UPS Backup Time Calculator — Battery Runtime
Load-shedding planning comes down to one question: how long will the battery carry the load? The honest answer involves two discounts vendors skip — you can only use about half a lead-acid battery’s capacity without killing it early, and the inverter loses another fifteen percent in conversion. This tool builds both in.
Why does my 150 Ah battery give half the backup the shopkeeper promised?
The promise multiplied Ah by volts and called it usable. Lead-acid batteries dying early when discharged past 50 percent, and inverter conversion losses, mean a "1,800 Wh" battery delivers roughly 750–800 Wh of real load support. This tool’s figure is the deliverable number, not the brochure one.
What load do typical backup essentials draw?
Two fans (160 W), four LED bulbs (40 W), router and a charging phone (25 W) — about 225 W for the standard Pakistani backup set. Add an LED TV and you’re near 300 W, which a single 150 Ah battery carries for roughly 2.5 hours.
Does a 24V two-battery setup double the backup of one 12V battery?
It doubles stored energy, so yes at the same load — and it halves current draw, which improves efficiency and battery life. Series 24V setups also unlock larger inverters; the wiring discipline (matched batteries, same age) is the catch.
UPS Backup Time Calculator
The real arithmetic behind backup time
Stored energy is Ah × V watt-hours; usable energy applies the 50 percent depth-of-discharge ceiling and ~85 percent inverter efficiency; runtime divides usable energy by load watts. Every disappointment with UPS systems traces to skipping one of the discounts or underestimating the load — the 100 W of "just a few things" that creeps to 350 W once the TV and the second room’s fan join. Measure the load honestly (the appliance audit from our unit-cost tool transfers directly) and the prediction holds.
Designing for your actual outage pattern
Size to your area’s real shedding schedule, not a worst case: two-hour urban outages need a single healthy battery for essentials; six-hour rural patterns justify 24V banks or the lithium upgrade; and anything beyond that is generator or solar-hybrid territory, where the battery becomes a bridge rather than the solution. The cheapest capacity you can buy is load reduction — swapping two old fans for BLDC models frees 80 W, which at typical backup loads is half an hour of runtime that cost Rs. 7,000 instead of a second battery’s Rs. 45,000.
The charging side nobody budgets
A UPS battery recharges from the grid after every outage, and that energy plus charger losses lands on the electricity bill — roughly 1.3 units to refill each unit drawn during backup. Daily-shedding households cycling a 150 Ah battery add 30–45 units monthly to the bill, worth knowing before blaming the fridge. It also sharpens the solar-hybrid case: panels recharging the same battery move that consumption off the bill entirely.
More questions answered
Capacity fade — a flooded lead-acid battery delivering rated Ah in year one commonly holds 60–70 percent by year three, faster if regularly deep-discharged or chronically under-watered. The runtime this tool predicts assumes healthy rated capacity; multiply by your battery’s age-adjusted fraction for realism.
Lithium (LiFePO4) tolerates 80–90 percent discharge against lead-acid’s 50, so a 100 Ah lithium delivers more usable energy than a 150 Ah lead-acid, lasts several times the cycles, and weighs a third as much. The purchase price is 2.5–3× — increasingly justified for daily-cycling load-shedding use.
Motors surge 3–6× their running watts at start, which trips small inverters and devours batteries. A fridge needs the inverter sized to its surge and adds 100–200 W of continuous load; pumps are usually better left to generator or grid. Decide deliberately — motor loads are why backup-time promises die.
Keep discharge above 50 percent, top distilled water monthly (flooded types), keep terminals clean, and avoid the killer pattern of leaving it flat overnight after an evening outage. Charging discipline matters more than brand — most "bad batteries" in Pakistan died of routine abuse.