Inverter, Solar and Backup Power Planning

Power cuts are less frequent in many Indian cities than they were twenty years ago, but backup power has become more important, not less. Families now work and study from home, keep medicines and food in the refrigerator, rely on Wi-Fi for everything from payments to school, and run water pumps, lifts and security systems that stop when the power does. A ten-minute cut is an inconvenience; a three-hour cut on a summer evening with a laptop meeting, a sleeping child and a full refrigerator is a problem that the house should have been designed to handle.

Most homes buy backup in a hurry: an inverter and battery chosen by a dealer, placed wherever there is space — often under the stairs, in a bedroom cupboard or beside the gas cylinder — and connected to whichever circuits the electrician can reach. The result is predictable. The inverter is either too small to run the refrigerator or so large that the battery drains in an hour. The battery is in a poorly ventilated corner. The air-conditioner is accidentally on backup and flattens it in minutes. And when the family later installs rooftop solar, they discover that their new on-grid system switches off during a power cut, exactly when they wanted it.

This article sets out how to plan backup power properly: listing and sizing the loads, choosing between inverter-battery systems, lithium and lead-acid batteries, on-grid, hybrid and off-grid solar, and generators; wiring a separate backup circuit; placing the equipment safely; and budgeting with indicative October 2026 prices that match this site's solar-ready home and net-zero guides. Electrical connection of any backup source is safety-critical and must be done by a licensed electrical contractor.

From candles to hybrid solar: how home backup evolved

For a long time home backup in India meant kerosene lamps, candles and, for the well-off, a small petrol or diesel generator. The home inverter with a lead-acid battery became common from the 1990s, first with square-wave and modified-sine-wave output that made fans hum and some electronics misbehave, then with pure sine-wave inverters that run modern electronics properly. Tall tubular lead-acid batteries, designed for deep discharge, became the standard partner. In apartment buildings, developers added diesel generator backup for lifts, pumps and common areas, and often a fixed backup allowance for each flat.

The last decade added two technologies. Rooftop solar, encouraged by falling panel prices, net-metering and the PM Surya Ghar: Muft Bijli Yojana launched in 2024, put generation on the roof. Lithium iron phosphate (LFP) batteries, falling in price with the growth of electric vehicles, brought compact, long-life storage that needs no topping up. Hybrid inverters combine the two: they run the house from solar, charge the battery, export surplus to the grid and keep chosen circuits alive during a cut. The design question has changed from which inverter to buy to which loads need backup, for how long, and from which sources.

Step 1 — List the loads that must run

Backup planning starts with a list, not a product. Walk through the home and write down what must keep working during a cut, its wattage (from the label or manual) and how many hours it needs. Lights, fans, the Wi-Fi router, phone chargers, laptops and the television are the usual essentials; the refrigerator is often added; a water pump, a microwave or a single bedroom AC may be added if the budget and battery allow. Geysers, ovens, induction cooktops, irons and multiple air-conditioners are high-wattage loads that are normally kept off an inverter, because they would require a very large inverter and drain any reasonably sized battery within minutes.

Step 2 — Size the inverter and the battery

Two numbers define an inverter-battery system: the inverter's capacity, which limits how much load can run at one time, and the battery's energy, which limits how long it can run. Inverters are rated in volt-amperes (VA). Because many loads draw some reactive current and motors draw a surge when they start, a common rule is to divide the total running watts by a power factor of about 0.7–0.8 and add a margin of 20–25 per cent for starting loads such as a refrigerator. Batteries are rated in ampere-hours (Ah) at a voltage; multiplying the two gives watt-hours. A 12 V, 150 Ah battery stores about 1.8 kWh nominally, but only part of that is usable.

How much is usable depends on the chemistry. Lead-acid batteries last far longer when they are not deeply discharged, so a design depth of discharge of about 50 per cent is a sensible planning figure for long life, although they can be discharged further occasionally. LFP lithium batteries are commonly used to about 80–90 per cent depth of discharge. The inverter also loses some energy in conversion — planning efficiencies of about 85 per cent for a home inverter with lead-acid and about 90 per cent with lithium are reasonable. Manufacturer data should always replace these planning figures for the final design.

The worked example shows the effect of two cheap decisions. Switching four conventional fans of about 70 W each to BLDC fans of about 35 W saves roughly 140 W, which on a three-hour backup is over 400 Wh at the load — close to a quarter of the battery requirement. Converting every light to LED before buying backup has a similar effect. Efficient appliances are the cheapest battery a home can buy.

Batteries: tubular lead-acid or lithium?

Tall tubular lead-acid batteries remain the most common choice in Indian homes because they are affordable, robust and widely serviced. Flooded tubular batteries need distilled-water topping up every few months and release small amounts of hydrogen while charging, so they need a ventilated location away from sparks and flames. Sealed lead-acid batteries need no topping up but generally offer fewer cycles. Lithium iron phosphate batteries cost more upfront, but they are much lighter and more compact, need no topping up, can be used more deeply, charge faster and typically last for many more cycles; they must come with a battery management system and certified cells, and should be kept away from heat. Over a long ownership period with frequent cuts or daily solar cycling, lithium often costs less per unit of energy delivered.

Step 3 — Decide the role of solar

Rooftop solar and backup are often confused, and the confusion costs money. A standard on-grid solar system — panels, an on-grid inverter and a net meter — reduces electricity bills by generating during the day and exporting surplus to the grid. But for the safety of line workers repairing the grid, an on-grid inverter must switch off when the grid fails (anti-islanding). During a power cut, even on a bright afternoon, a plain on-grid system produces nothing for the home. Backup requires a battery and an inverter able to run the house in island mode: a hybrid system, or a separate inverter-battery system alongside the solar.

A hybrid system makes sense where a home has suitable shadow-free roof space, cuts are frequent or long, and the family uses significant power in the evening. In the daytime it runs the house from the sun and charges the battery; in the evening it draws on the battery before the grid; during a cut it keeps the backup sub-board alive, recharging from the sun if the cut is in daylight. For a flat without its own roof, solar is usually not available individually, and a good inverter-battery system with efficient appliances is the right answer. On-grid systems cost roughly ₹50,000–75,000 per kW installed before subsidy, and need about 8–10 m² of shadow-free roof per kWp, as explained in this site's solar-ready home guide; a hybrid adds the cost of the battery and a hybrid inverter.

Step 4 — Wire a proper backup sub-board

The cleanest way to connect an inverter is a separate backup sub-board. The electrician groups the essential circuits — lights, fans, chosen sockets, the router socket, the refrigerator socket — on a small distribution board fed from the inverter output, while heavy loads stay on the main board fed only from the grid. This makes it impossible for someone to accidentally run a geyser or an AC from the battery, and lets the inverter be bypassed for maintenance with a changeover switch. Planning the backup circuits at the electrical layout stage costs very little; separating them after the walls are finished means chasing and rewiring.

Three safety rules apply to every backup connection. First, no backup source may ever feed back into the grid uncontrolled: generators connect only through a properly rated changeover switch (manual or automatic) that disconnects the grid before the generator is connected, and grid-tied inverters must meet the DISCOM's anti-islanding requirements. Second, protection must continue to work on backup: the backup sub-board needs its own MCBs and residual-current protection, and the inverter and battery need the protective devices and fusing specified by the manufacturer. Third, the earthing arrangement must be designed so that protection operates whether the house is on grid, inverter or generator. These are matters for a licensed electrical contractor following IS 732, IS 3043 and the Central Electricity Authority's safety regulations — not for improvisation.

Step 5 — Place the equipment safely

Inverters and batteries need a dry, ventilated, shaded location close to the main board, reachable for service, and away from water, heat and children. A utility area, a ventilated niche off the passage, or a dedicated space under the staircase in a bungalow all work if they meet those conditions. Bedrooms, airless cupboards, the space beside the gas cylinder or kitchen hob, bathrooms and places where water collects during the monsoon do not. Flooded lead-acid batteries, which release hydrogen while charging, need particular care with ventilation and should never be enclosed in a sealed cabinet.

Generators need even more care. Fumes from petrol and diesel engines contain carbon monoxide, which can kill in an enclosed space, so a generator must never be run indoors, in a closed garage or under a window; its exhaust must discharge away from openings and neighbours. New diesel generator sets in India must meet the Central Pollution Control Board's emission and noise norms and are supplied with acoustic enclosures; fuel storage must follow fire-safety requirements. In apartment buildings, the society's DG backup usually supplies a fixed load to each flat through a separate meter or circuit, and the flat's design should put the right circuits on that supply.

What backup costs in October 2026

The indicative figures below are installed or retail ranges for branded equipment in Indian cities. They vary with brand, warranty, capacity and installation conditions, and should be confirmed with local suppliers and installers. Wiring of a separate backup sub-board is priced like any other electrical work, using the per-point and distribution-board rates in this site's electrical cost guides.

Where home backup is going

Several changes are under way. Lithium storage is becoming the default for new installations as prices fall and integrated inverter-battery units appear in compact wall-mounted formats. Hybrid solar is moving from farmhouses into city bungalows, and the rules for net metering, battery storage and time-of-day tariffs continue to evolve state by state. Electric cars add both a new load and, in time, a potential storage source: vehicle-to-home systems exist internationally but are not yet a mainstream option in Indian homes. Smart energy monitors and app-controlled inverters let families see what is consuming their backup in real time. For designers, the practical lesson is the same as for solar readiness: reserve the space, the conduit routes and a separate backup circuit now, so that any of these technologies can be added later without breaking finished walls.

Rules for planning backup power

• List essential loads with their wattage and hours before buying anything; size from the list, not from the flat size.
• Convert to LED lighting and BLDC fans first — they are the cheapest way to extend backup time.
• Size the inverter at running watts ÷ 0.7–0.8, plus 20–25 % for motor starts; choose pure sine wave.
• Size the battery from energy needed ÷ (depth of discharge × efficiency): about 50 % DoD for lead-acid, 80–90 % for LFP.
• Wire backup circuits to a separate sub-board at the electrical layout stage; keep ACs, geysers and ovens off it.
• Remember that on-grid solar gives no backup; choose hybrid with a battery if backup from solar is the goal.
• Place inverter and batteries in a dry, ventilated, shaded niche near the main board — never beside the gas cylinder or in a sealed cupboard.
• Connect generators only through a changeover switch, never run them indoors, and use licensed contractors for every connection.

Frequently asked questions

What size inverter do I need for a 3BHK?

It depends on what must run, not on the number of rooms. For lights, fans, router, TV, laptops and a refrigerator — about 550–700 W — a 1.1–1.5 kVA pure sine-wave inverter is typical. Adding a water pump or more rooms pushes it to 2 kVA or more; running an air-conditioner usually needs a much larger inverter and battery or a hybrid solar system.

How long will a 150 Ah battery last?

A 12 V 150 Ah battery stores about 1.8 kWh nominally. Using about half of it for long life and allowing for inverter losses, it delivers roughly 0.75–0.8 kWh at the load — about 2.5 hours at 300 W, or about 1.5 hours at 500 W. Lithium batteries of the same nominal capacity deliver more, because more of their capacity is usable.

Will solar panels work during a power cut?

A standard on-grid solar system will not: its inverter must switch off when the grid fails, for the safety of line workers. To have power from solar during a cut, the system needs a battery and a hybrid or off-grid inverter that can run selected circuits independently of the grid.

Is a lithium battery better than a tubular battery for an inverter?

Lithium (LFP) batteries are lighter, more compact, maintenance-free, more deeply usable and usually longer-lasting, but cost more upfront. Tubular lead-acid batteries are cheaper and well proven but need ventilation and topping up. For frequent cuts or daily solar cycling, lithium usually makes more sense over its life; for occasional cuts on a tight budget, tubular is still reasonable.

Can I run an AC on an inverter?

Only with a correctly sized system. A 1.5-tonne inverter AC can draw well over 1 kW, so it needs an inverter of several kVA and a large battery, which drains quickly. Most homes keep ACs off the inverter and rely on a generator, society DG backup or a hybrid solar system with substantial storage if AC backup is essential.