Time-of-Day tariffs and the Indian C&I electricity bill

A flat tariff charges one rate per unit, whenever it was consumed. A Time-of-Day tariff divides the day into zones and prices each one differently, usually as a percentage adder or rebate applied to a base energy charge.

The regulatory intent is to move demand: discourage consumption when the grid is strained and encourage it when solar is abundant. The practical consequence for a consumer is that when you use energy is now a decision with a price attached — and for anyone selling solar or storage, that the value of a displaced unit depends entirely on which zone it came out of.

The zones, in the shape most orders use

Seasons

Many orders publish two sets of adders — a summer set and a winter set — and some define seasonal blocks tied to local demand cycles rather than the calendar (a paddy season, for instance). A model that applies one annual adder set misprices half the year.

The energy charge is the headline, but a C&I bill is assembled from several components, and a saving computed on the energy charge alone will be wrong in both directions.

kVAh or kWh — not the same bill

Some DISCOMs bill kVAh (apparent energy) rather than kWh (active energy). Under kVAh billing a site pays for its power factor: poor power factor inflates apparent energy above active energy, so the same machinery produces a larger bill.

This matters for sizing in two ways. Savings computed in kWh on a kVAh-billed site are understated. And a solar inverter that can supply reactive power may improve the power factor, producing a saving that has nothing to do with the energy it generated. Of the DISCOM vintages in this library, 26 bill in kVAh.

The landed rate, built up

What a displaced unit is actually worth is the landed rate — the published energy charge plus everything that rides on it:

Against a solar-hour rebated rate on the same base, the same arithmetic might land near ₹6.50. That difference — roughly ₹2.75 per unit between the cheapest and most expensive hour of the same day — is the spread storage is being paid to cross, and the reason it is worth crossing even after round-trip losses.

Ingro Sims ships a transcribed library of Indian DISCOM Time-of-Day structures and rates, read from the published tariff orders and carried with the order reference, its effective date and — in most cases — the page of the PDF each number came from.

What it does not hold

India has well over a hundred distribution licensees. This is not all of them, and it does not claim to be — it is the set covering the states where C&I solar and storage is actually being built.

The larger honest gap is the landed rate. Assembling one needs the electricity duty and the fuel adjustment published in a usable form, and almost no DISCOM publishes both: on the imported sheet, exactly two of fifty-five could reach a landed figure from their own published components. For every other DISCOM the base energy charge and its ToD structure are known, and the riders have to come from the site’s own bill — which is one reason a study starts from bills rather than from a rate card.

Every state regulatory commission issues a tariff order, typically annually. An order can change the base energy charge, the ToD adders, where the zone boundaries fall, the demand charge, and the duty.

The third of those is the one that quietly invalidates old work. If an order moves the evening peak from 18:00–22:00 to 19:00–23:00, every dispatch decision in a study built on the previous boundaries was optimised against hours that no longer carry that price. The totals still look plausible. They are simply computed on the wrong bill.

• Date every study. A proposal should name the tariff order it was priced against, not just the DISCOM.
• Re-run before you re-quote. A study from the last financial year is a starting point, not a number to put in front of a customer.
• Check the billing unit. kVAh and kWh schedules are not interchangeable, and a DISCOM can change which it bills on.
• Take the riders from the bill. Duty and fuel adjustment are where published schedules most often fall short of what the site actually pays.

The regulatory changes worth tracking right now — universal ToD since April 2024, and Maharashtra’s storage requirement from April 2026 — are covered in the regulation guide.