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Old 12th February 2023, 23:42   #1
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Optimising Residential Rooftop Solar Panels

Background



In our residential community many of us were interested in exploring rooftop solar panels. We were vaguely aware that systems with batteries were very expensive, maintenance heavy and didn’t make much sense. The feedback from friends and relatives abroad (mainly in California) was that rooftop solar with net-metering kind of works out for them. When similar schemes were announced in India, we got more serious but it required someone to take a closer look, find reliable vendors and make some recommendations.

That turned out to be me, when I came across the very informative thread on (Solar power! Turning my roof to a power plant) solar power on Team-BHP. I started making notes to share with my neighbours to discuss and decide. Then decided to expand on them and post here to help others in a similar position. I also ended up (Reason Underdetermines Life Decisions | Tata Nexon EV | Ownership Review) buying a Nexon EV, which got delivered just before our rooftop solar. It so happens that adding incremental capacity is cheaper and gives more savings due to reduction in higher slab rate units, thus improving Return on Investment (RoI). Basically increased electricity usage improves the RoI, but beyond that there is not much link between EVs and solar rooftop.

Net-metering



We took an optimisation approach to the problem, the idea was how to get the best bang for the buck while reducing as much of our carbon footprint. As already mentioned, the simplest and most popular option for residential rooftop solar systems is on-grid with net metering as this minimises the investment, maintenance and maximises the returns. This setup mainly has solar panels, inverters and a net-metering meter. It has no maintenance heavy components like battery, and the main components have a long life, the panels have a rated life of ~25 years and the inverters around 10 to 15 years. Only required maintenance is periodic cleaning of the panels to improve production.

The electricity generated by the solar panels are converted to AC with an inverter, exactly matching the frequency and phase of the grid supply. This can be thought of as a mini local power plant. When the local load is less than the local supply (say at noon), the excess power is injected into the grid, but when the local load is more than the local supply (say at night), power is drawn from the grid. The normal meter is changed to a bidirectional meter which keeps track of total imported and exported power. The biggest advantage of this is that with net-metering the grid effectively becomes a free battery with unlimited capacity, greatly reducing the upfront battery investment and periodic battery replacement cost.

On the billing date, imported minus exported units for the billing period are calculated. If this is positive, i.e. more units are imported then the consumer is billed for the net consumed units as per the slab rate along with fixed charges and taxes. When this is negative, i.e. more units are exported, then consumer gets paid for the net supplied units at the specified rate (which for BESCOM consumers is currently ₹4.02, this is around the lowest BESCOM slab rate for consumers). Example of both cases given in below table for BESCOM rural consumers.

Optimising Residential Rooftop Solar Panels-netmeteringexampletable.png

In Karnataka, there is a 9% tax on the usage charge, so the savings will be 9% higher than above. In addition there can be Fuel Cost Adjustment charges (FAC) per unit, which can be as high as 74 paise per unit. With net metering this will be applied only on net used units and in the above examples this can provide an additional savings of up to 200 rupees. The fixed charges will remain the same with net-metering and will be additional in both examples given above. If the fixed charges are for example ₹985, the bill amount before tax will be 985+81 = ₹1066 and 985-321.6 = ₹663.4.

The unit price charged by BESCOM in our area is ~₹5.76 (blended) for the first 200 units and ₹7.70 thereafter, but the BESCOM price for purchasing excess power is only ₹4.02 per unit. Therefore it is best to not have excess capacity (i.e. produce too many units in excess of consumption), as the earnings from the excess units are significantly less and will reduce return on investment (RoI). So it is better to continue to pay the fixed charges, instead of producing excess units to pay for the fixed charges.

Optimising returns



The optimal capacity for a given average monthly unit consumption can be calculated based on expected solar power production. In Bangalore a 1 kWp solar plant will produce a minimum of around ~1350 units per year (after accounting for cloudy days), i.e. 1 kWp plant is good for ~110 units monthly usage. The cost per kWp can vary from ~60,000 to even 1,00,000, depending on the type/quality of the panel, inverter technology and mounting structure type and quality. The table below gives the payback period (Pbk), loan payback period with 8% interest (Loan) and internal rate of return (IRR) for the investment. This is assuming 25 years life, inverter replacement at 10 years and a realistic price increase of 4% per unit every year (based on last 7 year rate hikes).

Optimising Residential Rooftop Solar Panels-roi.png

Even at a higher price range of ₹1,00,000 per kWp, the lowest IRR is close to ~10%, i.e. it gives better return than FD, debt fund or similar low risk investment option. The IRR improves with higher monthly units usage as more of the units will fall in the higher slab rate. The higher savings outweighs the proportionate increase in investment. Refer to this Google Sheet for the RoI analysis with formula for further details, you can change the parameters to what is applicable for your place and check the RoI.

At 200 units per month the blended cost per unit (including tax) is >₹6 and approaches ₹8 as usage increases. With cost per kWp of 80,000 the payback period reduces from 10 years to 8 years, with higher cost per unit like ₹32 per unit for DG, the payback period can be as low as 2 years.

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Panel Options



The main panel types are Monocrystalline, Polycrystalline, Passivated Emitter and Rear Cell (PERC) often combined with Monocrystalline, called Mono-PERC and thin-film. Very briefly Mono-PERC has the highest efficiency and highest cost, but the advantage is they take less space. Most vendors will offer 540W Mono-PERC panels.

Another option is bifacial panels, where the back is made of glass/transparent sheet instead of opaque material and suitable cells. This allows it to produce power with light entering from the back, this can improve the power produced by 15 to 20%. This makes sense only for frame mounted installations with a high albedo reflective floor (or places with snowfall in winter). It also seems to help on cloudy days when the light is more diffused and comes from all directions.

Panel Mounting Options



The best fixed mounting option for India is south facing with a slope equal to the latitude of the place, for Bangalore this will be ~13, but can be lower at 5 without much loss of efficiency. With modern panels each kWp requires an approximate shadow free area of ~6 sqmt (or ~64 sqft). A sturdy sloping south facing roof or open area with prefabricated standard structures offer cost effective mounting and cost per kWp will come to around 80,000 (with good quality equipment). More aesthetic and/or waterproof gazebo style mounting can cost 100,000 or more per kWp and may go well with bifacial panels.

Inverter Options



The main options are central string inverter or per panel micro inverters. String inverters are cheaper, but high voltage DC lines between the panels and inverter may not be efficient/safe for longer distances. They also don’t work well when there will be partial shadows. In contrast micro inverters are expensive, work well with partial shadows and don’t have any high voltage DC lines.

In our community almost all terraces are shadow free and the inverter can be housed very close to the panels, hence unaffected by string inverters disadvantages. It made sense to pick them due to the lower cost.

Hybrid system



With a net-metering setup in case of a power cut there will be no power production due to multiple reasons, 1) there will be no reference input AC for the inverters to match their AC output, 2) since the inverter is directly connected to the grid, any power it produces may endanger repair work and finally 3) it can’t directly supply any load as the production has to match consumption. So in case of a power cut only battery charging can be done. If your power backup is a UPS/Inverter + battery system, then a hybrid system (with additional cost) can charge that battery in case of power cut. But there will hardly be any savings or advantages, it might make sense only if you have very long duration or multi day power cuts and even then you may anyway need a generator (for rainy day power cuts). In which case you may be better off just using only the generator for backup. The cost of a small generator is much cheaper than the difference between a normal and hybrid system, it will have longer life and even fuel cost will be much cheaper than battery cost. I can’t think of any strong use case for a hybrid system.

Calculating consumption



To match planned production to consumption, it is important to accurately estimate consumption. To even out monthly variation it is best to take yearly consumption from the difference between current reading of two bills one year apart and divide by 12. Since the system will last for a long time, it would be better to think and add any increase in usage that can happen, e.g. an EV vehicle would add ~150 units/1000 km per month, a geyser can add ~15 units, similarly plan for any AC or other heavy power consuming devices that may get added. The first step in net-metering is approval and signing of a 25 year power purchase agreement with the electricity distribution company for the installed capacity, changing it currently requires complete rework and is not worth the hassle. Below is an example BESCOM bill with markings for illustration.

Optimising Residential Rooftop Solar Panels-bescombillmarkedup.png

First round of installations in the community



In the first round 4 houses have installed solar rooftop, many others are considering it in the next round. A prerequisite for residential rooftop solar in Karnataka is building owner and electricity connection should be in the same name. Many residents have not yet changed the electricity connection to their name and have to do that before taking this up.

Staircase Room Roof Installation in two houses

Optimising Residential Rooftop Solar Panels-staircaselabeled.jpg

1) Staircase room roof before installation, slope is West facing
2 & 3) 6 Mono PERC panels of 540 kWp = 3.24 kWp installed in 2 rows of 3 columns in one house
4) Same type 6 panels installed in 3 rows of 2 columns in another house on East facing slope.

Both will produce up to 350 units per month (after accounting for loss due to East and West facing slope). Cost per kWp is ~₹80,000. Advantage is it doesn’t take up any terrace area. But this is not recommended on North facing slopes as the losses will be much higher.

Standalone frame mount in the third house

Optimising Residential Rooftop Solar Panels-terra.jpg

6 Mono-PERC panels of 540 kWp (total 3.24 kWp) installed in 2 rows of 3 columns on standalone frame mounting. Produces up to 375 units per month with a South facing slope of ~5. Cost per kWp is ~₹85,000.

Standalone frame mount in the fourth house

Optimising Residential Rooftop Solar Panels-atrium.jpg

10 Bifacial panels of 395 kWp each (total 3.95 kWp) installed in 2 rows of 5 columns on a standalone frame with wood composite cladding and watertight roof mounting. Produces up to 460 units per month with a South facing slope of ~8.5. Cost per kWp is more than ₹1,00,000.

Common features

All four installations included the following:
  • Complete installation and commissioning, including full coordination with BESCOM and handling all paperwork.
  • DC, AC distribution box and Single phase string inverter to be kept on terrace
  • Lightning arrestor
  • Three earthing pits, one each for lightning arrestor, AC and DC
  • Solar generation meter next to LT panel
  • Replacement of current BESCOM meter with net metering meter given by BESCOM
  • App based monitoring
  • 1 year AMC

Current Production



It has been a few weeks since installation, winter is mostly clear skies in Bangalore this along with lower temperatures makes it ideal for solar panels. The 3.24 kWp Mono-PERC systems are producing ~17 units per day on clear days, this is ~5.3 units per kWp per day and is near peak production. We have assumed an average of ~3.7 units per kWp per day. We will need to wait for 1 year of data to get the average. The 3.95 kWp bifacial system is producing ~25 units per day, i.e. ~6.3 units per kWp per day. Below are the production curves from three of the installations from a good day.

Optimising Residential Rooftop Solar Panels-outputcomparision.jpg

Few observations from these graphs:
  • The Mono-PERC systems reach peak power of 2.5 kW, this is ~77% of the rated 3.24 kWp.
  • The south facing Mono-PERC curve (2) is symmetrical, whereas the west facing Mono-PERC (3) clearly produces more power after noon. On this day the south facing Mono-PERC produced 17.1 units, whereas the west facing one produced 16.8, this is hardly 2% loss.
  • The bifacial panels reach peak power of 3.3 kW, this is ~83% of the rated 3.95 kWp. We can also see from the flat top that it reaches this peak power faster and maintains peak production for longer. That is how it seems to be giving 6.3 units per kWp compared to 5.3 units per kWp for the Mono-PERC panels under similar conditions.
  • Another difference between the two south facing installations is, the Mono-PERC has a slope of ~5, whereas the bifacial has a slope of ~8.5, that is closer to the optimal of 13. This may contribute a little, but most of the difference is due to the type of the panel.
  • This was a bit unexpected, but looks like any non-dark floor under a tall frame mount structure seems to be sufficient to get ~20% increased production from bifacial panels. If the cost is similar or is not more than 20%, it might make sense to go for bifacial panels, additional benefit is it definitely looks good.

Conclusion



Everything has gone smoothly so far, the vendor has taken care of all coordination with BESCOM. Post commissioning, inspection was done and synchronization certificate has also been issued. Now we are awaiting the first net-metering bill to see if whatever production is shown in the app is actually getting reflected and resulting in projected savings. Will provide an update once we it.

What started out as simple notes has turned into this long post. Hope the information collected in one place is helpful to those thinking of installing rooftop solar. I would be glad to answer any questions you may have regarding this. In summary it is not very expensive to significantly reduce our carbon foot print and we do get decent financial returns for the investment.

Last edited by wocanak : 12th February 2023 at 23:50.
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Old 13th February 2023, 06:58   #2
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Re: Optimising Residential Rooftop Solar Panels

Thread moved out from the Assembly Line. Thanks for sharing!
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Old 13th February 2023, 10:49   #3
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Re: Optimising Residential Rooftop Solar Panels

This is a very timely post as I am planning to get Solar panels installed in the next 1 or 2 months. While I did some basic research, your post is very detailed and would help anyone thinking of getting solar panels installed. Another advantage of Solar panels if installed as standalone structure is that it can reduce the temperature for the residents in the floor below. Keeping this in mind I provided for pillar extensions on which I can mount the structure without disturbing the roof.

By the way who is the Vendor. I was thinking of going with Tata Power or Visakha industries. (Hyderabad)

Last edited by vamsi.kona : 13th February 2023 at 10:51.
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Old 13th February 2023, 10:55   #4
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Re: Optimising Residential Rooftop Solar Panels

Thank you for the great write-up.

One question. Does BESCOM pay you when you are net exporter or do they just keep adding credit to your account. What if you end up always being a net exporter? Will you get actual amount paid by BESCOM. Is there a settlement period?

I understand in Telangana, the board never actually pays you back. They just keep crediting your account and only way for us to make gain is to consume all we generate and try to be net neutral.
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Old 13th February 2023, 11:42   #5
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Re: Optimising Residential Rooftop Solar Panels

Thanks for these details, I am looking at solar panels at our house and this information is of much help.
I will check these panels out.
If okay, can you share the vendor details?
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Old 13th February 2023, 12:09   #6
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Re: Optimising Residential Rooftop Solar Panels

Quote:
Originally Posted by wocanak View Post

Hybrid system



With a net-metering setup in case of a power cut there will be no power production due to multiple reasons, 1) there will be no reference input AC for the inverters to match their AC output, 2) since the inverter is directly connected to the grid, any power it produces may endanger repair work and finally 3) it can’t directly supply any load as the production has to match consumption. So in case of a power cut only battery charging can be done. If your power backup is a UPS/Inverter + battery system, then a hybrid system (with additional cost) can charge that battery in case of power cut. But there will hardly be any savings or advantages, it might make sense only if you have very long duration or multi day power cuts and even then you may anyway need a generator (for rainy day power cuts). In which case you may be better off just using only the generator for backup. The cost of a small generator is much cheaper than the difference between a normal and hybrid system, it will have longer life and even fuel cost will be much cheaper than battery cost. I can’t think of any strong use case for a hybrid system.
Thanks for detailed summary. What was the cost difference between hybrid vs on grid inverter. When I got the quotes, its was around 20-30k only and if someone already has a power backup, they don't need to buy additional batteries. Hybrid setup is beneficial for areas where there are frequent powercuts in noon. I went with on-grid setup because while installing I thought the additional cost is not worth it but many days we get power cuts of couple of hours during day time when there is max potential of power generation.

Regarding the placement of panels, please make sure they are accessible for cleaning (at least weekly). I am seeing around 20-30% daily total capacity degradation whenever panels have dust on them (13-15 units vs 10-11 units for 3kwh setup)

Last edited by ritesh_44 : 13th February 2023 at 12:10. Reason: Removed extra headings
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Old 13th February 2023, 12:18   #7
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Quote:
Originally Posted by vamsi.kona View Post
This is a very timely post as I am planning to get Solar panels installed in the next 1 or 2 months. While I did some basic research, your post is very detailed and would help anyone thinking of getting solar panels installed. Another advantage of Solar panels if installed as standalone structure is that it can reduce the temperature for the residents in the floor below. Keeping this in mind I provided for pillar extensions on which I can mount the structure without disturbing the roof.
Reduced heat can be another benefit, but keep in mind that the best panels have only 20% efficiency, so heat will reduce by at least 20%. Some more heat reduction will also happen due to increased reflection, hard to say exactly how much. Often reduction by even 20 to 30% may be sufficient to make the worst summer feel like any other season.

If there will be ~2 meters between the roof and the panels, then please consider bifacial panels, especially if the premium (if any) is within 20%.

Quote:
Originally Posted by vamsi.kona View Post
By the way who is the Vendor. I was thinking of going with Tata Power or Visakha industries. (Hyderabad)
The installation was done by a Bangalore based company called EcoSoch the panels and structures were from HomeScape. I think the structures are made in India, the panels may be Indian or imported depending on what you choose and inverters are mostly imported. We mainly went with EcoSoch because they were very responsive and another colleague who had taken from them gave good feedback.

Quote:
Originally Posted by HappyRider View Post
Thank you for the great write-up.

One question. Does BESCOM pay you when you are net exporter or do they just keep adding credit to your account. What if you end up always being a net exporter? Will you get actual amount paid by BESCOM. Is there a settlement period?

I understand in Telangana, the board never actually pays you back. They just keep crediting your account and only way for us to make gain is to consume all we generate and try to be net neutral.
In the months where export is so much that, even after reducing fixed charges there is a balance, that amount will be paid directly to your bank account by BESCOM. For example if your net export is 200 units and fixed charges is say 500, 200*4.02 - 500 = 304 will be paid into your bank account. What I hear it takes a few months from installation for this process to get settled and it can be irregular in some sub-divisions.

When signing the power purchase agreement, we have to give a cancelled cheque to enable direct payment to bank.

Quote:
Originally Posted by int3 View Post
Thanks for these details, I am looking at solar panels at our house and this information is of much help.
I will check these panels out.
If okay, can you share the vendor details?
The installation was done by a Bangalore based company called EcoSoch the panels and structures were from HomeScape. Since you are Bangalore based, I will PM you the contact person details.

Quote:
Originally Posted by ritesh_44 View Post
Thanks for detailed summary. What was the cost difference between hybrid vs on grid inverter. When I got the quotes, its was around 20-30k only and if someone already has a power backup, they don't need to buy additional batteries. Hybrid setup is beneficial for areas where there are frequent powercuts in noon. I went with on-grid setup because while installing I thought the additional cost is not worth it but many days we get power cuts of couple of hours during day time when there is max potential of power generation.
Assuming it is a 3 kWp, it can at best product 2.5 units per hour, this will be around Rs 20 at a blended rate of 8 per unit, so it will take 1000 to 1500 hours to recover the additional 20 to 30K. If you are producing excess power then these units will only be paid at ~4 per unit, then it will take 2000 to 3000 hours to payback (without considering interest).

We have DG backup during power cut and get a DG bill from our association. I looked at ~6 months of DG unit consumption and it was only 10 to 15 units per month. I am guessing this will be around 10 to 15 hours of power cut a month, so we are talking 100s of month for payback. In East/rural Bangalore the power cuts are frequent, especially in summer, but mostly for very short duration.

I think it is best to keep whatever is your current backup, i.e. DG or inverter as is and save the extra hybrid investment. If you don't have any, then a simple independent inverter with few hours of backup should be more than sufficient.

Quote:
Originally Posted by ritesh_44 View Post
Regarding the placement of panels, please make sure they are accessible for cleaning (at least weekly). I am seeing around 20-30% daily total capacity degradation whenever panels have dust on them (13-15 units vs 10-11 units for 3kwh setup)
Yes, this is a good point. We tried to make it sure there is access for cleaning. I also tried simple cleaning after few weeks just to check how it is and what equipment to buy for cleaning.

Last edited by libranof1987 : 13th February 2023 at 17:39. Reason: Merging back-to-back posts. Kindly use the Quote+/Edit functionality to quote multiple posts.
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Old 13th February 2023, 13:48   #8
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Re: Optimising Residential Rooftop Solar Panels

Optimising Residential Rooftop Solar Panels-img_5505.jpg

The best way to optimize solar panels make them dual-axis tracking. A single axis would be fine too but dual looks cooler. See reference of my high school project many many fortnights ago. 40% more efficiency for 20% more cost
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Old 13th February 2023, 15:01   #9
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Re: Optimising Residential Rooftop Solar Panels

Let me share 4 year statistics of my solar generation for everyone's benefit. So I have used (14244-11375) + 8078 = 10947 units

Surplus: 11375 - 8078 -350 = 2947 units

For the above surplus, MSEB paid me: Rs 10951 (Approx Rs 3.71/unit)
2020: Rs 4080
2021: Rs 2600
2022: Rs 4271

My average consumption over last 4 yrs has been 305 units, for which I need to pay Rs 3175, That is 38000 per year!. And I paid 1.4L (excluding 43k subsidy for 3KW plant). Per that logic, I have crossed the breakeven!

Money talks apart, the "Sukh" of having a rooftop solar is, you need not bother about the monthly bill aspect when you raise your finger to switch on that light or AC! You see, I let go those 3000 units after leisurely using AC (ofcourse when required) on my top floor whole 4 years.
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Old 13th February 2023, 17:09   #10
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Re: Optimising Residential Rooftop Solar Panels

Very informative post. We are going in the same direction with solar panel installations in my hometown.
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Old 13th February 2023, 19:49   #11
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Re: Optimising Residential Rooftop Solar Panels

Extremely useful and informative post. Bookmarking


Does anyone know if CESC in Kolkata allows net metering and any recommended vendor in 700xxx who will handle this as a turnkey project like OP's?
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Old 13th February 2023, 23:09   #12
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Re: Optimising Residential Rooftop Solar Panels

Very timely thread and very informative. My house is almost getting completed and was planning to get solar power installed. Can you PM me the details of the contact person? I hope they can do a house visit and assess how many panels can be placed and provide a quote
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Old 14th February 2023, 09:15   #13
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Re: Optimising Residential Rooftop Solar Panels

Wonderful and in-depth analysis. however the costs seem to be much higher than what I have seen.
I installed 11 KW on-grid solar panels plus inverter on my rooftop in July 2021@ 46,000 per KW all inclusive. I went for a south facing installation, as is the norm here in Delhi NCR. I opted for a stilt installation to have the usable space underneath the panels.
Here are my observations (without detailed tracking)
  1. I have seen the power generation touch a max of 7.5KWh, mostly it hovers around 6.5KWh at the peak time
  2. my monthly bill in summers was around 10K, while in winters was about 4K - this has gone down to ZERO! I only paid around 2K for a month when the usage was very high
  3. Total yield as per the tracking app is INR 1.85 Lakh so far, in 20 months of operations.
  4. The rough breakeven is likely in 4.5 years, sooner if tariffs rise
  5. the thermal insulation is significant as the solar panels do not let through any light
  6. Although the raised installation allowed me to have usable space under the panels, the cleaning becomes very very difficult. In our climate the cells start showing significant dust accumulation within a week.
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Old 14th February 2023, 10:17   #14
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Re: Optimising Residential Rooftop Solar Panels

I am considering off-grid for couple of reasons.

1. Clean 230 voltage all time through the Solar inverter and full battery backup. BESCOM has roasted many of my appliances in past.
2. Has a lot of power outages in the area. No dependency on BESCOM and community generator. (Will add a provision to toggle to BESOCM for emergency)
3. On-grid can't go high capacity. I will need around 12.5kW (Including ACs and EV etc..)

Any reliable suppliers who can do off-grid?

And any strong reasons why I shouldn't consider off-grid. I know cost is one.
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Old 14th February 2023, 12:12   #15
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Quote:
Originally Posted by sj_koova View Post
I am considering off-grid for couple of reasons.

1. Clean 230 voltage all time through the Solar inverter and full battery backup. BESCOM has roasted many of my appliances in past.
2. Has a lot of power outages in the area. No dependency on BESCOM and community generator. (Will add a provision to toggle to BESOCM for emergency)
3. On-grid can't go high capacity. I will need around 12.5kW (Including ACs and EV etc..)

Any reliable suppliers who can do off-grid?

And any strong reasons why I shouldn't consider off-grid. I know cost is one.
Apart from initial cost, battery maintenance would be significant recurring cost. Also on days when there's no sun, the power generated would be low and might not be sufficient (In Bengaluru last year, there was hardly any sun for months)

You don't need high capacity on-grid setup, calculate the monthly consumption and install on-grid setup accordingly (1kW setup on average generates 4-5 units on a sunny day). Even if your sanctioned load from BESCOM is 12 or 15 kw, you don't need to install solar at same capacity. You can import power from grid whenever needed.
If you have longer power cuts (more than 1-2 hrs in day), better go with hybrid setup.
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