Frequently Asked Questions on Rooftop Solar.  தமிழில் காண

What are the types of Rooftop Solar Power Plants ?

There are two types of Solar Power Plants.

  • On-Grid (without Batteries)
  • Off-Grid (with Battery back-up)
On Grid System:

On Grid Solar Plants come without batteries.
The Solar Panels generate DC and an inverter converts that into usable AC form, and this out put is fed directly into the EB lines.
This type of Solar Plants can not be used as backup for EB.
Because, an On Grid System will not generate power when the EB power fails.
Government subsidy is available only for On Grid Systems.

Off Grid System:

Off Grid Solar Power Plants come with Batteries. They can be used as a backup for EB Power.
Government Subsidy is not available for such systems.

Why is an On Grid System switches off when EB power Fails?

OnGriDsystem-Pic Credtlighthousesolardotcom

This is a valid question. Let us first see how an On Grid System works.
As in the above picture, we have solar panels on the roof, when sunlight falls on these panels, electricity is generated.
But this generated electricity is DC (Direct Current), whereas most of our equipments like Fan,light,Fridge,TV etc work on AC (Alternating Current).
So, to convert this DC to into AC an INVERTER is used.
The AC current from this INVERTER is fed directly on to the EB lines ,at the Meter Point.
It acts almost like a mini Power Station, that actually feeds our EB lines.
So, the Solar Power that is generated, should match the characteristics of the EB power.
If the EB voltage is at 200 Volts, Solar Power Output should also be 200 Volts.
If the EB frequency is 49.8 Hz, Solar Power output should also be exactly 49.8 Hz.
This is achieved by a process called synchronization on the INVERTER.
For this a small portion of the EB line is fed into the INVERTER, which checks at the Voltage/Frequency and uses that to produce the same Voltage and Frequency using Solar Power.
Now imagine what will happen when the EB power fails.
The inverted would not know if it has to Generate 200 V or 210 V at 50 Hz or 49.0 Hz.
So it switches itself off.
There is another important reason why an On Grid Plant goes off when EB power fails.
We know that every now and then the EB technical team needs to do maintenance on a Distribution transformer or other electrical parts on the line.
So they switch off the supply to the area and start their work.
During such time, if any of the On Grid Solar Power plant is generating the Power, the EB maintenance person would receive a huge shock.
Hence to ensure the safety of the EB personnel, the Solar Power that feeds into the EB lines need to be switched off whenever EB power fails.

If an On Grid Solar Plant can not work during power failures, then why should we install it at all ?
Another valid question.
The simple, short answer is, ‘To Save Money’.
On Grid Plants would generate power for a period of 25 Years.
A 1 KW plant could save you up to Rs 1500 on every Electricity Bill.
That means a savings of Rs 9000 per year at the current EB Tariff.
If the EB Tariff goes up, our savings will also be more.
And an On Grid Solar plant needs very little maintenance all you need to do is just clean the panels with water every other week.. that is all.
They are environment friendly too, as they don’t emit any smoke, they don’t make any noise.
Besides all this, the Government Subsidy (upto Rs 50,000-) is available for On Grid residential plants.
So, if you have a reliable EB connection , it is highly recommended to go for an On Grid Solar Power Plant and make the empty spaces on your roof top earn money..!
What are the advantage/disadvantage of an On Grid System ?



Since On-Grid system does not require Batteries, we don’t have to do frequent maintenance/replace Batteries, and the cost would be considerably less.

Grid – Tied systems require the presence of EB power to make sure the power generated has the same characteristics (voltage / frequency etc) as the EB power.

The Generated Power would always be used, even if you do not switch on any light/Fan/TV. Because in an On-Grid system the Solar Generated Power is directly fed to the EB lines and you would be getting your electricity Bill reduced (using a Bi-directional Net Meter) to the extent of the power generated by the Solar Plant.

Hence when there is a power failure, the On Grid system stops working ,even though the sun is shining bright.The power generated by the solar Panel could not be used in this situation.

On-Grid system installation requires inspection/approval from the EB.

You need to apply, get the ‘Feasibility Report’ from the AE of your area and then install. After installation EB personnel would inspect, and allot a Net Meter , and only then the Solar Plant would become productive.

So this involves proper planning and some delay.

But this is a one- time trouble, and most of the Vendors providing the plant would help in this matter.

Why an Off Grid System is not recommended for residential use ?
Tamil Nadu Government subsidy under CIS is not available for Off Grid Systems.
Since this type of system, needs batteries, they require frequent maintenance.
A good Battery has a limited life ( 3 to 5 Years).. and needs replacement.
This adds up to the cost of the system.
Unlike an On Grid system, an Off Grid system would supply power to only a specific set of equipments (like 2 fans, 5 Lights etc).
If these equipment are not being used during the day time, the Solar Power would only be used to charge the battery.
But Batteries have limited capacity (say 150 AH or 200 AH).
So once the batteries are fully charged, the remainder of the Solar Power goes waste.
So an Off grid system is not efficient or economical.
This type should be used only when EB power is not available at all even for a few hours a day.
If the requirement is only to provide back-up, we must use regular (non-solar) inverter/UPS with adequate Battery.
This will be more economical.
What are the advantage/disadvantage of an Off Grid System ?



In an Off-Grid mode, the Solar Power plant does not require the presence of EB Power to operate. So during day-time, even if the EB power is cut, the Solar Plant will continue generating power.

Since Batteries are needed in Off-Grid mode, the cost of the plant is higher. Batteries need frequent maintenance / replacement.

This mode is suitable when EB power is un-reliable and there are frequent load-shedding /power failures for prolonged time.


Also in Off Grid mode the efficiency of the power plant is reduced. That means that though we have 1000 Watts Solar panels installed, we would not be able to extract the maximum, as we lose some power in the process of charging the batteries.

Also this mode would limit the type of load you can connect.

That is, only the load connected to the inverter would be working on Solar Power.

What is NetMetering ?

The regular Energy Meter in our homes work in one direction only.That is this meter records the amount of electricity drawn from the EB lines and used in the home.

A Net-Meter is a bi-directional meter. It works in two directions.


It records the amount of power drawn from the EB lines (import) and also the power sent to the EB lines (Export). So we get charged only for the difference.

For example, if in a month you consume 500 units of Power from EB and the solar plant generates 400 units, the meter will charge for only 100 units.

This works to our advantage when an On-Grid Power plant is used.

Even if we go on vacation, the power Generated by the solar plant will be completely exported and we get that much reduction in our EB Bill.

Net-Meter needs to be purchased from EB, and it costs around Rs.6000 for 3 Phase connection.

What is Accelerated Depreciation ?

The Government subsidy is available only for Residential consumers.

To incentivise, the use of Solar for other commercial users, the Accelerated Depreciation Scheme.

For commercial Organisations, who have taxable income Accelerated Depreciation could be used to claim tax benefits.

Accelerated depreciation is a method of depreciation used to depreciate the assets in a manner that a significant depriciation is allowed in the first few years. Accelerated depreciation can reduce costs during the project’s start up year.

In solar projects, accelerated depreciation is widely used as an incentive to lessen the burden of tax. In India, Accelerated depreciation (AD) allows investors, mostly setting up capacity for captive use, to take advantage of up to 80% of the project cost.

For a 100kWp installed with a capital cost of approximately INR 80 Lacs, a developer can write off payable tax for up to INR 24 Lacs in the first year of operation.

Visit the following sites for more details:

Understanding Accelerated Depreciation for Solar Projects:

Slideshare presentation:

How much space is needed to install a roof-top Solar Power Plant ?

Solar PV plants require a south-facing roof . The roof should also be free from shadows; portions of the roof that have shadows should be excluded while calculating the roof space available for the solar plant.

Shadows falling on solar panels reduce power generation, and may also damage the panels.

The kind of roof determines if solar panels can be installed.

Ability of the roof to support the weight of the panels and mounting structure even in strong winds is the critical parameter.

Concrete Roofs – Solar PV plants can be easily installed on flat concrete roofs.

Inclined roofs may pose a challenge depending on the angle and direction of the inclination

Metal Roofs – This will depend on the type of structure and the weight it can withstand Asbestos/Other Sheet Roofs – These are the least likely of the 3 to be able to support solar panels, though it is possible in some cases.

Approximately 100 Sqft is required for 1 kW of solar plant.

Do we get Government Subsidy for residential/commercial Roof-Top Power Plants ?

Yes. In Tamil Nadu, under Chief Minister’s Cash Incentive Scheme (CIS), for the first 10,000 installations, subsidy up to Rs.50,000/ (Rs 20,000 from State Government and Rs 30,000 from central government’s MNRE) is available.

But there are some conditions:

  1. This CIS is applicable only for residential service connections . (Tariff LA1A).
  2. Only On Grid Type of Solar Plants (without Batteries) are eligible for subsidy.
  3. Plants installed by the TEDA (Tamil Nadu Energy Development Agency) approved Vendors are only eligible for Subsidy.
  4.  On Grid Plant’s capacity should be 1 Kw, 2 Kw , 5 Kw or 10 KW.
  5. The maximum limit for the subsidy is Rs. 50,000/-


Since above scheme is available only for residential consumers ,for the benefit of Commercial (non residential) consumers Central Government gives Income Tax benefits through Accelerated Depreciation.

A 1 KW On Grid power plant typically costs about Rs.1 Lakh to Rs. 1.5 Lakhs. After deducting subsidy the cost for a 1 KW On Grid Power Plant would be Rs 50, 000 to Rs.70,000.

Contact us for specific price range and how to order.

Can we make an existing Inverter/Battery system to work on Solar ?

Technically speaking, yes, we can make an existing Inverter / Battery system to work with Solar PV panels in Off Grid mode provided the inverter and Battery bank are in good condition.

But practically such a system is not recommended for Residential users , since they cost more and not economical.


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