Work out the battery capacity and UPS size your load actually needs.
Buying a UPS or battery in Pakistan usually starts with a guess — and ends with a system that dies an hour into load shedding. This free calculator removes the guesswork. Enter the load you want to run and the backup time you need, and it works out the battery capacity in ampere-hours (Ah), the practical battery setup to buy, and the UPS size in VA that can actually carry your load. No sign-up, no cost — just the maths, shown step by step so you can check it yourself. It works for the 12V and 24V systems used in almost every Pakistani home, and if you also want the background reading, our guide on what size UPS you need explains the sizing logic in words.
Assumption used for every calculation: inverter efficiency 85% — a normal figure for home UPS units sold in Pakistan. Part of the battery's energy is always lost as heat in the inverter, so the battery must supply more than the load uses.
Required battery capacity: —
Recommended setup: —
Recommended UPS size: —
The calculator is only as honest as the load figure you feed it, so start by listing everything you want running during load shedding — not everything in the house, just what must stay on. Add up the watts. Most people overestimate wildly at this step and then buy a battery bank far bigger (and more expensive) than they need. The table below gives typical wattages for common appliances; treat them as typical values, not exact figures, because two fans or two fridges are never identical.
| Appliance | Typical watts |
|---|---|
| Ceiling fan | 80W |
| LED bulb / energy saver | 12W |
| LED TV (32–43 inch) | 100W |
| Fridge (running) | 150W |
| Laptop | 65W |
| Wi-Fi router | 10W |
| Washing machine | 500W |
A very common Pakistani backup setup — three fans, four LED bulbs, a TV and a router — comes to roughly 240 + 48 + 100 + 10, which is about 400W. Notice what is not on the list: the iron, the microwave, the air conditioner and the water motor. Heating and motor appliances draw so much that a normal home UPS either trips or flattens the battery in minutes. The honest approach is to size the system for lights, fans, TV and internet, and let the heavy appliances wait for grid power.
Here is the part battery sellers rarely explain. A battery marked 200Ah does not give you 200Ah of usable power. How much you may safely take out of it — the depth of discharge, or DoD — depends on the battery type, and regularly draining a battery beyond that limit shortens its life from years to months.
Think of a 200Ah lead-acid battery as a water tank you are only allowed to half-empty. Use more than half and the plates inside start to damage; do it every night during load shedding season and the battery is finished within a year or two. That is why the calculator uses 50% for ordinary lead-acid (flooded) batteries, 60% for tubular batteries — whose thicker plates tolerate deeper cycling — and 80% for lithium batteries, which are built for deep discharge. This single assumption is the difference between a system that lasts four years and one that disappoints you by next summer.
The second quiet thief is the inverter itself. Converting battery DC to household AC wastes energy as heat; 85% efficiency is a fair, standard assumption for home UPS units, and it is built into the calculator. Between DoD and efficiency, a "200Ah" battery on a 12V system honestly delivers about 1,020 usable watt-hours — not the 2,400Wh the label suggests.
Small systems — one battery, loads up to roughly 600–800W — are normally 12V: one battery, simple wiring, and most small UPS units are built this way. As loads and backup hours grow, 24V becomes the sensible choice. A 24V system uses two 12V batteries wired in series; the same energy then flows at half the current, which means thinner losses, cooler cables and an easier life for the inverter.
One point confuses many buyers: two 150Ah batteries in series make a 24V, 150Ah bank — the voltage doubles, the Ah stays the same, but the stored energy doubles because energy is volts × amp-hours. To add more backup to a 24V system you add a second series pair in parallel, which is why the calculator's 24V recommendations always come in pairs and fours. If you are buying a UPS of 1500VA or above, it will very likely be a 24V model, so plan the battery purchase in pairs from the start.
For most Pakistani homes the real contest is between tubular lead-acid and lithium. Ordinary flooded batteries are the cheapest to buy and the fastest to wear out under daily cycling. Tubular batteries cost more upfront but are designed for exactly our conditions — deep daily discharge during load shedding, slow recharge when power returns — and typically outlast flooded batteries by a wide margin. Lithium (LiFePO4) goes further: about 80% usable capacity, much lighter, no water topping, and a far longer cycle life, but at a significantly higher purchase price and with fewer local dealers in smaller cities.
A practical rule: if the system cycles every day and you can stretch the budget, tubular is the value choice and lithium is the comfort choice. Whichever chemistry you pick, the calculator's battery-type setting applies the right usable-capacity figure, so compare the two honestly before you buy. When you are ready to shortlist actual models, our brand guides cover the ranges sold across Pakistan — see the Exide battery guide, the Osaka battery range and the Phoenix battery range for types and capacities available locally.
A UPS is rated in VA, your load is in watts, and the two are linked by the power factor: watts ÷ 0.8 = VA. A 1000VA UPS therefore carries about 800W honestly, whatever the shopkeeper says. Run it at its limit daily and it will trip on fan-start surges or fail early. The calculator rounds your VA requirement up to the next common size for exactly this reason.
Car batteries are built to deliver a huge burst for a few seconds to start an engine, then recharge immediately. A UPS drains a battery slowly for hours — the opposite job. A car battery on a UPS will work for a few weeks and then fade badly. Always buy a deep-cycle or tubular battery designed for inverter use.
The most common sizing error of all: taking load × hours, dividing by 12, and buying exactly that many amp-hours. That ignores both the 85% inverter efficiency and the usable-capacity limit, and it under-sizes the bank by roughly half — the reason so many systems "should" give four hours but give two.
In a series pair or parallel bank, the weakest battery drags the rest down with it. Replace batteries of a bank together, and never pair a tired one-year-old battery with a new one.
Say your backup load is two fans, three LED bulbs, a TV and a router — about 300W — and you want four hours through the evening load shedding. The maths: 300W × 4h = 1,200Wh; ÷ 0.85 efficiency = 1,412Wh the battery must supply; ÷ 50% usable = 2,824Wh of nameplate capacity; ÷ 12V = 235Ah required. No single common size lands just above that, so the practical purchase is 2 × 150Ah batteries in parallel (300Ah total), and the UPS only needs to be a 1000VA model (300 ÷ 0.8 = 375VA, rounded up). This is the same sizing worked through in our Exide guide.
A larger home running five fans, six lights, a TV, a fridge and a router is near 800W. On a 24V tubular system: 800W × 4h = 3,200Wh; ÷ 0.85 = 3,765Wh; ÷ 60% usable = 6,275Wh; ÷ 24V = 261Ah required at 24V. Because 24V banks are built from series pairs, the practical setup is 4 × 150Ah batteries — two series pairs in parallel — with a 1000VA-class 24V UPS (800 ÷ 0.8 = 1000VA; in practice many buyers step up to a larger unit for surge headroom). Try both examples in the calculator above and change the battery type to see how much the chemistry alone moves the answer.
It depends entirely on your load. For a typical 300W load (fans, lights, TV, router) on a 12V lead-acid system, the honest answer is about 235Ah of capacity — practically, two 150Ah batteries in parallel. Double the load and you roughly double the Ah. Use the calculator with your own appliance list for a figure you can trust.
A 200Ah, 12V lead-acid battery honestly delivers about 1,020 usable watt-hours (200 × 12 × 50% usable × 85% efficiency). On a 200W load — say two fans and a few lights — that is about 5 hours. On a 400W load, expect roughly 2.5 hours.
A typical fridge draws about 150W while running, so the energy maths says a 200Ah battery can carry it for a few hours — but fridges draw a heavy surge each time the compressor starts, and that surge is what trips small UPS units. If the fridge must stay on backup, size the UPS generously and expect the fridge to consume most of your bank on its own.
Only as a desperate stopgap. Car batteries are starting batteries, not deep-cycle batteries; slow, deep UPS discharge damages them quickly. A tubular or deep-cycle battery costs more but is built for the job and lasts far longer, as covered in the tubular section above.
Usually a mix of ageing, repeated deep discharge below the safe limit, low water in flooded cells, and sulphation from sitting partly charged. Good habits recover some of it and protect a new battery — see our UPS battery maintenance tips.
For a 12V system, whichever gets you to the required Ah with the fewest, largest units is usually cleanest — one 200Ah beats two tired 100Ah units. For 24V you have no choice: batteries come in series pairs, so you add backup one pair at a time, and all batteries in the bank should be the same age and model.