Load shedding usually arrives as one- to two-hour cuts several times a day, and an IPS (an inverter with a battery bank) is the standard answer in Bangladesh, Pakistan, and much of South Asia. A system sized from a real list of essential loads and honest battery arithmetic runs through those cuts for years. One sized from a shop rule of thumb either dies in eighteen months or goes dark in the third hour.
List the essential loads honestly
Start with what must stay on, not what would be nice. Typical figures: a ceiling fan 70–80 W, an LED bulb 9–12 W, a Wi-Fi router plus fibre ONU 15–20 W, a 43-inch LED TV 60–80 W, a laptop charger 45–65 W, and phone chargers around 10 W each. A 250-litre refrigerator runs at 150–200 W but starts at 600–1,000 W for a second and only runs about a third of the time, so it averages 50–70 W over an hour while demanding a much larger inverter.
A common essential set is three fans (225 W), six LED lights (60 W), a router (20 W), a TV (70 W), and a couple of chargers (25 W): 400 W in total. Adding the fridge roughly doubles the inverter requirement and adds 250–300 Wh over a four-hour cut; a well-stocked fridge left closed holds temperature for four hours on its own, so decide before sizing whether it is in or out.
Convert watts into watt-hours
Energy is power multiplied by time. A 400 W load for 4 hours needs 400 × 4 = 1,600 Wh delivered to the appliances. The inverter loses some energy turning 12 V DC into 220 V AC; a realistic efficiency for a mid-range IPS is 85%, so the battery must supply 1,600 ÷ 0.85 = 1,882 Wh. Cheap square-wave units can be below 80%, which pushes that to 2,000 Wh.
Budget also for the inverter’s own idle draw, typically 10–25 W, or 0.25–0.6 kWh a day from the grid whether or not the power goes out.
Depth of discharge: tubular lead-acid versus LiFePO4
You cannot use all of a battery’s rated capacity without shortening its life. Tubular lead-acid batteries, the usual 150–200 Ah IPS battery, should not be discharged below about 50% on a routine basis. At 50% depth of discharge a decent tubular battery gives around 1,200 cycles; at 80% the same battery may give 500 or fewer. Lithium iron phosphate (LiFePO4) batteries are routinely cycled to 80% and rated for 2,000–4,000 cycles at that depth.
The stored energy you need is the delivered energy divided by the usable fraction. For the 1,882 Wh example: 1,882 ÷ 0.50 = 3,765 Wh of lead-acid capacity, or 1,882 ÷ 0.80 = 2,353 Wh of LiFePO4. This is why a lithium bank that looks 40% smaller on paper delivers the same backup.
12 V or 24 V, and the amp-hour maths
Amp-hours are watt-hours divided by bank voltage. At 12 V, 3,765 Wh ÷ 12 = 314 Ah, which means two 200 Ah tubular batteries in parallel (400 Ah). At 24 V, 3,765 Wh ÷ 24 = 157 Ah, which means two 200 Ah batteries in series. The same two batteries, wired differently, hold the same energy, but the 24 V bank halves the current: 400 W ÷ 0.85 ÷ 12 V is 39 A, while 400 W ÷ 0.85 ÷ 24 V is 20 A. Lower current means thinner cables, cooler connections, and less loss.
Most inverters above about 1,000 VA are 24 V for this reason, so if you may add a fridge later, buy 24 V now. For LiFePO4 the sum is 2,353 Wh ÷ 12 = 196 Ah, so a single 12 V 200 Ah lithium battery covers the 400 W, 4-hour case with almost nothing to spare; a 24 V 100 Ah unit does the same job.
Inverter VA versus watts and power factor
Inverters are rated in volt-amperes (VA), and appliances are rated in watts. The two are linked by power factor: VA = W ÷ PF. Fans, fridges, and old chargers run at a power factor around 0.8, so 400 W of load is 500 VA. Add 25% headroom for switching surges and hot days and you need 625 VA, which means a 650 or 800 VA inverter from the shelf. The calculator rounds to the nearest 50 VA above this figure.
If the fridge is included, size for its start-up surge. A compressor pulling 800 W for a second on top of 400 W of fans and lights needs an inverter whose surge rating covers 1,200 W, which in practice means a 1,000–1,200 VA unit with a stated peak of two times continuous. Check the datasheet for the peak figure, not just the headline VA.
Charging time and how often the power goes
A battery only helps if it is full when the next cut comes. Lead-acid should be charged at about 10% of its capacity: a 200 Ah battery at 20 A. Restoring the 50% you used takes 100 Ah ÷ 20 A = 5 hours of bulk charging plus 2–3 hours of absorption to finish. Many IPS chargers deliver only 10–15 A, so a 400 Ah bank at 12 A needs 200 ÷ 12 ≈ 17 hours to recover.
Now compare that with the outage pattern. If you lose power for two hours three times a day with three or four hours of grid between cuts, a lead-acid bank never reaches full charge, sits partly discharged all day, and sulphates within a year. You have two fixes: buy more lead-acid than the arithmetic says so each cycle is shallower and recharges faster, or move to LiFePO4, which accepts 0.5C (100 A into a 200 Ah battery) and can be refilled in two to three hours. Frequent short cuts favour lithium far more strongly than a single long nightly cut does.
Battery life and maintenance
Tubular batteries need distilled water topped up to the marked level every month or two, clean and greased terminals, and ventilation, because charging releases hydrogen. Keep them out of the sun: every 8–10 °C above 25 °C roughly halves lead-acid life, and a battery on a Dhaka balcony in May can be sitting at 40 °C. Expect three to five years of useful service with care, and less if the bank is regularly left flat.
LiFePO4 batteries carry a battery management system that stops over-discharge and over-charge, need no water, and usually last eight to ten years, but they still dislike heat and must be paired with an inverter that has a lithium charging profile. Whatever the chemistry, time the bank on a known load once a quarter; a battery that now gives 60% of its original runtime is telling you to budget for a replacement.
Common sizing mistakes
The usual errors are: using the fridge’s running watts and forgetting the starting surge; dividing watt-hours by the full rated Ah as if 100% depth of discharge were free; ignoring inverter efficiency, which quietly adds 15–20% to every figure; and adding a new battery in parallel with a three-year-old one, which drags the new battery down to the old one’s condition within months.
Also be wary of runtime claims on the box. “8 hours backup” usually means 8 hours at 100 W, one fan and two bulbs. Run your own numbers: battery Ah × voltage × depth of discharge × efficiency ÷ load in watts. A 200 Ah, 12 V tubular battery at 50% and 85% gives 200 × 12 × 0.5 × 0.85 = 1,020 Wh, which is 2.6 hours at 400 W, not 8. Match cable thickness to current, keep the inverter where air can move around it, and write the install date on the battery.
Sources
The figures and rules in this guide are checked against:
- Battery Maintenance (flooded lead-acid charging, watering, and depth of discharge) — Trojan Battery Company
- BU-808: How to Prolong Lithium-based Batteries — Battery University
- Batteries (lead-acid and lithium battery datasheets) — Victron Energy
- Solar Integration: Solar Energy and Storage Basics — U.S. Department of Energy