Electricity costs more at some hours of the day than at others. Time-of-day (ToD) tariffs are now national policy for commercial and industrial consumers, and in Tamil Nadu the Tamil Nadu Electricity Regulatory Commission (TNERC) sets how that works for high-tension (HT) consumers. A battery can take on part of the cost, but not all of it, and rarely by shifting energy alone.
Time-of-day tariffs have been mandatory for commercial and industrial consumers with a maximum demand of 10 kW or more since 1 April 2024.
The peak and night windows
Under TNERC Tariff Order No. 6 of 2025, HT consumers (except lift irrigation, temporary supply and EV charging services) pay 25% extra for energy used from 6 to 10 am and from 6 to 10 pm, and get 5% off energy used from 10 pm to 5 am. That's eight expensive hours a day, in two blocks.
| Time (24-hour) | Energy charge | What a battery would usually do |
|---|---|---|
| 06:00–10:00 | Normal rate + 25% | Discharge |
| 10:00–18:00 | Normal rate | Recharge, ideally from midday solar |
| 18:00–22:00 | Normal rate + 25% | Discharge |
| 22:00–05:00 | Normal rate − 5% | Charge |
| 05:00–06:00 | Normal rate | Hold, ready for the morning peak |
Two blocks matter for sizing. A battery charged only overnight has to cover both peaks from one charge, or it recharges between them at the normal rate. Midday solar is the natural way to refill it before the evening.
The demand charge and its floor
Maximum demand is measured in 15-minute blocks, and you're billed on the higher of your recorded maximum demand or 90% of your contract demand. So shaving peaks only saves demand charges if recorded demand is already above that floor, or if you formally reduce contract demand with the distribution company (DISCOM). Power factor matters too: the order sets penalties below an average of 0.90.
One 15-minute spike can set the demand for the whole month: a compressor start, a furnace cycle, or every machine switching back on after a power cut. A battery can cap those spikes, but only if the controls see them coming and the battery has charge left when they arrive.
Where a battery helps
- Shifting energy into both peaks: charging overnight at the rebate or from midday solar, then discharging from 6 to 10 am and from 6 to 10 pm.
- Capping 15-minute demand on peaky loads, where recorded demand sits above the floor or contract demand can be reduced.
- Covering dips and short outages, so sensitive processes keep running and the genset starts less often.
- Supplying reactive power when the power conversion system (PCS) has spare capacity, which helps hold the average power factor above 0.90.
Each job is worth something on its own. Together, on one battery, they are what makes the numbers work. That is why we model them together, with the assumptions shown, rather than quote a single saving. See the jobs one battery can do.
Where it doesn't, on its own
The difference between peak and off-peak energy prices is real but thin once round-trip losses are counted. A battery that only does time-of-day shifting rarely pays for itself in Tamil Nadu. Stack the jobs.
The reason is simple arithmetic. Every kWh delivered in the peak needs more than a kWh charged earlier, so part of the peak premium is spent before any of it is saved. Charging at the night rebate widens the gap a little, but not by enough to pay for a battery on its own.
A battery is also the wrong fix for some bills. A low power factor caused by a failed capacitor bank, or a contract demand set far above what the site uses, costs less to correct directly, and we'll say so.
What to check before you buy
- 12 months of bills, to see peak-hour energy, demand charges and any power-factor penalties month by month.
- 15-minute interval data from your meter, to see how tall and how wide the peaks are.
- Contract demand against recorded demand, to see whether the 90% floor is setting your bill.
- The peak-hour share of consumption: how much of your energy falls between 6–10 am and 6–10 pm.
- Genset hours and fuel records, if outages are part of the picture.
- Solar surplus: whether midday export or curtailment could charge a battery.
With these, we can say which jobs a battery can do on your site and what each is worth under the current order. See how storage sits between your site and the grid, and how to size a battery.