Tamil Nadu: the most valuable banked unit in the index, and a battery with no home

India Renewable Banking Index · Tamil Nadu · FY 2026-27

A banked renewable unit is worth more in Tamil Nadu than in any state in the index so far — but only to a captive consumer. A third-party buyer cannot bank at all. The value holds on real bills once a tax most models leave out is counted. And the State whose drafts would mandate storage has not yet decided which rules a co-located battery falls under.

The table

The same basis as Report No. 2: pre-open-access-charge marginal banking values, net of the stated banking charge, before cash network charges, surcharges and in-kind losses. They are not landed costs.

Rows are numbered on from the six rows of Report No. 2. All rows FY 2026-27.

Row 7: energy charge 750 paise, less the 8 per cent in-kind banking charge (Regulation 16(4)). Drawn in the peak blocks the same unit is ₹8.63; drawn at night, ₹6.56. Tamil Nadu has no fuel adjustment identified in its tariff, so row 7 is placed beside the Karnataka and Gujarat rows, not ranked against them.

Row 8 is not a price. Regulation 16(1) of the Green Energy Open Access Regulations, 2025 provides that there shall be no facility of banking for third-party purchase or sale. It is an absent entitlement.

Row 9: the proviso to Regulation 16(5) keeps twelve-month banking, April to March, for wind generators commissioned on or before 31 March 2018, "along with the applicable banking charges as decided in the previous tariff orders, i.e. 14% in kind". 7.50 × 0.86. The quantity on which the 14 per cent is levied follows those earlier orders, which are not re-read here; the row is Conditional on that point. Unused energy at the end of the banking period is bought at 75 per cent of the applicable renewable tariff (16(6)).

1. The number, and what it is not

The retail tariff order of 30 June 2025 prescribes an annual escalation from 1 July: the prevailing tariff increased by the April consumer price index, or 6 per cent, whichever is lower (paragraph 3.1.1.13). Applied at the provisional April 2026 figure, row 7 would be about ₹7.14.

It has not been applied. The same order says, at paragraph 1.5.1, that its tariffs "will be valid until issue of the next Order", and paragraph 1.4.1 records that the order was itself issued to implement the previous year's escalation. The Commission's tariff-order listing for 2026 carries no revision order; its last entry is dated 1 June 2026. On the order's own terms the rate in force is still 750 paise, and row 7 is ₹6.90.

The point reaches beyond Tamil Nadu. An escalation formula written into an order is not self-executing when the same order says its rates run until the next one. A model that steps the tariff up on 1 July is ahead of the bill. When the revision order issues, row 7 moves to about ₹7.14, possibly from 1 July 2026; whether the order will reach back to that date is not known.

2. Only a captive consumer can bank

In Karnataka the gap between a captive and a third-party banked unit is ₹2.26 of surcharges. In Tamil Nadu it is the whole unit. A third-party buyer's surplus in a fifteen-minute block has nowhere to go.

The additional surcharge follows the same line. The Commission allowed 10 paise for April to September 2025, on third-party and exchange purchases only, not on captive consumption (M.P. No. 13 of 2025, 29 April 2025), and directed the licensee to file every six months. It was reported to have sought ₹1.14 for October 2025 to March 2026. No order for any period after September 2025 was on the Commission's case-order listing when checked on 22 September 2026.

3. The tax nobody counts

Tamil Nadu levies two different electricity taxes on the same factory. Energy supplied by the licensee carries 5 per cent of the consumption charge (section 3(1)(a) of the Tax on Consumption or Sale of Electricity Act, 2003). Energy a consumer generates for its own use carries 10 paise a unit (section 3(1)(b) and (c)). A banked captive unit is the second kind. It avoids the first tax and pays the second.

That is not an inference from the Act. Five HT bills of a captive consumer for January 2023, adjusting captive wind energy, show a line headed "Self Generation Tax" equal to 10 paise times the adjusted units in every one of them, and a 5 per cent tax worksheet computed only on the energy actually taken from the grid. No bill image, consumer name or account number is reproduced here.

Carried through, for a captive solar consumer on the 50 per cent network-charge concession:

On the same steps a peak-banked unit drawn at peak is ₹7.79, and a night unit ₹5.61. Two cautions go with the ladder. It does not deduct in-kind line losses, which Tamil Nadu takes at actual rather than at a fixed percentage (Tariff Order No. 9 of 2020, paragraph 5.1.5), so it slightly overstates. And Report No. 2's Karnataka ladder carried no electricity tax; the two ladders should not be compared line by line.

4. The rule is applied on the bill

Tamil Nadu matches energy in fifteen-minute blocks and restricts where banked energy can be drawn: peak-banked in any block, normal-banked in normal hours, night-banked at night (Regulation 13(1), proviso). The same five bills show the wind adjustment carried slot by slot, each time-of-day slot separately. In Tamil Nadu the slot rule is not only in the regulation. It is on the invoice.

5. A battery with no home

The Central Electricity Authority has proposed that new ground-mounted solar and onshore wind carry co-located storage from 1 July 2027. In Tamil Nadu the instruments in force do not say which rules that battery falls under, and the drafts that would say so do not agree.

In force. For deviation settlement, Regulation 13(2) sends solar, wind and wind-solar hybrid generators to the 2024 forecasting regulations, where charges are capped at 3 paise a unit a year; other green generators to the 2019 deviation regulations, where the cap has been removed and, since 4 November 2025, the price reaches 1,000 paise a kWh below 49.90 Hz; and hydro, including pumped storage, outside deviation settlement. A co-located battery is named in none of the three.

The definitions add a second gap. Storage is "Green Energy" when it uses renewable energy, and so may take open access (Regulation 3(1)(i)). But "Banking" is defined only for energy from wind, solar and wind-solar hybrid stations (Regulation 3(1)(b)). A battery can use the network; on the text, its discharge is not a bankable source.

Two notified instruments then treat the same asset in different ways. The Grid Code, 2026 defines a Renewable Energy Generating Station as one "with or without Energy Storage System" (definition 140). The SLDC Functions and Accountability Regulations, 2026 require co-located renewable plant and battery to keep separate metering and scheduling, forbid commingling their energy accounting, and treat misclassification of battery discharge as renewable injection as potential gaming.

In draft. Three drafts of May 2026, none notified as at 21 September 2026, would sort the battery by the plant beside it. Beside solar or wind, deviation would be computed on net injection at the interconnection point, with storage charging deducted, within a 5 or 7 per cent band and a 7-paise cap. Beside a conventional plant, or standalone, it would be a generating station or a seller at market-linked charges with no cap. The third draft, on battery storage itself, sends every battery deviation, for the entire battery capacity, to the deviation regulations, claims precedence over other regulations for batteries, and proposes that new renewable projects of 5 MW and above install storage of at least 25 per cent of their capacity — against the 10 per cent the Central Electricity Authority has proposed. As drafted, it has not been reconciled with the other two.

Missing. The procedure that would settle the accounting — the SLDC's detailed procedure under Regulation 20, including an energy accounting procedure, due within ninety days of 18 September 2025 — was not traced, approved or in draft, on the Commission's or the SLDC's published listings as at 21 September 2026. Absence from a published listing is not proof that no consultation took place. In an order of July 2025 on delays in implementing deviation settlement since 2019, the Commission itself wrote that the regulation "would remain only on paper".

For a battery developer the questions before financial close in Tamil Nadu are therefore not about price. They are which regime, which meter, and which hour.

6. Dates that move this page

Limitations

1. In-kind losses. The ladder does not deduct line losses, which Tamil Nadu applies at actual. It overstates by the loss on the consumer's own voltage path.

2. Bill evidence. The five bills are one consumer, one month, 2023, captive wind, issued by the predecessor licensee. The Act has not changed; the bills show how the two taxes fall, not every case.

3. Network charges. The ladder uses the 50 per cent concession for solar under Tariff Order No. 9 of 2020 (paragraph 5.1.3); with renewable energy certificates the full charge applies (5.1.4).

4. Row 9. Conditional on the quantity on which the 14 per cent is levied under the earlier tariff orders.

5. Drafts. Section 5 describes proposals, not law.

6. Ranking. Row 7 is not ranked against rows carrying a fuel adjustment.

Source register

Every figure in this report comes from one of the instruments below.

How this was built

Every figure comes from a regulation, a tariff order, a commission order or a bill, read directly. Where the record is not settled the row says so. Corrections are welcome, and preferred with the document reference attached.