Battery Backup Calculator: How Long Your Battery Will Run Your Load

TL;DR: A battery backup calculator divides the usable energy stored in your battery, in watt-hours, by the load you run, in watts. The full formula is battery Ah × battery voltage × usable depth of discharge × inverter efficiency ÷ load in watts. On conservative assumptions (50% usable for lead-acid, 90% inverter efficiency), a 12V 150Ah tubular battery supplies about 810 usable watt-hours, roughly 3.7 hours for two ceiling fans, five LED lights and a Wi-Fi router. A 12V 100Ah lithium battery delivers about 1,080Wh at the socket in EcoFlow’s 2025 worked example^[1], close to double a lead-acid battery of the same rating. Treat every calculator answer as a ceiling, and check your battery’s rated depth of discharge before you rely on it.

How do you calculate inverter battery backup time?

Inverter battery backup time is the number of hours a fully charged battery can carry your connected appliances before the inverter shuts off. The simplest version of the sum comes from SOROTEC, a Chinese inverter maker, writing in 2025^[2]: backup hours equal Ah × battery voltage × inverter efficiency ÷ load in watts. That version assumes you can drain every amp-hour on the label, so it returns a best-case figure. A working calculator adds the usable share of the battery.

Depth of discharge (DoD)

the share of a battery’s rated capacity that is drawn out before it is recharged, which battery makers limit to protect battery life.

Usable energy is the watt-hours left once you apply that limit. Everything below runs on usable energy, never raw Ah, because an Ah rating means nothing until you know the battery voltage.

1. Add up the load in watts. List every appliance that will run during a power cut and add the wattage printed on each label. Where a label is missing, one Indian home load calculator^[3] uses defaults of roughly 80W per ceiling fan, 5W per LED bulb, 18W per LED tubelight, 65W for an LED TV and 335W for a fridge of up to 500L. Those are presets, not measured consumption, so your own labels come first.

2. Convert the battery to watt-hours. Multiply the Ah rating by the nominal voltage: a 12V 150Ah battery holds 1,800Wh, and two of them in series on a 24V inverter hold 3,600Wh. Lithium iron phosphate (LiFePO4) packs usually carry a 12.8V nominal rating, so a 100Ah lithium pack holds 1,280Wh.

3. Apply the usable share. Multiply the watt-hours by the depth of discharge your battery maker specifies. The next section explains why this input moves the answer most.

4. Apply inverter efficiency. Inverter efficiency is the share of battery energy that reaches your sockets as AC power. Power Queen’s 2026 runtime guide^[4] puts most pure sine wave inverters at about 90% to 95%, and budget units run lower. Truepower rates its Lithvon lithium inverters at above 85%.

5. Divide by the load. Usable AC watt-hours divided by load watts gives hours of backup. Read the result as an upper limit.

Tubular and lithium batteries give different usable energy from the same Ah rating

The usable share is where published calculators disagree most. Anker SOLIX, in a 2026 guide^[5], says LiFePO4 batteries often provide 80% to 100% usable capacity, while lead-acid batteries are usually limited to about 50%. Applied to a 12V 100Ah rating, Power Queen’s comparison gives about 1,080Wh usable from LiFePO4 against about 600Wh from lead-acid, which is why lithium can often run the same appliance nearly twice as long.

Indian tubular batteries are the thin spot in the evidence. No source in this research gives a tested depth of discharge for Indian tubular inverter batteries, and the 50% rule comes from US guidance written for deep-cycle lead-acid. A 2024 Hindi-language explainer by an Indian YouTube creator^[6] assumes instead that 80% of a 150Ah inverter battery is usable. The two figures differ because they come from different settings, conservative American deep-cycle practice against Indian installer habit, and neither publishes test data behind it.

This guide uses 50% as its tubular default and shows 80% alongside. For lithium it uses 90%, the top of the 80% to 90% range that SolarMathLab^[7] recommends; some sellers calculate at 100%, which makes their runtimes look longer. Your own battery’s datasheet overrides all three.

Three worked examples: a fan-and-lights home, a home with a fridge and a shop

Load in watts moves backup time further than battery chemistry does. Each example below uses calculator default wattages and a 90% inverter efficiency.

• Fan-and-lights home (218W). Two ceiling fans (160W), four LED bulbs (20W), one LED tubelight (18W) and a Wi-Fi router (20W) run together overnight. It is the lightest of the three loads, and the one a single battery handles best.

• Home with a fridge (383W average). The same fan-and-lights load runs alongside an LED TV (65W) and a fridge rated at 335W. A fridge compressor does not run continuously: one solar creator who metered their own fridge found it ran about 30% of the time^[8], so this example counts the fridge at roughly 100W on average. In summer heat, use the full 335W and expect far less backup.

• Small shop (337W). Four LED tubelights (72W), two ceiling fans (160W), a billing laptop (45W), a router (20W) and one phone charger (40W) make up the load. A shop needs this backup during trading hours.

Every runtime figure in this table is arithmetic from stated assumptions, not a laboratory measurement. Across the two 12V 150Ah rows, the depth-of-discharge assumption alone moves fan-and-lights backup from 3.7 to 5.9 hours. The 540Wh in the first row matches EcoFlow’s own working for a 12V 100Ah lead-acid battery.

How long will a 150Ah battery run my home?

A 12V 150Ah battery runs a 300W home load for somewhere between about 2.7 and 5.4 hours, depending on how much of the battery you count as usable. The top of that range is SOROTEC’s worked example^[9]: 150 × 12 × 0.9 ÷ 300 = 5.4 hours, under ideal conditions with no discharge limit. Counting 80% usable, the Indian explainer reaches about 3.84 hours, which it rounds to 4. Apply the 50% lead-acid default and the same sum gives 150 × 12 × 0.5 × 0.9 ÷ 300 = 2.7 hours.

That spread is the honest answer. A seller quoting 5.4 hours has done the arithmetic correctly and left out the discharge limit, so ask which usable share any quoted figure assumes.

Heavy loads shrink the figure further. On paper a 12V 150Ah battery holds 1,800Wh, enough for 1.8 hours at 1kW, but one contributor answering on Quora^[10] notes that at the roughly 80A such a load draws from 12V, the inverter’s low-battery cut-out would likely operate well within an hour. Lead-acid batteries deliver less than their rated Ah when drained quickly, so doubling the load on a tubular battery more than halves its backup.

A 12V 100Ah lithium battery supplies about 1,080Wh at the socket

A 12V 100Ah lithium battery supplies roughly 1,000 to 1,150 usable watt-hours once inverter losses are counted, enough to run a steady 100W load for about 10 to 11.5 hours. EcoFlow’s 2025 example reaches 1,080Wh by assuming full depth of discharge and a 90%-efficient inverter, while Power Queen reaches the same 1,080Wh with 90% depth of discharge and no inverter loss at all.

Anker SOLIX’s 2026 runtime table^[11] shows how fast those hours fall as load rises. It assumes a fully usable 1,280Wh battery feeding a 90%-efficient inverter, and Anker SOLIX calls the figures estimates, not guarantees.

Treat these as upper bounds, since a battery used to 90% rather than 100% gives about a tenth less. Anker SOLIX also says a single 100Ah lithium battery is usually not enough for whole-home backup or air conditioning; it suits a fan-and-lights load.

Truepower’s Lithvon Home ESS pairs a pure sine wave inverter with an inbuilt 12V 100Ah LiFePO4 battery rated at 1,280Wh, so the same arithmetic applies to it.

Why does real backup fall short of the calculator?

Every formula above assumes a steady load, a cool battery, new cells and a well-matched inverter. Power Queen’s own calculator states that actual runtime varies with inverter efficiency, start-up surge, ambient temperature, battery age and wiring loss. Evidence on the size of each effect is thin: every figure below comes from a seller or an individual, not an independent test.

• Start-up surges. Compressor appliances draw well above their rating as they start, and Tech Battery Solutions^[12] gives the example of a “600W” fridge pulling 1,000W at compressor start-up. Surges decide whether the inverter’s VA rating can carry the load, while watt-hours decide how long it runs. A higher-VA inverter does not add a minute of backup time; only more usable watt-hours do, as Truepower’s guide to inverter capacity explains.

• Heat. Eneronix^[13], writing about Nigerian cities and citing no underlying study, says lead-acid batteries lose 10% to 20% of rated capacity where temperatures regularly exceed 35°C. Many Indian summers run as hot, so treat that range as indicative and expect summer backup to fall short.

• Inverter standby draw. An inverter uses power simply to stay switched on, and Eneronix puts this at 30W to 80W for larger units, or 240Wh to 640Wh over an eight-hour night. Truepower specifies a no-load current below 2.5A on the Lithvon Home ESS, which is under 30W at 12V.

• Combined losses. SolarMathLab estimates that inverter, wiring and temperature losses typically cut runtime 5% to 15% below the ideal figure. A 2025 worked example from Cleversolarpower^[14] also counts the battery’s own efficiency and cuts a 60W TV estimate on 100Ah lead-acid from 10 hours to about 7.6, nearer a quarter. The two differ because the second includes energy lost inside the battery.

Size the battery from the hours you need

Sizing runs the formula backwards: required battery watt-hours equal load × hours ÷ inverter efficiency ÷ usable share. Divide that by the system voltage to get the Ah rating you need.

Take the small shop above, which needs 337W for 3 hours. That is 1,011Wh at the sockets, or about 1,123Wh drawn from the battery after a 90% inverter. A tubular bank counted at 50% usable needs about 2,247Wh of stored energy, roughly 187Ah at 12V or two batteries of about 94Ah each in series at 24V. Counted at 90%, a lithium battery needs about 1,248Wh, close to the 1,280Wh a 12.8V 100Ah pack stores.

Round up to the next battery size, never down, because heat and ageing take their share. Size the inverter separately from the battery: SOROTEC suggests choosing an inverter rated 20% to 30% above your total connected load to leave room for surges and added appliances.

Frequently asked questions