Inverter, UPS or a shared battery: power backup for flats, compared

An inverter is better when one flat has to act alone, and a shared battery is better when the building can act together. A home inverter or UPS needs nobody's permission and usually goes in within a day. A shared battery behind a shared rooftop plant needs the society to agree on the terrace and a battery room, but it can back up the whole flat and run it on solar instead of grid power.

Both can sit in the same building. Flats that join a shared system run on it, and flats that do not join keep their inverters and their DISCOM supply exactly as before.

A home inverter or UPS usually backs up a few lights, fans and sockets, and little else. An electrician wires it to selected circuits in your flat, and the rest of the flat goes dark in a power cut. The AC and the geyser are almost never on it, because they need a far larger inverter and battery than a flat normally keeps.

• It stores grid power, not sunshine. The battery charges from the grid, so it keeps the lights on but does nothing for your bill.
• It runs down in a long cut. Once the battery is empty, it waits for the grid to come back before it can charge again.
• The batteries wear out. Lead-acid inverter batteries are usually replaced every 3 to 5 years, so the cost comes back again and again.
• It takes space inside your home. The battery trolley sits in a corner of the flat with wires running to your board, and every flat in the building repeats it.

In a 40-flat building, that can mean 40 inverters, 40 sets of batteries and 40 service calls, each bought and replaced on its own schedule. One shared lithium bank does the same job once, for every flat that joins.

A home inverter is the right choice whenever a shared system is not possible for you. That is a real situation for many families, and an inverter answers it well.

• You rent your flat. An inverter is yours to take when you move, and it needs no agreement from the owner or the society.
• You need backup this month on a small budget. An inverter can usually be bought and installed in a day.
• Your flat is acting alone. If the society will not discuss solar or the terrace is not available, a shared plant cannot start, and an inverter is the backup you can control.
• Your outages are rare and short. If cuts in your area last minutes, a few lights and fans may be all you need.

A shared battery earns its place when owners plan to stay, the society is willing to talk, and backup for the whole flat matters, or when the building also wants lower bills. Our guide to solar for flats compares every way a flat can go solar, with and without a battery.

Grid-tied rooftop solar does not work in a power cut because its inverter switches off when the grid fails. This anti-islanding protection stops it sending power into lines that DISCOM staff may be repairing. Panels on the roof, and still no power in your home.

That is why solar alone does not replace an inverter. Backup needs a battery, and the question for a flat is whether that battery sits inside each home or is shared by the whole building.

A shared battery backs up every flat through a patented SPDU (Smart Power Distribution Unit) at each flat's meter. Protostar Fusion puts one shared solar plant on the roof and one shared lithium battery bank in the building, and installs one SPDU per participating flat in the meter room. Every flat gets a shared part of the solar plant and the battery bank, sized to its own bills when the system is designed. Capture. Store. Share.

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. A backup reserve, set aside from each day's share in the Nova app, is released only during a cut. When your share is used up, your SPDU switches your flat to its own DISCOM meter automatically until the next day. The street goes dark. Your homes don't.

The Fusion plant never connects to the grid, so there is no net metering and no export, and the battery is charged by the sun. Nothing inside your flat is rewired: all the work happens on the terrace, in the battery room and in the meter room.

Fusion uses lithium iron phosphate (LFP) batteries, the stable chemistry used in electric buses and grid storage. The bank lives in a ventilated battery room, professionally installed and monitored, not on a trolley inside anyone's flat. It carries a 10-year warranty and a 20-year rated service life, against the 3 to 5 years most homes get from a lead-acid inverter battery.

Maintenance changes too. Protostar looks after the whole building under one annual maintenance contract covering panel cleaning, battery health and performance monitoring, instead of every flat chasing its own UPS vendor.

The society has to agree to the use of its terrace, the meter room and space for the battery bank, because these are common property. Most societies decide this at a general body meeting, following their byelaws, and then give Protostar a no-objection certificate and a right-of-use agreement.

• Participation is flat by flat. Each participating home signs its own agreement and pays for its own share.
• Flats that do not join stay on their DISCOM supply, and keep their inverters if they have them.
• No net metering application, because the Fusion plant never connects to the grid.
• No work inside any flat. A typical retrofit is completed in about 3 months, including approvals.

We support committees with presentations, workings and documents for the meeting. See what shared solar looks like on a real Pune building in our Scorpio case study, or book a free rooftop survey to find out what your roof can support.