Inverter backup for a 2BHK: load list, VA rating, battery and backup hours
An inverter bought by the number on the box can leave you with two hours of backup in a long power cut when you expected four. Start from the load you will actually run and work forward from there.
By Bulan Sarkar · Updated
Add up the watts you will run during a cut. Three ordinary fans, six LED bulbs, a TV and a router in a typical 2BHK come to about 380 W. Divide by 0.8 to get about 475 VA, then pick a size where the load is near 70% of the rating: a 900 VA class, 12 V inverter. At full load a 150 Ah tubular battery gives about 3.8 hours by the battery makers' formula, 3.2 hours by our calculator and about 2.5 hours once you allow for the fast discharge. If you need three hours at full load, buy 200 Ah.
Source: Exide Home Star inverter user manual (India): models 700 to 1,625 VA at 12 V and 1,625 and 2,550 VA at 24 V, 10.5 V battery low cut-off, fast-charging current by model, load chart with fan and tube-light watts, battery safety precautions, checked Oct 2026.
The decision: size and buy the inverter yourself?
Yes, if you are choosing a ready-made home inverter and battery for the sockets and lights already wired for one, or replacing an old set. This page takes you from the load list to the battery Ah.
If the inverter works and only the battery is weak, skip the sizing. Replace the battery with one of the same voltage and Ah, rated at the same C-rate, and keep the inverter.
Get a licensed electrician to wire a new inverter into the house distribution board or to move circuits onto it. Get help too if the battery swells, smells of rotten eggs, or gets hot while charging, or if you want to run a pump, AC, geyser or fridge on backup. Those loads need the maker's own advice.
The steps, in order
Write the load list
Have ready:a torch to read rating labelsthe load list on the worksheet
Walk through the house and list only what has to run during a cut. Use the rating label wherever you can find one (volts × amps = watts). Where you can't read a label, use these sourced figures:
Item Watts, and where the figure comes from Ceiling fan, ordinary 70 to 75 W (Ministry of Power material); Exide and V-Guard size for 80 W Ceiling fan, BLDC 28 to 35 W (Ministry of Power material) LED bulb 7 W (Ministry of Power material) Tube light, old type 40 W (Exide load chart, V-Guard FAQ) TV up to 100 W (V-Guard FAQ) Wi-Fi router read the adapter label: 12 V × 1 A = 12 W in our example Our 2BHK example: 3 fans × 75 W = 225 W, 6 bulbs × 7 W = 42 W, TV 100 W, router 12 W. Total 379 W. Write a second, night-only list too (fans, two bulbs, router): 176 W.
Turn watts into VA
Have ready:the total watts from step 1
Luminous gives the conversion as VA = watts ÷ power factor and uses 0.8 in its example: 379 ÷ 0.8 = 474 VA. It also warns that 0.8 is not a fixed value for every home, so leave some room. V-Guard's sizing answer is stricter: the load should be about 70% of the rating (its own example puts 420 W on a 600 VA unit). 379 ÷ 0.7 = 541 VA. The common sizes above that are 700 and 900 VA. Take 900 VA. Exide's Home Star range has both at 12 V, and the extra room covers a mixer or an iron that someone switches on by mistake.
Pick 12 V or 24 V
Have ready:the VA from step 2the model's spec sheet
Home inverters of this size run on one 12 V battery: Exide's Home Star from 700 to 1,625 VA, V-Guard's Prime 1150 (1,000 VA) and Luminous's Eco Watt Neo 1250 Pro (1,100 VA). Bigger units, such as Exide's 2,550 VA, need 24 V, which means two batteries in series. One battery costs less to buy and to replace, so stay at 12 V unless your load list pushes you past what a 12 V unit can carry.
Work out the backup hours for each battery size
Have ready:the total wattstwo battery Ah figures to compare, such as 150 and 200
The battery makers' formula is backup hours = Ah × battery volts × efficiency ÷ load watts. Tata Green Battery puts the efficiency at around 80 to 90%. At 80% and 379 W, a 150 Ah battery gives 3.8 hours and a 200 Ah battery 5.1 hours.
These are the best-case numbers. Our calculator uses MNRE's specification figures instead (75% of rated capacity usable, 90% inverter efficiency), which give 3.2 and 4.3 hours. The discharge rate cuts it further. Exide and Amaron rate their Ah at C20, a 20-hour discharge, which is 7.5 A from a 150 Ah battery. A 379 W load pulls roughly 35 to 40 A, about five times that. As battery-monitor maker Victron puts it, "the faster the rate of discharge, the less capacity will be available", and it uses 1.25 as the default Peukert exponent for lead-acid. With that correction the 150 Ah battery gives about 2.5 hours and the 200 Ah about 3.6 hours.
Plan on the lowest figure. It is an estimate, and the real number depends on the battery's age and the room temperature.
Check the inverter accepts that battery and can charge it
Have ready:the inverter's spec sheetthe battery Ah you chose
Every inverter lists the battery range it supports. V-Guard's Prime 1150 spec table says 80 to 230 Ah, while the overview text on the same page says 80 to 220 Ah, so trust the table and confirm with the dealer. Check the charging current too. Exide's Home Star fast-charges at 15 A on the 700 VA model and 17 A on the 900 VA, so a 200 Ah battery takes longer to fill than a 150 Ah one after a long cut.
Compare batteries at the same C-rate
Have ready:the battery label or spec sheetthe warranty card
Two batteries can both say "150 Ah" and still differ. Exide's InvaTubular and Amaron's AM150TT54 quote Ah at C20. Luminous's LPTT 12150H solar battery is C10 rated, measured over 10 hours at a faster rate. The two labels don't measure the same thing, so ask the dealer for both batteries' Ah at the same rate. Write down the warranty split as well: Exide's "48+18" means 48 months of full warranty and 18 more pro-rata, and Amaron's 54 months is 36 free and 18 pro-rata.
Place it and look after it
Have ready:distilled watera corner with air flow, away from the stove
Follow the makers' own precautions. Exide's manual says to keep sparks, open flames and other sources of ignition away from the battery, to work in a well-ventilated area, and to refill cells only with distilled water. Luminous suggests checking the water level once every two to three months, and says checking every two to four weeks works well in many homes. Note each top-up and each long cut on the worksheet log.
What goes wrong
Variants: what changes the steps
Switching to BLDC fans
Swap the three 75 W fans for BLDC fans at 35 W and the load falls from 379 W to 259 W. The same 150 Ah battery then lasts about 4 hours with the fast-discharge correction, up from 2.5. If the fans are due for replacement, change them first and size the battery afterwards.
Night cuts only
Two fans, two bulbs and the router come to 176 W. A 700 VA unit with a 150 Ah battery gives about 6.5 hours with the fast-discharge correction, enough for a normal night cut.
Working from home
Add the laptop charger (read the wattage on its label) and keep the router on the list. Run the hours again; a laptop and router left on for a full working day can need the 200 Ah battery even when the fans are BLDC.
A bigger load and two batteries
If the load list goes past about 1,100 W, you are into larger units. Exide makes its 1,625 VA Home Star in both 12 V and 24 V and its 2,550 VA only in 24 V. A 24 V unit needs two identical 12 V batteries in series, bought together.
Tips and tricks
- Read the label, not the box. Every appliance has a rating sticker, and volts × amps gives you the watts. Photograph each one as you make the list.
- Size for the busiest ten minutes, when the TV, every fan and every light are on together. The average load will undersize it.
- If one shop quotes C10 and another C20, you are comparing different measurements. Ask both for the same rate.
- Look at the free part of the warranty first. In "48+18" or "36 free, 18 pro-rata", the first number is what you get without paying a share.
- Put a sticker on each socket wired to the inverter, so nobody plugs a geyser or an iron into one during a cut.
- Keep a backup log: how long each cut lasted and when the inverter beeped. When the hours start falling you will see it in the log.
What we would do
For a 2BHK with ordinary fans, we would buy a 900 VA class, 12 V sine-wave inverter and one 200 Ah tubular battery rated at C20. That gives about 3.5 hours at full load and a whole night on fans and lights. If the fans are due for a change, we would buy BLDC fans first, then size the battery against the smaller load; 150 Ah may then be enough. If you are also thinking about rooftop solar, read the rooftop solar plan before buying, because it has its own backup step and you don't want to buy batteries twice.
Print the working copy
Inverter backup worksheet: load, VA, battery, hours, from the Inverter Backup Planner. One A4 page with tick boxes and blanks; it prints in black and white.
- 1. Load list (runs during a cut)
- 2. Inverter
- 3. Battery
- 4. Backup hours
- 5. Backup log
Frequently asked questions
How many hours will a 150 Ah battery run a 2BHK?
Is a 700 VA inverter enough for a 2BHK?
What is the formula for inverter backup time?
How do I convert watts to VA for an inverter?
What does C10 or C20 mean on a battery?
The source we built this on
Also checked on 8 Oct 2026: Exide InvaTubular and tubular solar battery warranty pages; Amaron AM150TT54; V-Guard Prime 1150 (spec table and FAQ); Luminous Eco Watt Neo 1250 Pro, LPTT 12150H, its inverter-capacity guide and battery-water guide; Tata Green Battery's backup-time formula; Victron's note on battery capacity and the Peukert exponent; and Ministry of Power material on fans and LED bulbs (PIB). Backup hours are estimates worked from these figures, not measured results.
Wiring an inverter into the house distribution board is a job for a licensed electrician. Keep the battery in a ventilated spot away from flames and sparks, and top it up only with distilled water, as the makers say.