For a manufacturing company in Chennai, electricity is one of those expenses that quietly grows with production.
A factory may have motors running throughout the day, compressors supplying production equipment, pumps moving water, HVAC systems maintaining temperature, lighting across large production areas, CNC machines operating continuously, and office or warehouse loads running alongside them.
When electricity consumption reaches lakhs or even millions of units a year, a small reduction in the effective cost per unit can make a meaningful difference to the business.
This is where rooftop solar comes into the picture.
But one question comes up again and again among Chennai factory owners:
“How much can my factory actually save with rooftop solar?”
There is no single answer because the savings depend on the factory’s electricity consumption, rooftop area, solar capacity, electricity tariff, operating hours, system performance, financing and the applicable Tamil Nadu electricity arrangement.
Still, it is possible to understand the calculation clearly.
This guide explains how Chennai manufacturing businesses can estimate rooftop-solar savings and what they should check before investing.
Why Rooftop Solar Can Work Well for Chennai Factories
A major advantage for many factories is the timing of electricity consumption.
Solar power is generated during daylight hours.
Many manufacturing facilities also consume substantial electricity during daylight hours.
That creates a natural match.
A factory operating from 8 AM to 6 PM, for example, may be able to directly use a significant portion of its solar generation while production is running.
The Tamil Nadu rooftop-solar portal notes that grid-connected rooftop solar can generate electricity for consumption at the site, while excess generation can be handled through the applicable grid arrangement. TANGEDCO’s current FAQ also states that rooftop solar systems cannot generate at night, meaning the grid continues to supply electricity when solar generation is unavailable
For a factory, this means solar does not necessarily replace the electricity grid.
Instead, it can reduce how much electricity the factory needs to purchase from the grid.
The Simple Solar Savings Formula
The basic calculation is surprisingly simple:
Annual Solar Savings = Solar Electricity Used × Effective Electricity Cost
For example, if a solar plant produces 500,000 units a year and the factory effectively avoids ₹8 per unit of electricity purchases, the theoretical electricity-cost reduction would be:
500,000 × ₹8 = ₹40 lakh per year
But this is only an illustration.
Actual savings depend on how much of the solar generation is self-consumed, how excess electricity is settled, the applicable tariff and charges, and the specific consumer and metering arrangement.
That is why a professional feasibility study should use the factory’s actual electricity bills rather than relying on a generic online calculation.
How Much Electricity Can a Rooftop Solar System Generate?
A useful starting point is system capacity.
For example:
100 kW solar system
250 kW solar system
500 kW solar system
1 MW solar system
The annual generation will depend on solar radiation, orientation, shading, module efficiency, temperature, inverter performance and system losses.
TANGEDCO’s rooftop-solar FAQ gives a broad average of approximately 4 to 5 units per day per kW for grid-connected rooftop systems. (TNEB Limited)
That translates roughly to:
| Solar Capacity | Approx. Daily Generation | Illustrative Annual Generation |
|---|---|---|
| 100 kW | 400–500 units | 1.46–1.83 lakh units |
| 250 kW | 1,000–1,250 units | 3.65–4.56 lakh units |
| 500 kW | 2,000–2,500 units | 7.30–9.13 lakh units |
| 1 MW | 4,000–5,000 units | 14.60–18.25 lakh units |
These are planning-level illustrations, not guaranteed generation figures. Actual production at a Chennai factory will depend on the specific site and system design.
The official Tamil Nadu solar calculator similarly warns that actual production varies with roof orientation, tilt, shading, panel temperature, inverter efficiency and system losses.
Example: A 100 kW Factory Solar System
Let’s start with a smaller industrial installation.
Suppose a Chennai factory installs a 100 kW rooftop solar system.
If the system produces approximately 1.46 to 1.83 lakh units annually, the savings depend on the value of electricity that those units replace.
For illustration, assume the factory avoids ₹8 per unit of electricity purchases.
At 1.46 lakh units:
1,46,000 × ₹8 = ₹11.68 lakh per year
At 1.83 lakh units:
1,83,000 × ₹8 = ₹14.64 lakh per year
So a 100 kW system could potentially represent roughly ₹11.7–₹14.6 lakh of annual electricity-cost reduction under these illustrative assumptions.
But remember: this is not a quotation or guaranteed saving.
The factory’s actual tariff, self-consumption, export treatment, system performance and other charges need to be included.
Example: A 250 kW Factory Solar System
Now consider a manufacturing unit with more roof space.
A 250 kW system could produce roughly 3.65–4.56 lakh units annually using the same broad planning assumption.
At an illustrative avoided electricity cost of ₹8 per unit:
3.65 lakh × ₹8 = ₹29.2 lakh
to
4.56 lakh × ₹8 = ₹36.5 lakh
This gives a possible annual electricity-cost reduction in the broad range of ₹29–₹36 lakh, before considering the actual commercial structure and project costs.
For a factory with substantial daytime consumption, this can become a meaningful operating-cost consideration.
Example: A 500 kW Factory Solar System
A larger industrial facility may be able to accommodate a 500 kW rooftop plant.
Using the same broad generation range:
Annual generation: approximately 7.3–9.13 lakh units
At an illustrative ₹8 per unit avoided cost:
₹58.4 lakh to ₹73.0 lakh per year
Again, this is a simplified example.
A real project calculation needs to account for the actual tariff structure and how much solar electricity is consumed directly at the factory.
For a large factory, demand charges and other components of the electricity bill also need to be examined separately. Solar generation primarily offsets energy consumption; it should not automatically be assumed to eliminate every component of the electricity bill.
Example: A 1 MW Rooftop Solar System
Large manufacturing facilities sometimes have extensive roofs that can support much larger solar installations.
A 1 MW rooftop system could produce approximately:
14.6–18.25 lakh units per year
Using the same illustrative ₹8 per-unit avoided electricity value:
14.6 lakh × ₹8 = ₹1.168 crore
18.25 lakh × ₹8 = ₹1.46 crore
That means the theoretical annual energy-cost reduction could be around ₹1.17–₹1.46 crore under these simplified assumptions.
For a factory consuming many millions of units annually, a 1 MW system may still represent only part of its total electricity requirement.
That is why larger businesses often need to look beyond rooftop solar as well.
Your Factory’s Electricity Tariff Changes the Calculation
One factory may effectively avoid ₹7 per unit.
Another may have a different effective energy cost.
Another may have a different tariff structure altogether.
So two factories with identical 500 kW solar plants can have different financial outcomes.
This is why it is risky to say:
“A 500 kW solar system always saves ₹X lakh per year.”
It doesn’t.
The correct calculation should start with the customer’s actual electricity bill.
Look at:
- Energy charges
- Demand charges
- Applicable electricity duties and taxes
- Power factor-related components
- Other applicable charges
- Solar export/settlement treatment
- Operating hours
- Monthly consumption
Then calculate how much of the solar generation actually offsets purchased electricity.
Self-Consumption Is Extremely Important
Suppose your factory’s solar plant produces 1,000 units during a particular day.
If the factory consumes 900 units during those solar-generation hours, most of the generation can potentially be used directly.
But suppose the factory consumes only 400 units during that period.
The remaining generation may flow into the grid, depending on the applicable arrangement.
That excess electricity may not have the same financial value as electricity consumed directly by the factory.
This is why solar sizing should be based on load profile, not simply roof size.
A bigger solar plant is not automatically a better investment.
The goal is to find the capacity that makes sense for the factory’s consumption and commercial arrangement.
A Factory’s Daytime Load Can Make a Big Difference
Consider two Chennai factories.
Factory A
The factory operates mainly from 8 AM to 6 PM.
It runs multiple production lines during the day.
Its electricity consumption closely overlaps with solar generation.
Factory B
The factory has relatively low daytime consumption but operates heavy equipment mainly at night.
Both factories may have the same annual electricity consumption.
But rooftop solar can interact with their electricity demand very differently.
Factory A may have a strong direct-consumption opportunity.
Factory B may need to examine the applicable export arrangement, storage, operating schedule or other renewable-energy options.
This is why an energy audit and load analysis should come before the final system size.
How Much Roof Space Does a Chennai Factory Need?
Roof area is another important factor.
TANGEDCO’s FAQ gives a general planning reference of around 10 square metres of shadow-free area per kW for rooftop solar, although actual requirements depend on the system design and site conditions.
Using that broad reference:
| Solar Capacity | Approx. Shadow-Free Area |
|---|---|
| 100 kW | ~1,000 sq. m |
| 250 kW | ~2,500 sq. m |
| 500 kW | ~5,000 sq. m |
| 1 MW | ~10,000 sq. m |
This is only a preliminary planning estimate.
The actual usable area can be affected by roof structures, access pathways, equipment, shading, setbacks and mounting design.
A factory should never assume that its total roof area equals its solar installation area.
Roof Condition Matters
A manufacturing roof may look perfect from the ground but still require engineering inspection.
Before installing solar, check:
Roof age
Roof material
Structural strength
Waterproofing
Corrosion
Existing equipment
Maintenance access
Drainage
Wind loading
Future roof-replacement plans
This is particularly important for industrial buildings with older metal roofs.
If the roof needs replacement in a few years, the solar project should be planned accordingly.
Spending heavily on a solar installation and then having to dismantle it for roof replacement can create avoidable costs.
What About Chennai’s Heat and Weather?
Chennai receives strong solar radiation, but solar modules do not operate at their rated laboratory efficiency throughout the day.
High module temperatures can affect output.
Dust, salt-laden air in coastal areas, industrial pollution, bird droppings and other environmental factors can also affect performance.
This makes proper system design and maintenance important.
The official Tamil Nadu solar calculator specifically notes that actual generation can vary because of panel temperature, shading, inverter efficiency, roof orientation and system losses.
A good solar assessment therefore needs to look at the actual site rather than using only a generic Chennai generation assumption.
What About Rain and Monsoon Months?
Solar does not stop working during cloudy weather, but generation can be lower.
A grid-connected factory does not lose electricity supply simply because solar production falls.
The grid continues to provide the electricity required by the factory when solar generation is insufficient. TANGEDCO’s FAQ explicitly notes that rooftop solar does not generate at night and that grid power supplies the load when solar generation is unavailable.
This is one reason grid-connected rooftop solar can be practical for factories.
Solar reduces purchased electricity when it is producing.
The grid continues to provide flexibility.
Does Solar Eliminate the Factory’s EB Bill?
Usually, factory owners should not think of rooftop solar as automatically making the electricity bill zero.
The solar system reduces electricity purchased from the grid.
But the bill can contain multiple components.
There may also be demand-related charges and other applicable costs.
The treatment of excess generation depends on the consumer category and applicable metering mechanism.
TANGEDCO’s published guidance distinguishes between arrangements such as net feed-in and gross metering and sets out different treatment for consumer categories and capacities.
Therefore, a realistic solar financial model should calculate:
Current electricity bill
minus
avoidable electricity cost from solar
plus/minus
applicable solar and grid-related charges
rather than assuming the entire existing bill disappears.
What Is the Payback Period?
Payback is one of the first questions factory owners ask.
The basic calculation is:
Payback Period = Total Solar Investment ÷ Annual Net Savings
For example, if a hypothetical project costs ₹2.5 crore and produces ₹50 lakh in annual net savings:
₹2.5 crore ÷ ₹50 lakh = 5 years
That is a simplified example.
Actual payback should account for:
System cost
Financing cost
Annual generation
Electricity-price assumptions
Operations and maintenance
Performance degradation
Insurance
Applicable charges
Tax treatment
Replacement costs
The important point is that payback should be calculated using net project savings, not simply annual solar generation multiplied by an assumed electricity rate.
CAPEX vs Financing
A factory does not necessarily have to fund the entire solar project from its own cash reserves.
Businesses can evaluate different approaches.
CAPEX Model
The company purchases and owns the solar plant.
The initial investment is higher, but the company owns the asset and benefits from the electricity it generates.
Financed Solar
The company uses debt or another financing structure to fund the project.
The financial model then needs to include interest and repayment obligations.
Third-Party Models
Depending on the project and applicable structure, businesses can also investigate arrangements where a third party finances or owns the renewable asset and the factory purchases electricity under a contract.
The right approach depends on the company’s financial position and investment priorities.
What If the Roof Is Not Big Enough?
This is where the conversation becomes more interesting.
Suppose your factory consumes 10 million units of electricity every year.
Your roof can support a solar system generating approximately 1.5 million units.
That is still useful.
But rooftop solar alone cannot address the entire requirement.
The business could then evaluate other options such as:
Open-access renewable power
Purchase renewable electricity generated at an off-site project under the applicable regulatory framework.
Group captive
Participate in the ownership of a renewable project and consume electricity under the captive framework, subject to the applicable requirements.
Energy efficiency
Reduce the amount of electricity the factory needs.
Battery storage
Consider storage if the economics and load profile justify it.
This is why larger Chennai businesses are increasingly looking at energy strategy rather than only rooftop installation.
Rooftop Solar + Energy Efficiency Can Increase the Value
Imagine a factory spending ₹1 crore per year on electricity.
The company installs rooftop solar and reduces its electricity purchase.
But it also identifies:
Compressed-air leaks
Inefficient motors
Old lighting
Poorly controlled HVAC
Oversized pumps
Unnecessary idle loads
If energy efficiency reduces consumption as well, the business can reduce the amount of electricity it needs to purchase in the first place.
This creates two separate benefits:
Use less electricity
and
Generate more of the remaining electricity from solar
For manufacturing businesses, this combined approach can be more useful than treating solar as a standalone project.
What About Open Access in Tamil Nadu?
For larger consumers whose rooftop potential is limited, renewable open access is another option to investigate.
Tamil Nadu’s Green Energy Open Access framework provides a route for eligible consumers to procure renewable electricity through the grid, with provisions covering areas such as eligibility, connectivity, metering, energy accounting and banking.
The commercial calculation is different from rooftop solar because open access can involve additional network and regulatory charges.
So a factory owner should compare:
Rooftop solar landed cost
versus
Open-access renewable landed cost
versus
Current grid electricity cost
The lowest generation price is not necessarily the lowest final electricity cost.
What Should a Chennai Factory Owner Calculate?
Before approving a solar project, calculate these numbers:
1. Annual electricity consumption
How many units does the factory consume?
2. Daytime consumption
How much electricity is being consumed while solar is generating?
3. Maximum demand
What is the factory’s peak demand?
4. Available roof area
How much usable, shadow-free space is actually available?
5. Solar capacity
How much solar can realistically be installed?
6. Annual generation
How many units should the system produce?
7. Self-consumption
How much solar electricity will be used directly?
8. Export
How much may be exported under the applicable arrangement?
9. Effective electricity value
How much cost does each self-consumed solar unit actually avoid?
10. Net annual savings
What remains after O&M and other applicable costs?
11. Investment
What is the total project cost?
12. Payback
How long does it take to recover the investment?
These numbers provide a much more meaningful picture than simply asking for the price per kW.
A Practical Example for a Chennai Manufacturing Unit
Let’s put everything together with a hypothetical example.
Suppose a factory consumes:
50 lakh units per year
The factory has sufficient roof space for:
1 MW rooftop solar
Assume, purely for illustration, that the plant generates:
16 lakh units per year
If the factory can use most of that generation directly and its avoided electricity cost averages ₹8 per unit, the gross energy-cost reduction would be approximately:
16 lakh × ₹8 = ₹1.28 crore per year
Now suppose the project has annual operating and maintenance expenses and other costs.
The actual net savings would be lower than the gross figure.
The factory can then compare the net annual savings against the total investment.
This is how a professional solar feasibility study should work.
It should show the assumptions clearly.
It should not simply promise:
“1 MW solar will save ₹1.28 crore every year.”
The actual result depends on the factory.
What Can Change Your Savings?
Several factors can move the final number up or down.
Electricity Tariff
Higher avoided electricity costs can increase the value of solar generation.
Solar Generation
Better system performance produces more usable electricity.
Self-Consumption
Using more solar directly at the factory generally improves the value of generation compared with relying heavily on export settlement.
Roof Condition
Additional structural or roof work can increase project costs.
Shading
Shading can reduce generation.
Maintenance
Poor maintenance can reduce long-term output.
Factory Operating Hours
Daytime operations can improve solar utilisation.
Future Expansion
A factory expecting significant growth may have greater future electricity demand.
Regulatory Changes
Applicable metering, charges and electricity regulations can change over time.
This is why financial models should include sensitivity analysis rather than relying on a single number.
How Kinetiq Energy Can Help Chennai Factories
For a factory owner, the biggest challenge is often not finding solar panels.
It is understanding what solar capacity actually makes sense for the business.
At Kinetiq Energy, the process can begin with the factory’s actual energy data.
The assessment can look at:
Monthly electricity consumption
Electricity bills
Operating hours
Maximum demand
Daytime load
Roof availability
Roof condition
Electrical infrastructure
Solar generation potential
Energy-efficiency opportunities
Future expansion plans
From there, the business can evaluate an appropriate rooftop solar configuration and understand the expected energy generation and financial impact.
If rooftop space cannot meet the factory’s renewable-energy requirement, additional options such as open-access renewable power or group captive structures can also be evaluated where applicable.
The objective is not simply to install more solar.
It is to help the business make a better energy investment decision.
You can explore Kinetiq Energy’s solar and energy solutions at kinetiqenergy.com.
Questions to Ask Before Installing Rooftop Solar
Before signing an EPC contract, Chennai factory owners should ask:
The Most Important Number Is Not the System Size
When discussing solar, people often ask:
“Should I install 100 kW, 250 kW, 500 kW or 1 MW?”
That is not the first question.
The first question should be:
“How much electricity does my factory use, and how much of that electricity can solar realistically replace?”
Once that is understood, the appropriate system capacity becomes much easier to determine.
A factory with a smaller roof may need a smaller rooftop plant.
A factory with a large roof and heavy daytime consumption may justify a much larger installation.
A factory with extremely high electricity consumption may need rooftop solar plus another renewable-energy procurement model.