A brochure says "solar power". It is easy to imagine your own electricity bill disappearing. In an apartment building, that is rarely how it works. Here is a plain look at what rooftop solar actually does.
The roof is the limit
A roof has a fixed area. A taller building has more homes under the same roof. So the solar each home "gets" goes down as the building goes up. A low-rise, low-density building has more roof per family than a tall tower. That is simple arithmetic.
What it usually runs
In most buildings, rooftop solar supports common areas: corridor and outdoor lights, lifts, water pumps, and the STP. Those are paid for through your maintenance charges. So solar can help keep those charges in check, even if it does not touch your flat's own meter.
Why each flat rarely gets its own panels
Splitting a shared roof into many small systems, each with its own meter and wiring down to each flat, is complicated. Most buildings keep it shared.
Where the power goes
Solar power can be used directly by the building, or sent to the grid under net metering or similar schemes. In Karnataka, the electricity regulator, KERC, sets the rules. In Bengaluru, BESCOM manages the connection. Rules change from time to time, so ask what applies.
The quieter option: solar water heating
Solar water heaters use roof space to heat water for bathrooms. For a family, hot water for baths is a real daily electricity cost. Ask whether your flat is connected, and how backup heating works on cloudy days.
Questions to ask the builder
- What size is the solar system, and what does it power?
- Is it connected to the grid under net metering?
- Is there solar water heating for my flat?
- Who maintains the panels after handover?
Two words to know first
- kWp (kilowatt-peak): the size of a solar system, measured as its output in ideal test conditions.
- kWh (kilowatt-hour): the energy actually produced or used. This is the unit on your power bill.
A system's size in kWp tells you its capacity. The kWh it produces depends on sunshine, shade, dust, angle and upkeep.
A worked example: what rooftop solar can save (illustrative)
Every number here is an assumption, made up to show the arithmetic. Ask the builder for the real system size and its actual monthly output.
- System size: 30 kWp on a shared roof.
- Assumed output: 4 kWh per kWp per day. Daily output = 30 × 4 = 120 kWh.
- Monthly output: 120 × 30 = 3,600 kWh.
- Illustrative tariff: ₹8 per kWh. Monthly value = 3,600 × ₹8 = ₹28,800.
- Shared by 80 homes: ₹28,800 ÷ 80 = ₹360 per home per month, through lower common-area costs.
Now change one number. The same 30 kWp shared by 300 homes gives ₹28,800 ÷ 300 = ₹96 per home. Same roof, same panels, very different share. This is the roof-per-family arithmetic from above, in rupees.
A worked example: solar water heating (illustrative)
Say a family uses 100 litres of hot water a day, heated by 25°C. Heating 1 litre of water by 1°C takes about 1.16 watt-hours, which is basic physics. So 100 × 25 × 1.16 = 2,900 Wh, or about 2.9 kWh a day. Over 30 days that is about 87 kWh. At an illustrative ₹8 per kWh, about ₹696 a month of geyser power, before losses. Solar water heating can cover a large part of that on sunny days. Backup heating covers the rest.
Step by step: checking a building's solar claim
- Ask for the system size in kWp, in writing.
- Ask what it powers: lights, lifts, pumps, the STP, or flats.
- Ask about the grid link: is it connected under a net metering or similar scheme approved by the power company?
- Ask about solar water heating for your flat, and the backup.
- Ask who maintains it after handover, and whether there is a warranty or service contract.
- Ask for output records if the building is already running. Real kWh beats a brochure claim.
Common mistakes first-time buyers make
- Expecting a zero power bill. In most flats, solar helps common areas, not your own meter.
- Comparing system size without counting homes. Divide by the number of homes sharing it.
- Forgetting cleaning. Dusty panels produce less. Someone has to clean them regularly.
- Ignoring shade. Water tanks and nearby towers can shade panels for part of the day.
- Not budgeting for repairs. Inverters and other parts need replacing over time, through maintenance funds.
Your solar checklist
- System size in kWp.
- What it powers.
- Number of homes sharing it.
- Grid connection and metering.
- Solar water heating for my flat.
- Maintenance plan after handover.
Net metering in plain words
Under net metering, a special meter records both the power the building takes from the grid and the solar power it sends back. The bill is worked out on the difference. On a sunny afternoon, when common-area use is low, extra solar power goes to the grid. At night, the building draws power back. In Karnataka, the electricity regulator, KERC, sets the rules for this, and the local power company handles the connection. Ask the builder which arrangement the building has, and whether it has been approved.
How solar shows up in your maintenance bill
Common-area power is one of the largest parts of a building's running cost, alongside security, housekeeping and lift upkeep. When solar covers part of that power, the association pays the power company less each month. That saving can help keep maintenance charges steady. But panels and inverters also need cleaning, checks and, over time, part replacements. So a fair picture is solar savings minus upkeep. Ask the builder for both figures.
Low-rise buildings and solar
A low-rise, low-density building has more roof area for each home than a tall tower with the same land. That does not guarantee more solar, because the roof also holds water tanks, lift rooms and walkways. But it does mean the solar that is installed is shared among fewer homes. When comparing projects, look at system size per home, not just system size.
What about your own flat's bill?
Your flat's own meter covers your lights, fans, fridge, air conditioners and geyser. Rooftop solar on a shared roof rarely feeds that meter. So the best ways to lower your own bill are inside your home: efficient appliances, LED lights, a well-ventilated layout that needs less cooling, and solar water heating if the building offers it. When you compare flats, look at daylight and cross-ventilation as closely as the solar line in the brochure. They work for you every day, on every floor, whatever the size of the roof.
The short version
Rooftop solar in a flat usually helps the shared bill, not your own. Its value depends on system size, how many homes share it and how well it is kept. Ask for numbers, divide by the homes, and check the upkeep plan. Then judge it as one sensible feature among many, not as the reason to buy.
Keep the builder's written answers with your flat's papers for later.
When you add up the real monthly cost of a home, include maintenance alongside your EMI. Start with our EMI calculator.