UPS Battery Backup Time Calculator Guide for Home India 2026

Backup hours roughly equal battery volts times amp-hours times about 0.6 divided by your load in watts, so a 1500VA inverter with two 150Ah batteries runs a 500 watt load for around 4 hours. The 0.6 factor combines the usable depth of discharge and inverter losses, and it keeps your estimate honest.

This guide is researched from standard electrical arithmetic and typical Indian lead-acid battery behaviour, not from measuring your home. Before calculating, collect the real wattage of each appliance from its label, because listed ratings are often higher than what a device draws in normal use.

Prices below are approximate and can change during sales, so check the live price on Amazon before you buy.

Quick Pick: Use the formula hours = (battery Ah x battery voltage x 0.6) divided by load watts, then buy a 150Ah tubular battery pair at roughly ₹28,000-36,000 if the answer is below the cut length you face.

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Step 1: convert VA to watts

VA is apparent power; watts are the real power your appliances use. The conversion depends on power factor. Inverters are commonly around 0.8, so a 1500VA inverter delivers roughly 1,200 watts at best. UPS units for computers are often around 0.6 to 0.7, so a 1 kVA UPS gives roughly 600-700 watts.

Never load a UPS or inverter to its limit. Plan to keep the load at 70-80 percent of the watt rating so that the unit stays cool and efficient.

Step 2: add up your load

Rough figures: a ceiling fan at 70-80 watts, an LED bulb at 9-12 watts, a TV at 80-150 watts, a Wi-Fi router at 10-20 watts, a laptop at 50-70 watts, a desktop at 150-300 watts, and a refrigerator at 100-200 watts when the compressor is on. Do not include any AC or geyser in a home inverter plan.

Step 3: calculate battery energy

Multiply volts by amp-hours. A 12V 150Ah battery stores 1,800 Wh. Two in series for 24V give 24 x 150 = 3,600 Wh nominal. Multiplying by 0.6 gives about 2,160 Wh usable and delivered. That 0.6 comes from using only around 70 percent of a lead-acid battery to protect its life and losing around 15 percent in the inverter.

For a lithium battery, usable depth is around 90 percent, so use roughly 0.75 instead.

Step 4: divide by load

Backup hours = delivered Wh divided by load watts. With 2,160 Wh and a 500 watt load, you get 4.3 hours. At 300 watts it is 7.2 hours, and at 800 watts it is 2.7 hours. Notice that doubling the load more than halves the hours, since high current also reduces effective battery capacity.

Worked example one: a small flat

Load: two fans (150 W), four LED lights (40 W), a router (15 W) and a TV (100 W) equals roughly 305 watts. A 1100VA inverter on 12V with a single 150Ah battery holds 1,800 Wh, giving about 1,080 Wh usable. That is only about 3.5 hours. For more, step up to a 1500VA inverter with two batteries, such as a 1500VA inverter with two batteries, which gives about 7 hours at the same load.

Worked example two: a work-from-home desk

Load: a desktop PC and monitor at 250 watts and a router at 15 watts totals about 265 watts. A 1 kVA online UPS with a 12V 9Ah internal battery stores only 108 Wh, giving around 20-25 minutes. With an external 12V 100Ah battery, you get 1,200 x 0.6 divided by 265, about 2.7 hours. Browse UPS with external battery listings if you need that.

Worked example three: a CCTV and network system

Load: an NVR, a PoE switch and a router total around 110 watts. A 12V 100Ah battery gives around 1,200 x 0.6 divided by 110, about 6.5 hours. See our UPS for CCTV and NVR guide for design details.

Why real life gives less

Battery age cuts capacity. A three-year-old tubular battery may deliver only 70 to 80 percent of its rated Ah. Temperature matters too: cold reduces capacity, and heat shortens life. Peukert's effect means that high loads draw out less total energy than low loads from the same battery. If you run at near the inverter's limit, expect noticeably shorter runtimes than the formula suggests.

Also, battery Ah ratings are usually quoted at a 20-hour discharge rate. Discharging in four hours gives lower effective capacity.

Sizing for your cut length

Work backwards. If cuts last 4 hours and your load is 500 watts, you need 2,000 Wh delivered, which means 2,000 divided by 0.6, around 3,300 Wh of battery. That is roughly two 150Ah 12V batteries. For 6-hour cuts at the same load, you need about 5,000 Wh, which means two 200Ah batteries or more. A compare of 200Ah battery for inverter listings shows the cost step.

Recommendation

Add your load honestly, apply the 0.6 factor, and leave a 20 percent margin for battery ageing. A 1500VA inverter with two 150Ah batteries (roughly ₹30,000-45,000 for the set) covers about 4 hours at 500 watts. If the number comes up short, reduce the load first, since trimming a 200-watt appliance is cheaper than buying another battery.

If your total lands under roughly 600 watts, a smaller 1100VA inverter with battery can be enough and costs less than the 1500VA class.