Extracting solar power from your terrace: An FAQ

Solar energy is the primary source of energy for all living things on earth. It is the cleanest and greenest form of energy available. However, except on a small scale, it is not really used to power the inanimate machines that have increasingly become part of our everyday lives. This is primarily because one needs to convert the solar energy in to some other form of energy – either mechanical or electrical energy – and till recently this was neither very energy-efficient nor cost-efficient.

Over the last several years, though, the solar panels, which convert solar energy to electrical energy, have been becoming better and cheaper. As a result it has become an increasingly common source of energy – especially in rural areas – where there is often no other choice.

In urban areas, however, they have not taken off on the same scale – partly because the initial cost is still quite high – and partly because there is always the option of relying on the electricity grid.

Reliance on the grid has its drawbacks. As is clear to any resident of Bangalore – there just is not enough energy being produced. As a result, the government has to resort, in the long term, to producing energy by ‘dirty’ methods such as coal plants or nuclear power plants, or potentially environmentally harmful methods like massive hydroelectric plants, or, in the short term, to load-shedding at times of shortage, or to cut electricity to rural areas to light up the urban areas.

The hapless consumer then has to resort to buying gadgets like inverters – which may temporarily solve the problem for the consumer – but in fact do not reduce consumption at all – since one is just consuming more energy when it is available and storing it for later – like the ‘sump and pump’ system for water supply that has become commonplace.

The future, therefore lies in finding an alternative source of energy – an analogue of rain-water harvesting if you like – going off the grid. While technologically it has been viable for a while, there have been new developments on the regulatory side which have made it possible to not only go off the grid – but supply energy back to the grid. It is this that changes the equation completely and makes it much more cost effective – in fact profitable in the long run – to install a solar power plant on one’s roof.

The Karnataka Energy Regulatory Commission (KERC) has recently announced rates at which the electricity companies (ESCOMs) in Karnataka, will buy excess solar energy from individuals and commercial establishments.

In this write up, I will explain what I understand of the situation. Namely, I will try to give an answer to the question – Should I install solar panels on my roof?  The quick answer is yes – if you have the space and some money to spare. However, here is a more detailed analysis. 

Some questions and answers:

`Watts’ the difference between Power and Energy? Energy, as we have all learned in school, cannot be created or destroyed. Physicists measure it in joules but BESCOM and electricity companies in general use this mysterious quantity called units. Power is energy per unit time and is measured in watts or kilowatts.

Often these things are confused – since you get a ‘power bill’ not an ‘energy’ bill, while you are told to reduce your energy consumption. I too was quite confused when I started trying to understand the situation. I did not want to sound like a complete idiot if I called an expert – so read up a little – and did some calculations which the readers might find useful.

For the sake of making the calculations easy – I have made some approximations of the actual numbers. Let us assume that the cost of a unit of energy is around Rs 5.

  • What we do, when we ‘consume’ electricity or consume energy is convert it from one form to another. For instance – chemical energy is converted to thermal energy when you  heat up milk and water for your morning’s coffee using an LPG stove. When I use the words ‘generate’ or ‘produce’ what I mean is convert from one form to another.
  • An electricity bill is determined by units of electrical energy consumed. One unit is the same as one kilowatt-hour. So if you turn on a 100W bulb for an hour, it will consume 0.1 units.
  • A typical middle class household, equipped with the usual accoutrements of middle class life – TV, washing machine, storage water heater and pressure washer – living on a 30 x 40 plot, consumes around 300 units of electricity a month. This corresponds to around 10 units a day and a monthly bill of around 300 x Rs. 5 = Rs. 1500.
  • Solar panels are typically rated in watts. For instance, a 2 x 1 square metre solar panel is usually rated around 250 watts. This means, in one hour it will generate 250 watt-hours of energy. Or in a day, with 8 hours of sunlight, it will generate 2 kilowatt-hours of energy – or 2 units.
  • So if your daily consumption is around 10 units, you will need 10/2 = 5 panels to produce it. Since each panel is 2 square meters, you need 5 x 2=10 square meters  ( ~ 110 square feet ) of roof space for it. 

What are the new proposals? The new proposals of the KERC is that the ESCOMs will buy excess solar energy from you at a certain rate. Here is how it works.

  • Suppose, as above, you consume 10 units a day – hence you need 5 panels to produce it. However, you might have a much larger terrace – suppose, for instance your terrace is about 55 square metres ( ~ 600 square feet), it is more than enough to accommodate 20 panels. Hence you would be producing 20 x 2 = 40 units per day.
  • The energy is supplied to the grid and then supplied back to you – and you get paid the net change. For instance, if you consume 300 units a month but supply them 30×40=1200 units you get paid for the balance of 900 units.
  • There is also the alternative – depending on the kind of inverter you have – of simply storing some of the energy for your consumption and supplying the excess to the escom.
  • Suppose you generate 40 units of energy and consume 10 – you will supply BESCOM with 30 units of energy a day.
  • BESCOM will pay you Rs. 9.52 per unit, so this will amount to 30 x Rs. 9.52 a day = Rs. 285.6 per day. This means 365 x Rs. 285.6 = Rs. 1,04,244 per year!
  • Added to this, you will also be saving on your electricity bill which at Rs. 1,500 a month amounts to Rs. 18,000 a year. So that brings the total to around Rs. 1,20,000 a year.
  • The cost of installing such a unit is around Rs. 7 lakhs unsubsidised. So earning Rs. 1,20,000 a year means you will pay off the investment in about 6 years After that, you will earn Rs. 1,20,000 a year.
  • There is also the option of getting a subsidy. That would reduce the initial cost by 30% but you will get a lower price in return – Rs. 7.20 per unit. However, the catch with that is, you might be forced to buy your panels from certain approved sellers and it is possible that the quality could be inferior.

I should remark that I have made several simplifying assumptions in order to make the calculations easier. The most glaring of them is that I have assumed 8 hours of sunlight. To my understanding the experts assume around 6 hours of sunlight per day. Also, the cost of a unit is not constant – there are slabs – and  the price changes depending on the consumption.

However, it is more or less the case that you will be able to pay off your initial investment in about 6 or 7 years and after that it is all profit.

Are there any other benefits?

  • Since you have an additional layer over your roof, your roof will not get direct sunlight – hence will remain cooler in the hotter months and you will save on cooling costs and sleep better.
  • Solar panels typically last around 20 years and do not substantially degrade. They require minimal maintenance – need to be occasionally cleaned to remove dust. This might be problematic if you lived in a desert – but in Bangalore it should be fine. In fact, by installing solar panels you would be doing your bit to stop Bangalore turning in to a desert! 
  • Unlike solar water heaters that require direct sunlight, photovoltaic solar panels do not require direct sunlight – so they can still generate power in the shade.

So if you have a few lakhs and a roof to spare – perhaps from that fortuitous Bitcoin investment – then it’s well worth investing in panelling your roof with solar panels. Not only will you make some money in the long run – you will have a planet to spend it in.

Okay, I’m convinced, what do I do next? 

Not so fast. Here it gets a little murkier.

  • KERC is a regulatory board, so their recommendations have to be accepted by BESCOM and other electricity commissions.
  • In order for the electric supply companies (ESCOMs) to buy power, one has to enter into a contract with them.
  • In order to sell power to the ESCOMs one has to get the panels installed by approved installers.
  • It might also be possible to rent out your terrace to a third party who will install the panels and make a contract with BESCOM. This will save you on the initial investment and the trouble of signing a contract – but of course you will earn less.

What are the technical requirements? 

Though the situation is still not entirely clear as far as the regulations are concerned, I thought I would investigate this the technical and feasibility aspects a little more. I called up a solar panel company and they sent over someone from a company which does the installation. After speaking with him, I learned there are a few more technical issues to consider.

  • You need to have a roof which gets sunlight from the South. The panels will be angled towards the South. ( yes, the sun still rises in the East – but for us – since we are in the Northern hemisphere – sunlight mostly comes from the South.)
  • Unlike solar water heaters which use heat, solar power panels generate power from luminosity, so you do not necessarily need direct sunlight – but of course in direct sunlight it is brighter – so the panels will generate more power.
  • The inverters come in several types – Export Units (EUs) and Export-Storage Units (ESUs). EUs are essentially for supplying power back to the grid and not storing it for your home consumption. ESUs allow you to decide how much you want to store and how much you want to sell to the grid.
  • There are panels of varying quality. Important things to look for are operating temperatures. Some panels designed for Europe, for instance, have an operating temperature of −20◦C to 35◦C which may not be sufficient for India – where it can get much hotter.
  • A factor which influences panel life is the thickness of the glass – the thicker the glass the stronger it is.
  • Older inverters had a problem whereby they could overcharge the batteries on sunny days which would cause the batteries to degrade quite quickly. Modern inverters incorporate technology which avoids that by reducing to a trickle charge as the battery fills up.
  • Another factor to be considered is while batteries and panels can be added later, the inverter has a specific capacity. So if you plan on adding panels later to sell to the grid, for instance, it makes sense to get a larger capacity inverter at the outset.
  • While you can use the battery solar energy to power up a pump and refrigerator – especially if it is one of those new ‘inverter’ models – you cannot use it to run a washing machine or anything which requires a lot of energy in a short time. So if you use a washing machine, you cannot be entirely off the grid. 
  • There is a certain amount of loss – around 15% – of power in the storage and reconversion process. Solar panels generate a DC current which charges the batteries and this has to be converted to AC to be supplied to the grid or to be consumed by your gadgets. So for instance, if you have a panel rated at 280 Watts for the purpose of computing the number of panels required you should assume that it produces about 250 watt-hours of energy.

Bureaucracy and time delays notwithstanding, if you have a roof and some money to spare – it is perhaps worthwhile to install a solar plant on your roof. You do not need too many panels so the cost is not so much. The cost of a plant which produces 10 units a day – which would suffice for most peoples daily needs – is around 2.5 lakhs. The price does not scale linearly – since there are some absolute costs like the cost of the inverter and wiring which does not increase substantially when you add more panels – so the more panels you add the per unit costs come down.  

Though I must confess that I have not yet installed solar panels on my roof – I am waiting for more clarity on the issue. (I would like to thank K.P Yogendran and B.V. Sreekantan for their comments)

Edison and Tesla

Everyone knows Thomas Alva Edison – the ‘Wizard of Menlo Park’ – invented the light bulb, right? It is completely false. The incandescent light bulb was invented by several people – none of whom were Edison. Edison simply marketed it. He was, in the language of our time, a patent troll.

In fact, Edison, from all accounts, was a terrible person. The real pioneer in the field of electricity distribution was the Serb-American Nikolai Tesla. As you are perhaps aware, electricity supplied to us today and most of our gadgets use Alternating Current (AC). However, the first electrical gadgets – like the lightbulb marketed by Edison used Direct Current (DC) – like a battery. Edison was blindly adamant on the use of DC rather than AC for electricity transmission. Tesla believed that AC was better. It developed in to a bitter feud called the `War of Currents’ in which Edison tried desperately to prove Tesla wrong.

There is an interesting story – apparently people noticed that pet dogs and cats around Menlo Park were suddenly disappearing in large numbers. It turned out that Edison was offering local school kids a quarter for every dog or cat they could find. He would then send his cronies around to give public demonstrations where they would electrocute the dogs and cats using AC to purportedly show its danger! Such were the lengths Edison would go to!

As it turned out AC was much better that DC to transmit electricity over long distances, so Tesla eventually won out. However, Edison became a very rich man and Tesla died in poverty – simply because he did not care about patenting his ideas. For him, pleasure came out of working in his laboratory and not from the money his inventions could make. In fact, it is believed that he invented many things for which he is not usually credited – like the radio, AC induction motor and  radar – much before the others did. One of the most fascinating things he was working on – but could not achieve because of a loss in funding due to bad press – was wireless electricity. Only now is it becoming a reality in the form of wireless mobile phone chargers. On a larger scale it would be quite revolutionary.

Electricity in Karnataka

What is perhaps not so well-known is that the Kolar Gold Fields, about a 100km East of Bangalore, was the third place in India to be electrified. In 1902, the British built the Shivanasamudram project to supply electricity to the gold mines. Till fairly recently it used AC current with a frequency cycle of 25Hz – so many gadgets had to be modified to be used there. The tragedy is that today, 110 years later, K.G.F is  connected to the regular grid, and their electricity supply is intermittent at best!

Related Articles

In the offing: Plans to produce solar power in Bangalore homes
Bengaluru college adopts solar power

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About Ramesh Sreekantan 6 Articles
Ramesh Sreekantan is a Mathematician at the Indian Statistical Institute. He takes a keen interest in cycles - both the algebraic and mechanical kind.


  1. I’m not sure. I haven’t done it myself as it didn’t make financial sense for me. Is it not possible for you to directly use the power you are generating?

  2. I have installed a 3KVA solar power unit on the rooftop and since November 2016 I have been running around for the approval. They find some or the other excuse not to even acknowledge the application. What could be wrong with the BESCOM? Still pending…:( since its totally for export on to the grid, the captial is totally wasted…

  3. As it was mentioned there should be quality in the product & also especially the modules should be with good quality cells if the cells are B graded Please do not go for it , Be specific with the Kind of cells used in the modules you buy, it is bound to be degraded fast

  4. I’m not really an expert on solar panels but as you point out, if you
    use LED bulbs ( 20 W LED is very bright ) you will halve your
    consumption and as a result, your lights will work for twice as long.
    From what I understand you should be generating about 3.6 units a day
    ( if you have 6 hours of sunlight and am assuming that your panels
    lose some efficiency and generate 200W net ) which I think should
    suffice to run your fans, lights and TV for several hours. ( The email address you provided does not seem to work)


  5. I have installed 3 (Three) 250 W and 24 V solar Panel of Vikram brand . and solar charge controller of 24 V ,30 A and it is attached to microtek inverter. My Electricty consumption is 10 Units paer day. I am using 40 W 5 Tubes and three Fans and One T.V. and one CCTV on it. As I have experianced that if The Electricty is closed, I will save 5 units paer day But in the night , My Batteried charged on Electricy. Please let me know that , If I have to use these 24 hours,what type of panel are required and how may Batteried are required and what type of solar charge controller is required.I think that if LEd Tubes are to be used to reduced the Load of solar power. Your guidance is necessary to increase my hour of 10 to 12 hours of solar use. Thanking you, your reply is awaited with strong recommendations.

  6. I think you will still need to be connected to the grid for certain things which require short bursts of high power – like a washing machine. Though perhaps there are some new machines which can overcome this. I suggest you contact one of the solar power companies ( AnuSolar, Trienergy etc. ) and see what they say. I’m not sure what the rules are regarding bypassing BESCOM and only using solar power ( i.e going off the grid ) are – I dont see why they or BBMP should care if you dont use BESCOM power.

  7. Basavaraj Bhumannavar-Bangalore. I would like to know that for my new building, without getting Bescom power connection, can I install rooftop solar panels, generate and consume myself the power without supplying to grid? Any body can clear my doubt.

  8. Vasanthkumar – I made some assumptions to simplify the calculations – the main point I wanted to make was that it pays for itself in about 6 years. I know 2 units is a bit on the high side – as is 8 hours of sunlight in Bangalore.

    Ram – Perhaps the best thing to do is to contact one of the numerous solar power companies. Just search for Solar panels Bangalore and you should be able to find something. They might be able to give you the data you want. Or you could contact TERI. As I said, I’m waiting for more clarity on the situation.

  9. Excellent FAQ!! Thanks Ramesh.
    1. I’m interested to know more about ESU inverters which I think are better suited for my residence.
    2. Has BESCOM started entering such contracts with consumers? When do KERC recommendations become binding on BESCOM?
    3. Can one find research data for Bengaluru such as luminosity of sun over an year that can demonstrate impact of weather conditions such as rain, clouds etc?

  10. From FAQ – “Solar panels are typically rated in watts. For instance, a 2 x 1 square metre solar panel is usually rated around 250 watts. This means, in one hour it will generate 250 watt-hours of energy. Or in a day, with 8 hours of sunlight, it will generate 2 kilowatt-hours of energy – or 2 units”.

    Please correct me if I am wrong on my following observations – What is the ground reality on these assumptions? Generation of 2 units would be achievable only if the solar panels are facing direct solar radiation which will be at its best in the noon. But other times, its efficiency is stated to be hardly 18 to 20 per cent because of oblique radiation. Earth rotates on its axis at a speed that does not exactly allow solar radiation on a particular substance even for a few seconds. This being the reality, one cannot expect sustained generation of quality power unless the solar panels are equipped with sun-tracking technology which is unaffordble for supply of electricity for grid purpose by individual roof top panels arrangement. But individual households can install roof top panels, store whatever power is generated in batteries, have inverters and then use the same in the evening hours so that they can cut down consumption of power supplied through ESCOMS and reduce their bills and also help the nation save power. In the present day context, one unit saved is equal to 3 units to be generated – Rs.1000 crores are required for installing a power plant for generation of 1 MW power. People who can afford to install solar panels will help fight global warming.

  11. “In order to sell power to the ESCOMs one has to get the panels installed by approved installers” – This is incredulous! After all they should only be concerned about the current given to them. It is the house owners who should be concerned of the quality of their installation. Does this spell fraudulent contracts being favoured?

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