Inverter & Battery Backup Calculator
From the appliances you run in a power cut to the inverter, batteries and generator.
Try before you buy.
- Free preview: the inverter rating or the backup time in full, with the first rows of the battery table (up to 3); every other figure is hidden.
- Locked until you unlock it: download and copy.
- Unlock: Pro pass, ₹179 for 30 days or ₹79 for 1 week, a one-time payment unless you choose automatic renewal. Unlock: Pro pass, $5.99 for 30 days or $2.99 for 1 week, a one-time payment unless you choose automatic renewal.
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Appliances in a power cut
Watts from each rating plate. Hours: how long it runs during the cut (a fridge’s compressor runs part of the time).
Inverter
Batteries
Generator for the same loads
| System | Current at full load | Bank needed | Batteries | Wiring |
|---|
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Results are estimates from the formulas shown, not a professional design or certification. Have a qualified engineer verify anything safety-critical (structures, electrical installations, gas or pressure systems).
About the Inverter & Battery Backup Calculator
List the appliances that must keep running in a power cut — lights, fans, the router, the fridge, a pump — with their watts and how long each runs. The calculator gives the inverter or UPS rating in VA (running watts ÷ power factor, plus the headroom you choose), the starting peak when a motor kicks in, the battery bank in Ah at 12, 24 or 48 V and how many batteries to wire in series and parallel, and — for comparison — the generator kVA that would cover the same load.
It also works the other way: give it the batteries you have and a load, and it gives the backup time. Lead-acid batteries deliver less at high currents; tick Peukert’s law to allow for it. Efficiency, power factor and depth of discharge are figures from your equipment’s datasheets — the defaults are only starting values. An estimate for choosing equipment, not an electrical design.
Without a pass, the free preview shows the inverter rating or the backup time in full (the rating in VA, or the hours the batteries you have will last), with the first rows of the battery table; the complete result, Copy result and the CSV come with a Pro pass.
How to use it
- Add each appliance from the list or as your own, with its watts from the rating plate, how many, and the hours it runs during the power cut. Mark motors (fridges, pumps, mixers, air conditioners) and their starting surge.
- Set the backup time you want, the inverter’s power factor and efficiency, and the headroom to leave.
- Choose the battery type, its voltage and Ah rating, the depth of discharge from its datasheet, and the system voltage — or let the table compare 12, 24 and 48 V.
- Read the inverter rating, the battery bank and the generator size. Switch to Backup time to check batteries you already have.
- Copy result or Download CSV to take the figures to your dealer or electrician — with a pass, or after unlocking this result.
Examples
9 × 4, 75 × 2, 80 (3 h), 10 and 150 W; power factor 0.8, 20% headroom
Running 426 W → 426 ÷ 0.8 = 533 VA, with headroom 639 VA. Energy 1,624 Wh. The fridge starting at 3× takes the peak to 726 W (908 VA).
85% inverter efficiency, lead-acid at 50% depth of discharge, 12 V 150 Ah batteries
1,624 ÷ 0.85 = 1,911 Wh from the battery ÷ 12 V ÷ 0.5 = 318 Ah → three batteries in parallel at 12 V, or 2 × 2 = four at 24 V (159 Ah). With Peukert’s law (k 1.2, C10) it needs 330 Ah — still three.
85% efficient inverter, 80% depth of discharge
1,800 Wh × 0.8 ÷ (300 ÷ 0.85) = 4.08 h. Rated at C20 with Peukert k 1.25 it is about 2.9 h: at 29 A the battery gives less than its 150 Ah.
Running 426 W ÷ 0.8 = 0.53 kVA, × 1.25 headroom = 0.67 kVA; the fridge starting needs 726 W ÷ 0.8 = 0.91 kVA, so at least about 0.9 kVA.
Common uses
- Choose a home inverter and how many batteries to buy for the hours of backup you need.
- Check how long the batteries you have will last with the fridge on.
- Compare a 12 V and a 24 V system before buying.
- Size a standby generator for the same essential loads.
The formulas
- Running load: P = Σ watts × quantity (everything on at once).
- Inverter rating: VA = P ÷ power factor × (1 + headroom). A rating plate that says 900 VA / 720 W has a power factor of 0.8.
- Starting peak: P + (starting multiple − 1) × the largest motor’s watts — compare it with the inverter’s surge rating in its datasheet.
- Energy: E = Σ watts × quantity × hours run; the battery supplies E ÷ inverter efficiency.
- Battery bank: Ah = E ÷ efficiency ÷ system voltage ÷ depth of discharge; batteries in series = system voltage ÷ battery voltage, strings in parallel = Ah needed ÷ battery Ah, rounded up.
- Backup time: t = V × Ah × depth of discharge ÷ (load ÷ efficiency).
- Peukert’s law (lead-acid): a battery rated C Ah at the H-hour rate lasts t = H × (C ÷ (I × H))^k at a current I. Below the rated current (C ÷ H) the rated capacity is used, which keeps the estimate on the safe side.
- Generator: kVA = the larger of P ÷ PF × (1 + headroom) and the starting peak ÷ PF.
Choosing the figures
Power factor and efficiency are on the inverter’s datasheet; the calculator starts at 0.8 and 85%. Depth of discharge is how much of the battery you use: lead-acid batteries last longer when they are not run flat, so 50% is a cautious start; lithium iron phosphate (LiFePO4) batteries are often rated for deeper cycles — the calculator starts at 80%. Use your battery’s datasheet. Battery ratings are given at a discharge time — C10 means the Ah are measured over 10 hours, C20 over 20 — and lead-acid batteries deliver less when drawn faster; Peukert’s exponent k from the datasheet allows for it. A higher system voltage halves the current: a 24 V bank carries half the amps of a 12 V one for the same power, so thinner cables and smaller losses.
Motors and the starting surge
Fridges, freezers, pumps, mixers, washing machines and air conditioners draw several times their running power for a moment as their motor starts. An inverter or generator that covers the running watts can still trip on that peak. The calculator assumes the largest motor starts while everything else runs; give each motor’s starting multiple from its datasheet (the list starts at 3× for compressors and pumps). Inverter-type air conditioners and BLDC fans have electronic drives that usually start more gently.
Limitations
- An estimate for choosing equipment — not an electrical design. Cables, fuses, earthing and the charging system need an electrician and the equipment’s instructions.
- Appliance wattages in the list are examples; the rating plate of your appliance is what counts. Fridges and pumps cycle on and off, so their average draw is lower than their rating.
- Batteries lose capacity with age and in the cold; a margin on the battery bank allows for it.
- Charging the bank again after a cut needs enough charger current and time, which this calculator does not size.
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Frequently asked questions
What do I get without a pass?
Without a pass, Inverter & Battery Backup Calculator shows the inverter rating or the backup time in full, with the first rows of the battery table (up to 3); every other figure is hidden. Until you unlock it, the result can’t be downloaded or copied. A Pro, Premium or Ultimate pass, a one-time payment unless you choose automatic renewal, unlocks the full result. The pricing page lists the passes and their prices.
What size inverter do I need?
Add the watts of everything that runs together, divide by the inverter’s power factor and add headroom. 426 W of lights, fans, TV, router and fridge need 426 ÷ 0.8 = 533 VA, or about 640 VA with 20% headroom — and its surge rating must cover the fridge starting.
How many batteries do I need for my inverter?
Energy for the backup ÷ inverter efficiency ÷ battery voltage ÷ depth of discharge gives the Ah. For 1,624 Wh at 85% and 50% depth of discharge that is 318 Ah at 12 V — three 150 Ah batteries in parallel, or four (two strings of two) for a 24 V inverter.
How long will a 150 Ah battery last?
A 12 V 150 Ah battery holds 1,800 Wh. At 80% depth of discharge and 85% inverter efficiency it runs 300 W for about 4.1 hours — less, about 2.9 hours, once Peukert’s law is allowed for at that current with a C20 rating and k = 1.25.
What is the difference between VA and watts?
Watts are real power; VA is voltage × current. With motors and electronics the two differ by the power factor: watts = VA × PF. Inverters and generators are rated in VA (or kVA), so divide your watts by the power factor to compare.
Should I choose 12 V, 24 V or 48 V?
The same power at twice the voltage needs half the current, so larger systems use higher voltages to keep cables and fuses manageable. The calculator’s table shows the current, the Ah and the batteries needed at each voltage; your inverter decides which it accepts.
What size generator runs my house in a power cut?
At least the larger of the running load ÷ power factor plus headroom and the starting peak ÷ power factor. For the 426 W example with a fridge starting at 3×, that is about 0.9 kVA; add the other loads you want to run.