Tap the things you run during load-shedding — or type your total load directly — then pick your battery. You get your real backup time in hours and minutes, using the honest rule that a lead-acid battery only gives about half its charge.
Wattages are typical values for common Pakistani appliances — a new DC fan uses less, an old fan more. You can always type the exact total below instead.
The honest maths: usable power = volts × Ah × usable share × efficiency. A 12V 200Ah lead-acid battery holds 2,400 Wh on paper, but only gives about 1,020 Wh in real life — that is why a 400 W load lasts about 2 hours 33 minutes, not 6 hours. At 24V, enter the Ah of one battery — two of that size are used in series. Two batteries in parallel on 12V? Add their Ah together.
Tap the result to copy it
Backup assumes a healthy, fully charged battery. An old battery, or one that did not fully recharge between two load-shedding spells, will give less.
Backup times worked out with this calculator's formula — 85% inverter efficiency. The lead-acid / tubular battery gives 1,020 Wh usable; the lithium gives 1,632 Wh usable:
| Load | Lead-Acid / Tubular | Lithium |
|---|---|---|
| 200 W — about 2 fans, lights & router | 5h 6m | 8h 10m |
| 400 W — fans, lights, TV & router | 2h 33m | 4h 5m |
| 600 W — above + fridge & laptop | 1h 42m | 2h 43m |
Want longer backup? The only real fixes are a bigger battery, a lithium battery, or a smaller load — switching off one fan adds more time than most people expect.
Check it yourself: 12 × 200 × 0.5 × 0.85 = 1,020 Wh usable → at a 400 W load, 1,020 ÷ 400 ≈ 2 hours 33 minutes — not the 6 hours the paper figure suggests.
This battery backup time calculator works from the charge you can actually use — not the full number printed on the battery — in five quick steps:
The calculator assumes a healthy, fully charged battery. Real backup drops when the battery is old or sulphated, when it did not fully recharge between two load-shedding spells, or when the actual load is higher than estimated — a fridge, for example, draws extra power each time its compressor starts.
A 12V 200Ah battery stores 12 × 200 = 2,400 Wh in total. A lead-acid or tubular battery should only be drained halfway, and the inverter loses about 15%, so the usable power is about 2,400 × 0.5 × 0.85 = 1,020 Wh — enough for a 400 W load for about 2 hours 33 minutes. A lithium battery of the same size gives about 1,632 Wh usable, because 80% of it can be used safely.
From the same Ah rating, lithium gives roughly 60% more backup time, because you can safely use about 80% of a lithium battery against only about 50% of a lead-acid or tubular one — and lithium also charges faster between load-shedding spells. It costs much more upfront, but lasts many more years.
Usually yes, if the inverter is big enough. A fridge draws about 150 W while running but needs a short surge of 600–900 W each time the compressor starts, so the inverter must handle that surge. This calculator counts the fridge at 150 W; if your UPS trips when the fridge starts, the inverter — not the battery — is the limit.
A 24V system uses two batteries in series, so the bank stores twice the energy of a single 12V battery of the same Ah — and it handles heavy loads (above roughly 1,200–1,500 W) more comfortably, which is why bigger inverters are usually 24V. For a few fans and lights, 12V is fine.
On a 12V UPS with a lead-acid / tubular 200Ah battery (about 1,020 Wh usable): about 5 hours 6 minutes at a 200 W load (2 fans, lights and a router), 2 hours 33 minutes at 400 W (fans, lights, TV and router), and 1 hour 42 minutes at 600 W (adding a fridge and laptop). A lithium 200Ah battery lasts about 60% longer — about 8 hours 10 minutes at 200 W. The table above shows the full comparison.
Yes, completely free. All calculations happen inside your own phone or computer — nothing you type is uploaded anywhere.