How to Calculate Backup Time in Tubular Battery Inverter — with the Free Calculator
Forget the formulas given to the industry for years — they don’t match reality. Our mentor Kunwer Sachdev — the “Inverter Man of India” and the “Solar Man of India” — designed a free Battery System Calculator for the ease of selection of battery and inverter, with the exact backup time chart, built on his own lab discharge tests. This article shows you how it works and how to use it.
When you ask a shop “how much backup will I get?”, you get brochure mathematics. The truth, proven in real discharge testing, is that a battery never delivers its label — a new 150Ah tubular battery delivered 93Ah at 400W and only 52Ah at 1000W when tested to the 10.5V cutoff. So instead of giving you another wrong formula, this article gives you the tool that does the honest calculation for you.
Step 1 — Pick what runs during a power cut
Open the Battery System Calculator and simply tap + on your appliances — home or shop. Every appliance carries its real Indian-market wattage: a ceiling fan at 75W, LED bulb at 9W, refrigerator at 150W. Notice the word “cycles” on the refrigerator, AC and geyser — the calculator knows these switch off after reaching temperature, so it counts their real average draw, not the nameplate. That alone is why its answers beat every formula on the internet.
Step 2 — Tell it how many hours of backup you need
Choose your backup target and press 🔍 Calculate my system. Behind the button, the calculator separates two numbers most people mix up: the connected load (which sizes the inverter) and the average drawn (which decides the backup).
Step 3 — Read your honest answer
The result gives you four numbers, and each one matters:
- Inverter needed (in watts) — as BIS mandates, you buy inverters by continuous wattage, never by VA. The figure includes 15% steady headroom.
- System voltage — 12V, 24V, 48V or higher; heavy loads need higher banks.
- Battery required (tubular, C20) — the Ah you should actually buy.
- Real backup — taken from lab discharge curves, not arithmetic. The comfort gauge shows whether your battery will live in its Comfort, Working, Stress or Cliff zone — and how that changes battery life and water-topping from yearly to monthly.
The measured chart behind it
These are the real discharge tests the calculator stands on — resistive loads through a home inverter, run to the 10.5V cutoff:
| System | Battery | Load | Measured backup |
|---|---|---|---|
| 12V | 150Ah tubular | 400W / 600W / 800W / 1000W / 1400W | 2h 50m / 1h 30m / 1h 00m / 40m / 14m |
| 12V | 200Ah tubular | 400W / 600W / 1100W | 4h 20m / 2h 26m / 58m |
| 24V (2×150Ah) | 150Ah tubular | 800W / 1200W / 1600W / 2000W | 3h 00m / 1h 40m / 1h 00m / 26m |
| 48V (4×150Ah) | 150Ah tubular | 1600W / 2400W / 3200W / 4000W | 3h 05m / 1h 35m / 1h 05m / 30m |
| 96V (8×150Ah) | 150Ah tubular | 2000W / 4000W / 6000W / 8000W | 4h 00m / 1h 50m / 45m / 20m |
The lesson hidden in this chart: a 1200W load on one 12V battery gives 25 minutes — on a 24V pair it gives 1h 40m. For heavy loads or long hours, add batteries before you add hope.
Free · No registration · Zero affiliate bias
⚡ Try the Battery System Calculator
Pick your appliances and get your inverter watts, system voltage, battery Ah and honest backup time — in seconds.
Open the Calculator →
⚡ The backup calculator, sizing methodology, charts and underlying battery discharge test records are original works of Kunwer Sachdev, first published on inverterindia.com in June 2026. © 2026 Kunwer Sachdev. All rights reserved.
If your loads have outgrown tubular batteries altogether, see the difference between tubular and lithium batteries and Su-vastika’s retrofit lithium battery range.
Disclaimer: Su-vastika does not manufacture or sell tubular batteries. This article and the calculator are educational — published to help buyers select the right battery and inverter honestly, whatever brand they choose. Su-vastika manufactures inverters, lithium battery systems and energy storage solutions.
Disclaimer: This article is written by Kunwer Sachdev, mentor of Su-vastika. Kunwer Sachdev is no longer associated with Su-Kam Power Systems Ltd. in any capacity. Anyone dealing with Su-Kam should be aware that Kunwer Sachdev has no association with the Su-Kam brand or company.
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