For many HT (High Tension) factories, workshops, commercial buildings and other high-consumption electricity users in Chennai, the electricity bill is not simply about how many units are consumed. When those units are consumed also matters.
This is where Tamil Nadu’s C5 consumption slot becomes important.
If your electricity bill shows significant C5 consumption, you may be wondering: Can a rooftop solar system offset those units? Does solar generated during the daytime reduce night-time C5 consumption? And what do Tamil Nadu’s current solar regulations actually allow?
The short answer is: rooftop solar can reduce your overall electricity cost, but you should not assume that every C5 unit will be offset on a one-to-one basis by daytime solar generation. The actual financial benefit depends on your tariff category, metering mechanism, solar capacity, consumption pattern and the applicable TNERC rules.
What Is C5 Consumption in Tamil Nadu?
C5 is the night-hour time-of-day (ToD) slot used in Tamil Nadu’s tariff structure for HT (High Tension) consumers. Under the FY 2025–26 tariff structure, the HT time-of-day slots are: C1 (6 AM–10 AM) and C2 (6 PM–10 PM) as peak periods, C4 (10 AM–6 PM) as the normal daytime slot, and C5 (10 PM–6 AM) as the night-hour slot. This time-of-day structure applies specifically to HT consumers; LT (Low Tension) consumers are billed under a different tariff mechanism and do not have C1–C5 slots.
This creates an important challenge for rooftop solar.
Solar panels generate most of their electricity during daylight hours. Your factory may therefore produce substantial solar power between roughly morning and evening, while C5 consumption happens mainly at night.
For example, imagine a Chennai manufacturing unit that consumes:
- 10,000 kWh during daytime hours
- 4,000 kWh during C5 night hours
- 14,000 kWh total in a billing period
A rooftop solar system may generate 10,000 kWh during the day. That does not mean the factory has automatically eliminated all 4,000 kWh of C5 consumption.
The electricity bill must be evaluated according to the applicable solar settlement and tariff mechanism.
Can Solar Directly Offset C5 Units?
Not in the simple way many businesses assume.
A rooftop solar plant produces electricity when sunlight is available. If your factory is operating during the day, the first and most valuable benefit generally comes from self-consumption.
If the solar system generates 500 kWh at noon and the factory is simultaneously consuming 500 kWh, that solar energy can directly serve the factory’s load.
But at 2 AM, the rooftop solar system is normally producing no electricity. The factory must continue drawing electricity from the grid for its C5 load.
This means solar sizing should not be based only on your total annual consumption. Your hourly load profile matters.
Then What Happens to Excess Daytime Solar?
This is where Tamil Nadu’s rooftop solar regulations become important.
TNERC’s Grid Interactive Solar PV Energy Generating Systems framework allows eligible consumers to use mechanisms such as net metering or net feed-in depending on their consumer category and connection. For non-domestic consumers, the applicable mechanism is generally net billing/net feed-in rather than domestic net metering.
Under a net feed-in arrangement, the solar electricity used within the premises reduces the need to purchase electricity from the grid. Excess solar electricity exported to the grid is accounted for separately and receives the applicable feed-in value.
That distinction is extremely important.
It means you should not simply calculate:
Total solar generation – total C5 consumption = C5 savings
The actual bill impact depends on how imported electricity and exported solar electricity are financially settled.
What Do Tamil Nadu Regulations Actually Allow?
Tamil Nadu’s 2021 Grid Interactive Solar PV regulations provide an important framework for rooftop solar.
For eligible consumers other than hut and agriculture categories, net billing/net feed-in can be used up to the sanctioned load or contracted demand, subject to the applicable capacity limit. The framework also provides for gross metering for eligible larger systems.
For commercial and industrial consumers, this is particularly relevant because solar generation and grid consumption are not necessarily treated as identical units at the same financial value.
Under net feed-in, imported electricity is valued at the applicable retail tariff, while exported solar electricity is credited at the applicable feed-in tariff. This means exporting a unit of solar is generally not financially identical to avoiding a unit of grid electricity through direct self-consumption.
That is why businesses should aim to maximize solar self-consumption wherever practical.
The Network Charge You Should Not Ignore
There is another important component that many businesses overlook: network charges.
Tamil Nadu’s rooftop solar framework provides for network charges for consumers/prosumers using net metering, net billing or net feed-in mechanisms, with the applicable treatment determined by the Commission. For HT industrial and commercial consumers, this currently works out to approximately ₹1.27 per unit — applied to solar units settled through the network, on top of whatever tariff or feed-in value applies. On a large industrial system, that is not a rounding error; it belongs in the ROI calculation from day one, not as a footnote discovered after commissioning.
This matters when calculating the actual return on investment of a commercial or industrial rooftop solar plant.
A solar proposal that says:
“Your factory consumes 1 lakh units, so install a system that generates 1 lakh units.”
may sound attractive, but it is not necessarily the best financial strategy.
The right calculation should consider:
Daytime consumption + solar generation profile + export potential + tariff slots + network charges + demand charges + applicable solar settlement mechanism.
What About C5 Consumption Specifically?
Scenario 1: High Daytime Consumption, Low C5 Consumption
This is usually an excellent situation for rooftop solar.
If your factory operates heavily between 9 AM and 6 PM, a large percentage of your solar generation can be consumed directly on-site.
That can reduce grid purchases substantially without relying heavily on exported energy.
Scenario 2: High Daytime and High C5 Consumption
Solar can still provide significant savings.
However, the solar plant primarily reduces daytime grid consumption. The factory will continue drawing electricity during C5 hours unless it has another source of power, such as suitable energy storage or another approved electricity arrangement.
Scenario 3: Low Daytime Consumption and High C5 Consumption
This requires more careful planning.
Installing a very large rooftop solar plant may result in more daytime surplus generation being exported rather than consumed internally.
In this situation, simply increasing solar capacity may not produce the best financial return.
A detailed load analysis becomes essential.
Can Battery Storage Help With C5?
Potentially, yes — and for HT consumers, the case is stronger than simply “shifting” solar into the night.
Because C1 (6 AM–10 AM) and C2 (6 PM–10 PM) are the peak tariff windows, a battery that specifically targets those two slots captures the highest-value savings per unit, rather than displacing grid electricity at whatever rate happens to apply after dark. This is a different, and often better, way to think about battery economics than “store daytime solar, use it at night.”
A C1/C2/C5-aligned battery cycle typically looks like this:
- Charge from solar generation during the day (C4, 10 AM–6 PM)
- Discharge during C2, the evening peak (6 PM–10 PM)
- Recharge from the grid during C5, the night/off-peak window (10 PM–6 AM), when rates are lowest
- Discharge again during C1, the morning peak (6 AM–10 AM)
The diffally into C2, and topping the battery back up on cheap C5 power before C1, avoids the peak rate twice in a single day — which is where a battery’s payback period actually improves.
That said, this still doesn’t mean every HT facility with C5 consumption needs a battery. Battery storage introduces additional capital expenditure, efficiency losses, replacement considerations, operating requirements and safety requirements.
So a battery should not be added simply because a factory has C5 consumption.
The economics should be calculated against the actual tariff structure, operating profile and — specifically — how much of the facility’s load falls inside the C1/C2 peak windows, since that’s what determines how much a battery actually has to work with.
Should Chennai Businesses Size Solar Based on Annual Consumption?
Not blindly.
This is one of the most common mistakes businesses make when evaluating rooftop solar.
Consider two factories, both consuming 12 lakh kWh per year.
Factory A operates mainly from 8 AM to 6 PM.
Factory B operates three shifts and has substantial consumption between 10 PM and 6 AM.
Their annual consumption is identical.
Their ideal solar strategy may not be identical.
Factory A may be able to consume a large percentage of its rooftop solar generation directly.
Factory B may export more daytime solar while continuing to purchase substantial electricity during night hours.
Therefore, 12-month electricity consumption is only the starting point.
A better solar assessment examines monthly bills and, where available, interval or time-slot consumption data.
What Chennai Industries Should Check Before Installing Solar
Before signing a rooftop solar EPC contract, businesses should ask for a proper energy assessment.
At minimum, review:
1. C5 consumption
How many units are you consuming between 10 PM and 6 AM?
2. Daytime consumption
How much electricity is being consumed while the rooftop solar system is producing power?
3. Solar generation estimate
What will the proposed system realistically generate at your Chennai location?
4. Self-consumption ratio
How much of the generated electricity will actually be used inside the facility?
5. Exported energy
How much electricity is expected to flow to the grid?
6. Applicable settlement mechanism
Is the project being implemented under the appropriate net feed-in, net billing, net metering or other applicable mechanism?
7. Network charges
Have the applicable network charges been included in the financial model?
8. Demand charges
Will the solar project meaningfully affect the facility’s demand profile, or will demand-related charges remain largely unchanged?
9. Roof capacity
Is there sufficient shadow-free roof area for the proposed system?
10. Future electricity consumption
Will production expand after installation?
These questions can make a major difference to the project’s actual ROI.
A Simple Example
Imagine a Chennai industrial facility consuming 1,20,000 kWh per month.
Its average monthly profile is:
- Daytime consumption: 80,000 kWh
- Night/C5 consumption: 40,000 kWh
Suppose a rooftop solar system generates approximately 70,000 kWh per month.
At first glance, someone might say:
120,000 – 70,000 = 50,000 kWh grid consumption
But the real calculation is more nuanced.
If most of the 70,000 kWh is generated while the factory is operating, a significant portion can be consumed directly.
If production is low during weekends or holidays, more solar may be exported.
Meanwhile, the factory could still consume close to 40,000 kWh during night-time hours.
The final electricity bill therefore depends on the actual consumption pattern and the applicable settlement rules—not simply the difference between annual consumption and annual solar generation.
The Key Takeaway for Chennai Businesses
So, can rooftop solar offset C5 consumption?
Rooftop solar can reduce the overall electricity cost associated with a facility that has C5 consumption, but daytime solar generation should not be treated as a simple one-for-one replacement for every night-time C5 unit.
Tamil Nadu’s regulatory framework distinguishes between electricity consumed from the grid and electricity exported by the rooftop solar system. For non-domestic consumers, the net feed-in/net billing structure and applicable network charges can materially affect the economics.
The most financially sensible approach is therefore to design the solar system around the actual load profile, rather than simply matching the system size to annual electricity consumption.
For a Chennai factory, warehouse, commercial building or industrial facility, the question should be:
“How much of my solar power can I use when I need it, and what happens to the rest?”
That question gives you a much better basis for deciding the right rooftop solar capacity.
How KinetiQ Energy Can Help
At KinetiQ Energy, we believe rooftop solar should be evaluated as an energy-performance investment—not simply as a panel installation.
For Chennai businesses, our approach can start with your electricity consumption, operating hours, tariff structure and available rooftop area. The goal is to understand where your electricity cost is actually coming from and design a solar solution around your real operating conditions.
If your facility has significant C5 consumption, don’t assume that a bigger solar plant is automatically the answer.
Not Sure How Much of Your C5 Load Solar Can Offset?
Send us your last 12 months of bills and your operating hours. We’ll show you the real numbers before you invest, rather than sizing a system off your annual units alone.
Not Sure How Much of Your C5 Load Solar Can Offset?
Send us your last 12 months of bills and your operating hours. We’ll show you the real numbers before you invest, rather than sizing a system off your annual units alone.