require procurement and legal lead times. If you want a BESS operational before the July 2026 municipal tariff event, commercial commitments need to happen now.
• Consider the tariff trajectory, not just today's rate. With an 8.83% increase already approved for April 2027, any investment model built on today's tariff understates your future savings. Use a realistic escalation curve — NERSA's own MYPD 6 determinations give you the forward schedule.

The arbitrage window has never been wider, the hardware has never been cheaper, and the regulatory trajectory has never been more certain. The CFO who waits for perfect information is the one paying peak-rate grid tariffs while their competitor locks in a decade of cost certainty.

Sources & References

• Eskom: Eskom Implements NERSA Decision for Financial Year 2027 — eskom.co.za (March 2026): https://www.eskom.co.za/eskom-implements-nersa-decision-for-financial-year-2027/
• Business Day: Nersa Approves Steeper Eskom Tariff Hikes After Admitting Calculation Errors (February 2026): businessday.co.za
• Newcastillian News: NERSA Approves Eskom Tariff Increase — Eskom Response (March 2026): newcastillian.com
• The South African: How Much More You'll Pay for Electricity from 1 April 2026 (March 2026): thesouthafrican.com
• Eskom Distribution: Tariffs and Charges — Schedule of Standard Prices FY2026: eskom.co.za/distribution/tariffs-and-charges
• Ember Energy: How Cheap Is Battery Storage? (October 2025): ember-energy.org
• BloombergNEF via HighJoule: Global BESS Cost Forecast 2026–2027 (November 2025): highjoule.com