Inverter Battery Backup Time Calculator (100/150/200Ah)
Enter your load and battery size and get your real backup time in seconds — plus the simple formula, worked examples for 100Ah, 150Ah and 200Ah, and the honest reasons your backup runs shorter than the maths says.
Short answer first: with a single 12V battery, your backup time = (battery Ah x 12 x inverter efficiency x usable fraction) / load in watts. For a typical lead-acid setup that works out to roughly 6 watt-hours of real, usable energy per Ah — so a 150Ah battery running a 300W load gives you about 3 hours, and a 200Ah battery on the same load gives about 4. Use the calculator below for your exact load and battery, then read on for the formula, worked examples for 100Ah, 150Ah and 200Ah, and why real backup runs 10-25% below the textbook figure.
| Battery (12V) | At 200W | At 400W | At 600W |
|---|---|---|---|
| 100Ah tubular | ~3.0 hrs | ~1.5 hrs | ~1.0 hr |
| 150Ah tubular | ~4.5 hrs | ~2.3 hrs | ~1.5 hrs |
| 200Ah tubular | ~6.0 hrs | ~3.0 hrs | ~2.0 hrs |
Approximate backup hours by battery size and load (12V, ~85% inverter efficiency, ~60% usable for lead-acid)
To put those wattages in context: a typical Delhi home running-cut load of two ceiling fans, four to six LED lights, a TV and a router sits around 200-300W, which is why a 150Ah battery is the default choice for a 2-3 BHK flat. Push the load to 400-600W — add a water pump, a couple more fans, or a small fridge — and you can watch the hours collapse. Backup is a tug-of-war between how much energy is in the tank (Ah) and how fast you pour it out (watts), and the table above is just that ratio made concrete.
Calculate your own backup time
Enter your load in watts and your battery's Ah and chemistry below. The calculator already bakes in realistic inverter and depth-of-discharge losses, so the result is much closer to your actual clock time than a back-of-envelope guess.
Backup Time Calculator
InteractiveEstimate how long your battery will keep your load running during a power cut.
Estimated backup
3 h 4 min
At this steady load.
Usable energy
918 Wh
1 × 12V × 150Ah × 60% × 85%
If load doubles
1 h 32 min
Backup roughly halves as load rises.
Assumes ~85% inverter efficiency and usable depth-of-discharge of 60% (lead-acid tubular/flat). Real backup depends on battery age, temperature and the exact load running at the time — a tired 3-year-old battery can give 30-40% less. For an exact figure for your home, talk to our team.
The backup time formula, explained
The full formula reads: Backup time (hours) = (Battery Ah x Battery voltage x Inverter efficiency x Usable fraction) / Load in watts. Every term is a real-world correction to one simple idea — energy out divided by power drawn. Each term below explains exactly what it corrects for and why it matters.
- Battery Ah x Voltage = energy in the tank. Amp-hours alone are not energy; you have to multiply by voltage to get watt-hours (Wh). A single inverter battery is 12V, so a 150Ah battery holds 150 x 12 = 1,800Wh on paper. (Two batteries in series make a 24V system, four make 48V — but the maths is identical once you use the right voltage.)
- Inverter efficiency (~85%) = the conversion tax. Your appliances run on 230V AC, but the battery stores 12V DC. The inverter has to convert DC to AC, and that conversion is never free — a decent home inverter is about 80-90% efficient, so we use 0.85 as a sensible middle figure. The other ~15% is lost as heat in the electronics.
- Usable fraction / Depth of Discharge (~60% for lead-acid) = how much you can safely take out. You cannot drain a lead-acid battery to zero without wrecking its life. To get a reasonable number of cycles, tubular and flat-plate batteries should only be drawn down to about 50-60% depth of discharge in everyday use, so only ~0.6 of the rated capacity is genuinely usable. Lithium (LFP) batteries flip this — they comfortably allow 80-90% usable, which is a big part of why they punch above their Ah rating.
- Load in watts = how fast you are emptying it. This is your actual appliance draw, not the inverter's VA rating. If you only know VA, multiply by the power factor (commonly around 0.7-0.8 for a mixed home load) to get watts.
Combine the three constants for a standard lead-acid setup and you get a handy shortcut: 12 x 0.85 x 0.60 = about 6.1 watt-hours of usable energy delivered per rated Ah. A quick field estimate is therefore: Backup hours = (Ah x 6) / load watts. Roughly 6 Wh per Ah for lead-acid — commit that number to memory.
Worked examples: 100Ah, 150Ah and 200Ah
Take a 150Ah, 12V tubular battery running a 350W load — two fans, lights, TV and a router, a very ordinary Delhi flat during an evening cut:
- Energy in tank: 150 Ah x 12 V = 1,800 Wh.
- Apply inverter efficiency: 1,800 x 0.85 = 1,530 Wh that actually reaches your sockets.
- Apply usable fraction: 1,530 x 0.60 = 918 Wh you can safely use without harming the battery.
- Divide by load: 918 Wh / 350 W = about 2.6 hours of real backup.
The headline 1,800Wh shrank to 918Wh of genuinely usable energy before we even divided by the load — that is the gap between the brochure and reality, entirely down to efficiency and depth of discharge, not a faulty battery. The same battery on a lighter 200W load stretches to 918 / 200 = about 4.6 hours; push it to 600W and it drops to 918 / 600 = about 1.5 hours. Same battery, three very different answers — backup is always load-dependent.
| Battery (12V, lead-acid) | Usable energy (approx) | Backup at 300W | Backup at 500W |
|---|---|---|---|
| 100Ah | ~612 Wh | ~2.0 hrs | ~1.2 hrs |
| 150Ah | ~918 Wh | ~3.0 hrs | ~1.8 hrs |
| 200Ah | ~1,224 Wh | ~4.1 hrs | ~2.4 hrs |
How the 6 Wh-per-Ah shortcut plays out across common battery sizes
Why your real backup is 10-25% less than the calculator says
Every formula above assumes a brand-new battery, a comfortable temperature, and a gentle discharge. Real life rarely offers all three at once. Budget for 10-25% less than the clean calculation — and more once the battery is a few years old.
| Factor | Effect on backup |
|---|---|
| Fast discharge (C-rating) | A 150Ah battery is usually rated at C20 — a gentle 20-hour drain. Pull a heavy load and you discharge in 2-3 hours instead, and Peukert's effect means you get noticeably fewer usable Ah. Heavier load = less efficient = double penalty. |
| Ageing | Capacity fades with every cycle. A 3-4 year old tubular battery often holds only roughly 70-85% of its original Ah, so the same load that once gave 3 hours now gives barely 2. |
| Heat | Counter-intuitively, a hot Delhi evening does not cut tonight's runtime — warm acid reacts more readily and can even give a few extra minutes. The danger is to lifespan, not to a single evening: sustained heat sharply accelerates plate corrosion and water loss, so a battery cooked in a closed balcony cupboard ages out years sooner and loses capacity permanently. Keep it ventilated and topped up through summer. |
| Surge / startup loads | Motors (fridge, pump, cooler) draw 2-3x their running watts for a second or two at startup. That spike does not change the average much, but it stresses the inverter and can trip an undersized one. |
| Wiring & connection losses | Thin or long DC cables and loose, corroded or sulphated terminals drop voltage and waste energy as heat before it ever reaches the inverter. |
| Low charge / undercharging | If the last grid spell was short, the battery may not have reached 100% before the next cut. You cannot pour out energy that was never put in. |
What changes your real backup
The single biggest surprise for most people is the C-rating point. The Ah on the label was measured during a slow 20-hour test, but a real power cut drains the battery far faster — and a battery gives up fewer usable amp-hours when you ask for them quickly. Two batteries both marked 150Ah can deliver different real backup depending on whether that 150 was measured at C20 or C10; check before comparing quotes on price alone.
How to get more backup time
If the number you got is too low, you have four honest levers — and the cheapest one is usually the last thing people try.
- Reduce the load. The formula divides by watts, so shaving the load is the fastest free win. Move heavy items (fridge, AC, pump, geyser) off the inverter, switch the lighting to LED, and run only what you truly need during a cut. Cutting load from 400W to 250W lengthens backup by over 50% with no spend at all.
- Add Ah (bigger or more batteries). A 200Ah battery gives almost exactly double the backup of a 100Ah at the same load — capacity scales nearly linearly. Two batteries in series (a 24V system) double the energy but require an inverter that supports that voltage, so check compatibility before buying.
- Switch to lithium (LFP). Lithium allows ~80-90% depth of discharge versus ~60% for lead-acid, so a lithium pack delivers far more usable energy per rated Ah, lasts many more cycles, and handles heat and fast discharge better. It costs more upfront but often wins on cost-per-cycle over its life.
- Right-size from the start. Decide the hours you need first, then size the bank to the load — not the other way round. Our full sizing calculator works backwards from your desired backup to the Ah and number of batteries you need, which is the correct order to do it in.
A 1-ton AC draws roughly 1,000-1,200W continuously, which would flatten even a 200Ah battery in well under an hour while ruining its cycle life. Fans, lights, TV, fridge and Wi-Fi are what an inverter battery is built for; whole-home AC backup needs a much larger bank or solar. For long backup on a heavy load, the answer is almost never a bigger single battery — size the system correctly: essential watts first, then target hours, then the bank.
Where to next
Frequently Asked Questions
How long will a 150Ah battery last?
On a typical 200-400W home load (a few fans, lights, TV and router), a healthy 150Ah, 12V tubular battery gives roughly 2.5 to 4.5 hours. The formula is Backup = (150 x 12 x 0.85 x 0.60) / load watts, which works out to about 918Wh of usable energy — so at 200W that is about 4.5 hours, at 300W about 3 hours, and at 400W about 2.3 hours. A heavier load, an older battery or a very hot day will pull these figures down further.
What is the formula for inverter battery backup time?
Backup time (hours) = (Battery Ah x Battery voltage x Inverter efficiency x Usable fraction) / Load in watts. For a standard 12V lead-acid setup use efficiency around 0.85 and a usable fraction around 0.60, which simplifies to a handy shortcut: Backup hours = (Ah x 6) / load watts. So a 150Ah battery on a 300W load is roughly (150 x 6) / 300 = 3 hours.
How long will a 150Ah battery run a 1 ton AC?
Honestly, well under an hour — and you should not do it routinely. A 1-ton AC draws around 1,000-1,200W continuously. A 150Ah battery holds only about 900Wh of usable energy, so the maths gives barely 45-55 minutes, and discharging that fast and that deep wrecks the battery's life. Running AC on backup needs a much larger battery bank or a solar system; see our guide on running an AC on inverter or solar for what it actually takes.
Does a 200Ah battery give double the backup of a 100Ah?
Roughly yes, at the same load — capacity scales almost linearly, so 200Ah delivers close to twice the hours of 100Ah. The small catch is the discharge-rate (C-rating) effect: under a heavy load the larger battery discharges at a gentler relative rate and can do a touch better than exactly double, while at very light loads the two track almost perfectly. For planning purposes, treating 200Ah as double 100Ah is a safe estimate.
Why is my actual backup less than the calculator says?
Because the clean formula assumes a brand-new battery, a mild temperature and a gentle discharge — and real life offers none of those at once. Ageing (a 3-4 year old battery often holds only 70-85% of its rated Ah), fast discharge under heavy load (the C-rating/Peukert effect), startup surge from motors, and voltage drop in thin or corroded wiring all chip away at the result. Heat is the odd one out: it does not cut tonight's runtime, but it shortens the battery's overall life. Budget for 10-25% less than the textbook number, and more once the battery is a few years old.
How do I convert my battery Ah into watt-hours?
Multiply Ah by the battery voltage. A 12V, 150Ah battery holds 150 x 12 = 1,800 watt-hours (Wh) on paper. But that is the gross figure — after inverter conversion losses (~15%) and the safe usable limit for lead-acid (~60% depth of discharge), only about 900Wh is genuinely usable. Lithium batteries allow 80-90% usable, so they deliver far more real energy from the same nameplate Wh.
How many hours of backup do I need for my home?
Match it to your typical cut length plus a margin. In most of Delhi NCR, planned and unplanned cuts run from a few minutes to 2-3 hours, so 3-4 hours of backup on your essential load covers the vast majority of situations comfortably. The right way to size is to add up your essential watts first, decide the hours you want, then work backwards to the Ah using our sizing calculator — rather than buying a battery and hoping it lasts.
Will a bigger inverter give me more backup time?
No. Backup time depends on the battery's energy (Ah x voltage) and your load, not the inverter's VA rating. A bigger inverter only lets you run more appliances at once (higher peak watts) and handle bigger startup surges — but for a given load and battery, it does not add a single minute. To extend backup you need more battery capacity, a lower load, or a higher-efficiency chemistry like lithium.
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