Buying the inverter is the easy part. The job that fails is the wiring: a thin cable on a heavy load, no changeover, or the battery sitting in a closed loft with no air.
We wire the UPS into the points you actually need through a cut — lights, fan, Wi-Fi — and we say so on the call if the board itself has to be opened first.
In Bangalore flats the usual brief is one changeover for the hall and bedroom lights, a separate line for the Wi-Fi, and the battery cupboard left with air. Send a photo of the board and say which rooms must stay on.