Solar for flats: every way a flat in India can go solar, compared

Yes, you can install solar for your flat, but flats face two problems bungalows do not. One terrace serves every flat in the building, so the roof space per flat shrinks with every floor. And one plant on one meter has no built-in way to give each flat its fair part of what it makes. Five options answer them differently.

• Your own panels and net meter, on a patch of terrace the society gives you.
• Common-area solar on the society's meter, for lifts, pumps and common lights.
• Virtual net metering, which shares the bill credits from one plant. Maharashtra notified it in 2024, but MSEDCL is not approving applications, so it is not an option in MSEDCL's area today.
• Protostar's grid-connected microgrid: one shared plant with an SPDU at every flat's meter, offered in Maharashtra.
• Protostar Fusion: a shared plant, a shared battery bank and an SPDU per flat, for backup in power cuts.

The right choice depends on the building. A flat with a sunny patch of terrace pays the least cash up front with its own system, if the subsidy is sanctioned in time. A society that only wants smaller common bills can start with common-area solar. When every flat should benefit from one roof, a shared plant with SPDUs splits it fairly, and when power cuts matter, only a battery keeps homes running.

You can put your own solar panels on the society terrace if the society agrees to give you part of it and that part gets enough sun. You install a grid-connected system on your own electricity connection, and the DISCOM fits a net meter that records what you export and what you import. This is the route the PM Surya Ghar subsidy for individual homes is built for.

The limits appear in apartment buildings. Dozens of separate installations, each spaced for its own shadow, need more shadow-free roof than most buildings have. Whoever installs first takes the sunniest corner, and later flats inherit the shade of the water tank and the lift room. Each system brings its own inverter, approval, warranty and service call, so a 40-flat building can end up managing 40 of each.

Like all grid-tied solar, it also switches off in a power cut. Backup means a second system inside your flat, usually a UPS whose lead-acid batteries run a few lights and fans and need replacing every 3 to 5 years.

Common-area solar does not reduce a flat's own electricity bill. It is connected to the society's common meter, so it cuts the bill for lifts, water pumps, and staircase and compound lights, and residents feel the saving only through lower maintenance charges. Under ordinary net metering, credits earned on the common meter stay on the common bill and cannot be passed to individual flats.

It is the simplest route for a society: one decision, one installation, one net meter on the common connection, and PM Surya Ghar support for a housing society's common facilities. Its size is capped at the common connection's sanctioned load under MERC's rooftop regulations, and it switches off in a power cut like any grid-tied system, so the lift and the pumps still depend on the grid or the diesel generator.

If residents later want solar on their own bills as well, the roof is better planned once for both. Protostar designs lifts, pumps and common lights into the roof plan from the first drawing.

Not in practice. Virtual net metering is legal in Maharashtra, but MSEDCL, which supplies most of the state outside Mumbai, is not approving applications, even after MERC penalised it in October 2025. MERC notified it for housing societies on 29 August 2024, in the Second Amendment to its rooftop solar regulations. Under virtual net metering (VNM), a plant's exported energy is credited to the bills of the participating flats and the common connection, in a sharing ratio the members agree, with a Lead Person signing for the group.

MSEDCL has an online application form, but it is not approving applications, and it has challenged the regulations in the Bombay High Court. Three housing societies had to take it to MERC in Case No. 222 of 2024 to get VNM enabled. On 14 October 2025, MERC found that MSEDCL had kept the applications pending and penalised it under Section 142 of the Electricity Act. As of 12 October 2026, the case awaits a final order, and no commissioned VNM system appears in MERC's public record.

The same MERC rules allow a society's grid-connected micro-grid, and that needs no energy accounting by MSEDCL. Protostar's SPDUs do the accounting in hardware, flat by flat, and the building can add one common battery bank for every flat. A battery cannot be built into a VNM layout at all, because each flat draws only from its own DISCOM meter. Even when it works, VNM is accounting, not distribution. It adjusts bills after the fact, so it cannot allocate stored solar to a flat at 9 pm, and the plant switches off in a power cut like any grid-tied system. Our guide to virtual net metering in Maharashtra covers the clauses, the case and what a society can do instead.

A shared solar plant splits its power fairly with an SPDU at every flat's meter. Think of your society's water tank. One tank sits on the roof, yet every home gets its own fair supply, because the sharing is built into the plumbing. Protostar's patented SPDU (Smart Power Distribution Unit) does the same for sunshine. It sits in the meter room, one per flat, and switches that flat's supply between the shared plant and the flat's own grid connection.

Each flat's share is sized to its own bills when the system is designed. Its SPDU meters and enforces that share, and you see your balance in the Nova app, like a prepaid recharge. When your share is used up, the SPDU moves your flat to its own meter automatically.

Protostar builds this in two ways. Protostar Fusion adds a shared battery bank, as the next section explains. In Maharashtra, Protostar also offers a grid-connected microgrid without a battery: the shared plant is net metered on the society's common meter, and every flat has its own SPDU and share. It costs less than Fusion and cuts every participating flat's bill, but like all grid-tied solar it pauses during a power cut. Nothing inside your flat is rewired.

Grid-tied rooftop solar does not work during a power cut. A grid-tied inverter's anti-islanding protection switches it off when the grid fails, so that it never sends power into lines that DISCOM staff may be repairing. Panels on the roof, and still no power in your home.

Backup needs storage. One way is a UPS in every flat, which usually covers a few lights and fans. The other is Protostar Fusion, now offered to housing societies: shared rooftop solar, one shared lithium iron phosphate (LFP) battery bank and an SPDU for every flat. Every flat gets a shared part of the solar plant and the battery bank. The plant never connects to the grid, so there is no net metering and no export.

With Fusion, your home runs on sunshine by day and on stored sunshine into the night. In a power cut, Fusion is built to keep the whole flat running on its share, ACs included, with the system sized to your building's loads. A backup reserve, set aside from each day's share in the app, is released only during a cut. When your share is used up, your SPDU moves your flat to its own DISCOM meter until the next day. The street goes dark. Your homes don't.

PM Surya Ghar covers grid-connected rooftop solar on a flat's own connection, on a housing society's common facilities, and under virtual net metering once the DISCOM approves the arrangement, which MSEDCL is not doing today. It pays nothing towards batteries and excludes off-grid systems. For a household, the central subsidy is ₹30,000 per kW for the first 2 kW and ₹18,000 for the third, so ₹78,000 at most, or ₹85,800 in special category states. For a society's common facilities, including EV charging, it is ₹18,000 per kW up to 500 kW, at 3 kW per house, and that limit includes plants residents install themselves. The amount is worked out on panel capacity alone, the panels must be made in India from Indian cells, and the scheme's implementation period runs to 31 March 2027 (MNRE guidelines, checked 4 October 2026).

The society rate applies only to a connection dedicated to common facilities, so it cannot fund a plant that supplies the flats. Fusion is off-grid by design, so the scheme does not cover it, and Protostar engineered it to pay for itself without any subsidy, with one roof used as one plant and one shared battery bank. Fusion pays back in about 5 years for homes, without subsidy. Our solar savings calculator gives a first estimate for your building.

A housing society always has to approve the use of its terrace, because the roof is common property. Most societies decide it at a general body meeting, following their byelaws. The rest depends on the option.

• Your own panels: permission for your part of the terrace, then your own net metering application to the DISCOM.
• Common-area solar: a resolution to spend society funds, and net metering on the common connection.
• Virtual net metering: the participating members, a Lead Person to deal with the DISCOM, and a sharing ratio in the agreement. MSEDCL is not approving VNM applications today.
• Protostar grid-connected microgrid: terrace use, and the common meter as the net metering connection, which Protostar applies for on the society's behalf. Each flat joins individually.
• Protostar Fusion: a no-objection certificate and right-of-use agreement for the roof, the meter room and space for the battery bank. Each flat signs its own agreement and pays for its own share. There is no net metering application, because the plant never connects to the grid.

We support committees with presentations, workings and documents for the meeting, and handle the paperwork after that. A typical retrofit is completed in about 3 months, including approvals, with no work inside any flat.

Shared rooftop solar for flats has been working in Pune since 2019. Protostar's first building was Scorpio at Fortune Estates in Hadapsar. Its 24 flats share a 41.28 kWp rooftop plant, every flat gets 100 free units of solar a month, and homebuyers paid no green premium. The SPDU was invented there, and the building is now a Harvard Business Publishing case study (IM037B), written at IIM Bangalore in 2024. Read the Scorpio case study.

At 7 Plumeria Drive in Pune, the SPDUs delivered 9.6 kWh of solar per flat per day from 2.5 kW of panels per flat, measured, not modelled, and cross-checked within 2% against the independent site meter. Protostar's SPDUs have powered about 300 flats in Pune, all on grid-connected plants without batteries, and MERC's Statement of Reasons for the 2024 rooftop rules quotes Protostar's SPDU proposal, as our impact page shows. Real roofs. Real bills. Real savings.