Solar on a high-rise: what a roof can power, and what it cannot

Rooftop solar on an apartment tower is one of the few sustainability claims that can be checked with arithmetic rather than argued about. The arithmetic is unflattering, which is presumably why so few brochures do it. Here it is, along with what a rooftop array in a high-rise genuinely does, who regulates the connection, and what to ask.

The geometry: fixed roof, rising home count

A photovoltaic array generates in proportion to the area it occupies and the sunlight falling on it. On a tower, that area is the roof — and the roof does not grow when you add floors.

Follow the consequence. A four-storey building and a twenty-storey building on the same footprint have the same roof. The second has five times as many homes under it. Generation per home in the taller building is therefore a fifth of what it is in the shorter one, before any other consideration. Add the parts of the roof that are not available for panels — lift machine rooms, overhead tanks, staircase headroom, plant, safety access, and the setbacks needed so that one row of panels does not shade the next — and the usable area shrinks further.

This is not an argument against rooftop solar on a tower. It is an argument against a specific claim: that a rooftop array will meaningfully power the flats. It will not, and any buyer can establish that for themselves by asking two questions — how much roof is actually available for panels, and how many homes are under it.

What a rooftop array realistically serves

Once you stop expecting it to run the flats, rooftop solar in a high-rise becomes a sensible and worthwhile piece of equipment, because the community has a substantial electrical load of its own that has nothing to do with individual homes.

• Common-area lighting — corridors, lobbies, staircases, basements, landscape and street lighting. A continuous load, much of it at night, and the largest single common-area consumer in many communities.
• Water pumps — the transfer pumps lifting water to overhead tanks and the pressure and booster sets. A daytime load, which suits solar well.
• Lifts — intermittent but significant, and running through the daylight hours.
• The treatment plant — blowers and pumps in a sewage treatment plant run more or less continuously and are a genuinely heavy common load.
• Clubhouse and amenity loads — gym equipment, air conditioning, pool filtration.

That is a real bill, it is paid out of the maintenance fund by every resident, and reducing it is a benefit shared by the whole community. It is a smaller and less romantic claim than "solar-powered homes", and it has the advantage of being true.

Why individual-flat solar in a tower is a different problem

Buyers occasionally ask whether their own flat can have panels. In a tower, three separate obstacles stand in the way, and the first is not technical.

• The roof is not yours. Under the definition of common areas in section 2(n) of the Real Estate (Regulation and Development) Act, 2016, the terraces of the project fall within the common areas, in which each allottee holds an undivided proportionate share. Occupying a slice of that roof with private equipment is a decision for the association, not for an individual owner.
• Allocation is unsolvable at scale. If every home wanted a share of the roof, the share available to each would be too small to be worth the switchgear, and any allocation would advantage whoever asked first.
• Metering. Connecting generation on a common roof to an individual flat's meter requires a metering and settlement arrangement that the distribution licensee has to accept.

Facades and balconies are sometimes proposed as an alternative surface. Vertical surfaces receive markedly less sunlight than a horizontal roof at this latitude, they shade each other, and any external installation is an alteration to the elevation that the association's rules will usually govern. Treat any such offer with the same scepticism you would apply to any other retrofit promise.

Where the electricity goes, and who sets the rules

Three arrangements are in common use, and the difference between them decides who benefits.

• Behind-the-meter self-consumption. The array feeds the common-area supply directly, reducing what the community draws from the grid. Simplest arrangement, no export, and the benefit shows up as a smaller common-area bill.
• Net metering. Generation offsets consumption on the same connection and any surplus is exported and accounted against imports, on terms the regulator sets.
• Gross metering. Everything generated is exported and metered separately, and the community is credited for it at the applicable rate while continuing to buy its own consumption.

Which of the three is available, on what terms, and with what limits, is not a matter for a developer to promise. The distribution licensee — BESCOM across most of Bengaluru — administers the connection, and the Karnataka Electricity Regulatory Commission issues the regulations and tariff orders that govern it. Both act by order, and orders are revised. This post prints no rate and no capacity limit for exactly that reason: the useful thing is to know which two bodies to check, and then to read what they currently say.

One practical consequence for a buyer: the arrangement chosen affects the association's paperwork for years. Ask which one the project has applied for, and whether the connection is already sanctioned or merely intended.

Solar water heating: the quieter option

The option that often does more work in a Bengaluru building gets a fraction of the attention, because it does not photograph as well.

Heating water is one of the heaviest electrical loads in an Indian household, and a solar thermal collector converts sunlight into heat far more efficiently than a photovoltaic panel converts it into electricity. For the same area of roof, a thermal system therefore displaces a larger quantity of grid consumption than a PV array does — and it displaces it in the flats, where the load actually sits, rather than only in the common areas.

It comes with its own constraints, which is why it deserves questions rather than enthusiasm: it needs a distribution network from the collectors down to the homes, it needs a backup element for cloudy weeks, it competes for the same roof as the PV array, and it needs maintenance that somebody has to own after handover. But in a city with Bengaluru's climate and a monsoon that is cool rather than freezing, it is frequently the better use of a square metre of roof — and a project that has thought about both, and can explain how it split the roof between them, has done real engineering.

What to ask a developer

All of these are answerable from documents before booking. None of them requires a promise about the future.

• How much roof area is allocated to solar, and how much of the total roof is that? Ask to see it marked on the terrace plan.
• What is the array's rated capacity, and which loads is it connected to — common areas only, or something else?
• Which connection arrangement has been applied for, and has the distribution licensee sanctioned it?
• Is there a solar water heating system, and if so, which homes does it serve and what is the backup?
• Who maintains the array — cleaning, inverter servicing, monitoring — during the promoter's maintenance period, and does that contract pass to the association?
• Is generation monitored, and will the figures be visible to residents?
• What warranty exists on the panels and, separately, on the inverters, and who holds the documents?

The last two matter more than they look. A rooftop array with no monitoring is a system nobody can prove is working, and an array whose warranty papers were never handed over is one the association cannot claim against. Both are common, and both are avoidable by asking in advance.

The honest summary

Rooftop solar on a high-rise is a good idea, sized by geometry rather than by ambition. It should be described as what it is — a reduction in the community's shared electricity bill, maintained by the community after handover — and not as a way to run your air conditioner off the sun. A developer who says the smaller thing is telling you something checkable, which is worth more than the larger claim.

How energy, water and green cover are treated across SUYUG's developments — stated per project, with the IGBC status each one actually holds — is set out on the sustainability page, and the recurring questions buyers ask about services and handover are answered on the FAQ page.