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Inverter & Battery Calculator

Find the inverter VA and battery Ah for your home, or how long your battery will last.

Appliances to run on backup
  • W
    ×
  • W
    ×
  • W
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  • W
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Watts are typical running values: check the label or manual for yours. Total load: 331 W
I want to find
hours
How long power cuts usually last where you live.
Battery type
Home inverters up to about 1,100 VA mostly run on one 12 V battery; bigger ones on 24 V or more.
%
Lead-acid lasts more cycles when it isn't drained fully: 50–80% is the usual range.
%
Typically 80–90% for home inverters.
PF
VA = W ÷ power factor. 0.7–0.8 for most home inverters; 0.9–1 for some sine-wave models.
%
Spare capacity for starting currents and future appliances.
Inverter size
600 VA
331 W ÷ 0.8 = 414 VA, + 20% headroom = 497 VA
Battery for 3 h backup
150 Ah
Needs 139 Ah at 12 V: 331 W × 3 h ÷ (12 V × 70% × 85%)
Total load
331 W
Energy per hour
0.33 kWh
Battery energy
1,800 Wh
rated, before losses
Rule-of-thumb estimate. Lead-acid Ah ratings are measured at a slow 10–20 hour discharge, so heavy loads and ageing batteries give less backup.

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Results are for general guidance. Please check anything important before relying on it. Terms of use

How to Use

  1. List the appliances you want to run during a power cut. Edit the watts and quantity, remove what you don't need, and add more from the list.
  2. Choose whether you want the battery size for a backup time or the backup time of a battery you already have, and enter the hours or the battery's Ah.
  3. Pick the battery type (lead-acid or lithium) and the bank voltage (12 V for one battery, 24 V for two).
  4. Adjust the depth of discharge, inverter efficiency, power factor and headroom if you know better values. The defaults are typical for Indian home inverters.
  5. Read the recommended inverter VA and the battery Ah or backup time, with the working shown under each figure.

Choosing an inverter and battery for your home

A home inverter setup has two parts. The inverter decides how much load you can run at the same time, and it's rated in VA. The battery decides how long that load can run, and it's rated in ampere-hours (Ah). People often buy a big inverter with a small battery, or the other way round. This inverter battery calculator works out both from the appliances you actually run during a power cut, so you don't make that mistake.

Step 1: total load

Add up the running watts of everything you want on backup. Typical values: ceiling fan 70–80 W (BLDC fans about 30 W), LED bulb 9 W, LED tube 20 W, LED TV 50–100 W, Wi-Fi router about 10 W, fridge 100–200 W while the compressor runs. The label on the appliance gives the exact figure.

Step 2: inverter size (VA)

Inverter VA = total load (W) ÷ power factor
Recommended = VA × (1 + headroom), rounded up to a common size

Most home inverters have a power factor of about 0.7–0.8, so a 600 W load needs at least 750–860 VA. Leave about 20% headroom for appliances with motors, which draw a surge when they start, and for anything you add later.

Step 3: battery capacity (Ah)

Battery Ah = load (W) × backup hours ÷ (battery voltage × depth of discharge × inverter efficiency)
Backup hours = voltage × Ah × depth of discharge × efficiency ÷ load (W)

Depth of discharge is how much of the battery's charge you actually use. Lead-acid batteries last longer if they aren't drained fully, so the calculator uses 70% by default. LiFePO4 lithium batteries can use 90% or more. Inverter efficiency, typically 80–90%, accounts for losses when the inverter converts the battery's DC to 230 V AC.

Worked example

3 ceiling fans (75 W), 4 LED bulbs (9 W), an LED TV (60 W) and a Wi-Fi router (10 W):

  • Load: 225 + 36 + 60 + 10 = 331 W
  • Inverter: 331 ÷ 0.8 = 414 VA, plus 20% = 497 VA, so a 600 VA inverter
  • Battery for 3 hours at 12 V: 331 × 3 ÷ (12 × 0.70 × 0.85) = 139 Ah, so a 150 Ah battery
  • Backup from that 150 Ah battery: 12 × 150 × 0.70 × 0.85 ÷ 331 = about 3 h 14 min

Typical setups

InverterBattery bankSuits
600–900 VA12 V, one 100–150 Ah battery1BHK: 2–3 fans, lights, TV, router
1,000–1,100 VA12 V, one 150–200 Ah battery2BHK: 3–4 fans, lights, TV
1,450–2,000 VA24 V, two batteries3BHK, a fridge, longer backup
2,500 VA and above24–48 V, several batteriesBigger homes, water pump; usually paired with solar

These are typical combinations. Check the input voltage and the battery sizes the inverter supports in its manual.

Things to keep in mind

  • Motors and compressors (fridge, cooler, pump, mixer, washing machine) draw 3–5 times their running power for a moment at start-up. A pure sine-wave inverter handles them better and runs fans more quietly.
  • Ah ratings are measured at a slow discharge (over 10 or 20 hours for lead-acid). At heavier loads a lead-acid battery delivers less, so real backup can be shorter than the formula shows.
  • Batteries age. A lead-acid battery gives noticeably less backup after a few years. Topping up distilled water on schedule (for flooded tubular batteries) helps it last.
  • Avoid running an AC on a home inverter unless it was designed for that. Even a 1.5 ton inverter AC needs a large inverter and battery bank.

Frequently Asked Questions

About 3.5 hours on a 300 W load. A 12 V, 150 Ah battery stores 1,800 Wh. Using 70% of it through an 85%-efficient inverter gives about 1,070 Wh. That runs a 300 W load (3 fans, a few LED lights and a TV) for about 3.5 hours, a 500 W load for about 2 hours, or a 150 W load for about 7 hours.

Usually an 800–1,100 VA inverter with one 150–200 Ah battery. That covers 3–4 fans, LED lights, a TV and a router (300–450 W). If you also want to run a fridge, choose a larger, preferably pure sine-wave inverter (around 1,450 VA or more) because of the fridge's starting surge.

Divide the watts by the inverter's power factor. Most home inverters have a power factor of 0.7–0.8, so 400 W ÷ 0.8 = 500 VA. Then add about 20% headroom and pick the next size up, here a 600 VA inverter.

In most ways, yes, but it costs more. Lithium (LiFePO4) batteries can use more of their capacity, charge faster, need no water top-ups and usually last more cycles, but they cost more up front. Tubular lead-acid batteries are cheaper and widely serviced. For the same backup you need fewer Ah with lithium.

Yes, on a suitably sized inverter. A fridge compressor draws a short surge of several times its 100–200 W running power when it starts. Use an inverter with enough headroom (sine-wave preferred), and avoid starting the fridge, a pump and a mixer at the same moment.