Inverter Battery Buying Guide: Sizing, Chemistry and What Lasts

Bengaluru outages are shorter than they used to be, but they are also less predictable, and a lot of people now work from home where a two-hour cut means two hours of lost work. Getting the inverter right is mostly arithmetic.

Sizing: two numbers, not one

An inverter has a capacity in VA, which determines how much you can run at once, and a battery in Ah, which determines how long you can run it. People tend to focus on the first and under-buy the second.

Add up the wattage of everything you want running during a cut. Four fans at 75W, six LED lights at 10W, a TV at 100W and a router at 15W comes to roughly 475W. Divide by 0.8 for inverter efficiency and you need about 600VA — so a standard 900VA unit has comfortable headroom.

For runtime: a 150Ah battery at 12V holds about 1,800Wh, of which you should use no more than half if you want the battery to last. That is 900Wh usable, which at 475W gives just under two hours. Want four hours? You need two batteries, not a bigger inverter.

Why your backup time halved

This is the most common service call we get, and the answer is almost always the battery rather than the inverter. Lead-acid batteries lose capacity steadily and then sharply, typically after three to four years of regular cycling. Deep discharges, high ambient temperature and skipped water top-ups all accelerate it.

Before you replace anything, ask for a load test. It takes fifteen minutes and shows you the actual capacity remaining. We have lost count of the customers who were told their inverter had failed when a ₹14,000 battery was the entire problem — and, occasionally, the reverse.

Never mix old and new batteries

If you have two batteries in series and one fails, replacing only the failed one feels economical. It is not. The old battery limits the pair, the new one is repeatedly overcharged to compensate, and you typically lose the new battery within a year. If the bank is over two years old, replace the set.

Tubular, SMF or lithium?

Three chemistries, three different trade-offs:

• Tall tubular — around ₹14,000 for 150Ah, lasts 4 to 5 years, needs water top-ups every few months, tolerates deep discharge well. The default choice for most homes.
• SMF (sealed maintenance-free) — similar price, no watering, but shorter life under deep cycling. Best where the inverter is indoors and ventilation is limited.
• Lithium (LiFePO4) — roughly double the upfront cost, 8 to 10 year life, no maintenance, and you can safely use 80 percent of its capacity rather than 50 percent. Over ten years it usually works out cheaper per usable unit.

The lithium question

If you plan to stay in the same home for the next decade, lithium is the better financial decision as well as the more convenient one. If you are in a rented flat and may move in two years, tubular remains the sensible choice.

One caveat: lithium needs a compatible inverter or an appropriate charge profile. Fitting a lithium bank to an inverter configured for lead-acid will damage it. Check this before buying.

Maintenance that actually matters

Top up with distilled water only, every three to four months on flooded batteries. Keep the terminals clean and greased. Make sure the battery has ventilation and is not in an enclosed cupboard. Do not leave the inverter switched off for weeks at a time, because a battery sitting at partial charge sulphates.

That is genuinely the whole list, and following it is the difference between four years and six.