⚡ What Battery System Do You Need?

Assumptions & disclaimer

I built this calculator to be honest, and honesty includes telling you exactly where its numbers come from and where their limits are.

What is measured: the 12V/150Ah curve (400–1400W), the 12V/200Ah lab runs (March 2021), and the 24V, 48V and 96V banks at 150Ah — all real discharge tests to 10.5V cutoff with resistive bulb loads through a home inverter, logged at 5–15 minute intervals in winter 2020–21.

What is estimated: results below 400W and above the tested load range (extrapolated from the measured curves, capped at 88% of nominal energy); the 120V bank (scaled from my 96V tests with a 2–5% series-string allowance); batteries other than 150Ah and 200Ah (scaled by specific load); and all parallel-string configurations.

Appliance figures are typical Indian market values — your fan, AC or geyser may differ; check the nameplate. Cycling appliances (refrigerator, AC, geyser, room heater, washing machine, microwave, laser printer, photocopier, coffee machine, deep freezer) are counted at typical average draw because compressors, fusers and thermostats switch off after reaching temperature. In rare extreme conditions of continuous running — a heatwave with the AC never resting, a geyser refilling continuously — expect less backup than shown.

Motor surge is not modelled. Water pumps, AC compressors and refrigerators draw 3–5× their rated watts for a moment at start. The watt figure I recommend includes 15% steady headroom, but your inverter must also carry the surge — confirm its surge rating with the manufacturer.

All recommendations assume new, healthy, fully charged tubular batteries with an honest Ah label. A 3-year-old battery holds roughly 70% of its rating. Temperature also moves capacity — batteries give a little more in heat (and age faster) and noticeably less in cold. Water-topping intervals are drawn from my three decades of field experience and vary with charger quality and climate.

This is guidance, not a guarantee. Results are indicative estimates for planning and comparison. Final system selection, wiring and installation must be done by a qualified professional. Battery brands differ; I name no brands and earn nothing from any purchase you make — zero affiliate relationships, zero sponsorships, zero commercial bias.

The measured data behind this calculator

These are my actual discharge test results — bulb (resistive) loads through a home inverter, run to the 10.5V cutoff, timed with a logger. The calculator interpolates between these points, scales by capacity, and accounts for the real average draw of cycling appliances. Read the full article with the test video.

The principles this calculator follows

A battery never delivers its label. In my tests a new 150Ah battery delivered 93Ah at 400W — and only 52Ah at 1000W. The harder you load it, the less total energy it gives. That is why these answers are lower than the brochure: they are real.

Cycling appliances draw less than their nameplate. A refrigerator compressor, an AC, a geyser thermostat, a laser printer's fuser — they all switch off after reaching temperature. The calculator counts them at their real average draw, so backup is honestly longer than rated-watt arithmetic. Only in rare extreme conditions of continuous running will you see less.

Buy inverters by watts, not VA. VA has no fixed relationship to real output in Indian inverter standards — which is exactly why BIS mandates the continuous wattage on every nameplate. The calculator tells you the watt figure to demand.

Every battery has a comfort zone. A 150Ah is comfortable up to about 300W; a 200Ah up to 400–500W. Inside the zone you get the best capacity, water topping once a year, and the longest life. Beyond it the gauge turns amber, then red — and the water-topping schedule tightens from yearly to monthly.

For heavy loads or long hours, add batteries before you add hope. A 1200W load on one 150Ah battery gives 25 minutes; on a 24V pair it gives 1h 40m — four times the backup from twice the battery. Long backup needs energy, not wishful thinking, which is why the calculator recommends parallel strings when one string cannot reach your target.

One warning: all of this assumes the Ah on your label is honest. In India's unorganised segment, it often is not — weigh the battery before you buy.