IPS / UPS backup battery calculator guide
What does this calculator help you figure out?
When the grid drops for two hours at a time in Dhaka, Karachi, Lagos, or Johannesburg, the question is not whether to buy an IPS or home UPS but how many amp-hours of battery it needs. This calculator takes the watts you want to keep running, the hours you want them to run, and the battery bank voltage, then works back to battery capacity. It divides the watt-hours by inverter efficiency, because the inverter wastes some energy turning 12 V DC into 220 V AC, and by the depth of discharge the battery chemistry tolerates: half the nameplate for tubular or flooded lead-acid, 80% for lithium iron phosphate. It also tells you how many batteries of the size sold near you to connect in parallel, the inverter VA rating to buy, and how long one battery alone would last. Charging time, battery cost, and the generator alternative are outside its scope.
How is the result calculated?
Energy to store (Wh) = load (W) × hours ÷ (inverter efficiency ÷ 100). Battery capacity (Ah) = Wh ÷ (bank voltage (V) × depth of discharge), with DoD 0.5 for lead-acid and 0.8 for lithium. Batteries = ceil(Ah ÷ Ah per battery). Inverter (VA) = load ÷ 0.8 × 1.25, rounded up to 50 VA.
Worked example
Worked example: two ceiling fans, a router, LED lights, and a television add up to 400 W, and the household wants 4 hours of backup on a 12 V bank of 200 Ah tubular batteries. Energy delivered is 400 × 4 = 1,600 Wh; at 85% inverter efficiency the batteries must supply 1,600 ÷ 0.85 = 1,882 Wh. Lead-acid should stop at 50% depth of discharge, so the bank needs 1,882 ÷ (12 × 0.5) = 314 Ah. Two 200 Ah batteries in parallel give 400 Ah, and a single 200 Ah battery would run the load for 200 × 12 × 0.5 × 0.85 ÷ 400 = 2.6 h. The inverter should be 400 ÷ 0.8 × 1.25 = 625 VA, rounded up to the next 50 VA size. The calculator displays 314 Ah, 2 batteries, and a 650 VA inverter.
Units and conversion notes
Load is in watts, taken from appliance labels or a plug-in meter; hours is how long a single outage lasts, not the daily total if the battery recharges between cuts. Bank voltage is 12 V for a single battery, 24 V for two in series, and 48 V for four; the result is amp-hours at that voltage, so a 24 V system needs half the Ah of a 12 V one for the same energy. Battery size is the nameplate Ah at the C10 or C20 rate printed on the label.
What does the result mean?
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on backup load, backup hours needed, battery bank voltage, battery type, inverter efficiency, battery size available, market / jurisdiction, effective date, data source, update owner and review date, so start there if the ips / ups backup battery calculator returns something you did not expect.
Common mistakes to avoid
Good to know: nameplate Ah is measured over 10 or 20 hours. Drawing 400 W from a 12 V battery is about 39 A after inverter losses, which empties a 200 Ah tubular battery in nearer 4 hours than the 5 the arithmetic suggests, so treat the result as a minimum and round up. Do not size the bank on the inverter rating; a 1,000 VA inverter with a 100 Ah battery gives a long runtime for a fan and minutes for an iron. Refrigerators and water pumps need two to three times their running watts for a moment at start-up, which sets the inverter VA even though it barely affects Ah.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Common questions
Frequently asked questions
How many Ah of battery do I need for 4 hours of backup?
Multiply the watts by the hours, divide by about 0.85 for the inverter, then divide by battery voltage times usable depth of discharge. For 400 W and 4 hours on 12 V lead-acid that is 1,600 ÷ 0.85 ÷ (12 × 0.5) = 314 Ah, so two 200 Ah batteries. With a 200 Ah lithium battery at 80% usable, one battery is enough.
Why is my inverter rated in VA and not watts?
VA is apparent power, the volts times amps the inverter must actually push. Motors and some electronics draw current out of phase with the voltage, so the real watts are only about 80% of the VA. A 650 VA inverter is therefore good for roughly 520 W of mixed load, and the 25% headroom in this calculator covers fan and refrigerator start-up.
Is a lithium battery worth it for an IPS?
A LiFePO₄ battery lets you use about 80% of its capacity instead of 50%, weighs less than half as much, and typically lasts 2,000 to 4,000 cycles against 800 to 1,500 for a good tubular battery. Its purchase price is two to three times higher, so it pays off where outages are daily. It also needs an inverter with a lithium charging profile.
How long will a 150 Ah battery run a fan?
A 75 W ceiling fan on a 12 V, 150 Ah tubular battery has 150 × 12 × 0.5 = 900 Wh of usable energy. After 85% inverter efficiency that is 765 Wh, or about 10 hours for one fan. Two fans and a few LED lights at 200 W would bring it down to roughly 3.8 hours.
Should I use 12 V or 24 V for my backup system?
Above about 1,000 W of load, move to 24 V or 48 V. At 12 V a 1,000 W load pulls close to 100 A through the cables and terminals, which needs very thick wire and wastes energy as heat. A 24 V bank halves the current for the same energy, and the Ah figure this calculator gives also halves.
How long does it take to recharge an IPS battery?
Roughly the Ah replaced divided by the charger current, plus two to three hours of absorption for lead-acid. Refilling the 157 Ah used from two 200 Ah batteries in this example with a 15 A charger takes about 10 to 13 hours, so a battery discharged every evening may not be full by the next outage without a larger charger.
