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Inverter and Battery Calculator

The inverter VA, battery Ah and battery count for the backup you want, and the solar panels that would refill it each day.

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Quick Answer

Divide the watts you want to run by 0.8 to get the inverter size in VA. For the battery, multiply those watts by the backup hours, divide by 0.9 for inverter losses, then divide by the share of the battery you are allowed to use: 75% for lead-acid and 85% for lithium LiFePO4, the limits in MNRE's off-grid specifications. So 300 W for 4 hours on a 12 V lead-acid battery needs 148 Ah: one 150 Ah tubular battery and an inverter of at least 375 VA. Put in your own load below. The calculator also gives the solar panel wattage that would refill the battery each day in your state.

Figures checked Oct 2026. Source: MNRE: Technical Specifications for Solar Photovoltaic Lighting Systems & Power Packs (Off-grid Solar Applications Scheme 2016-17): LiFePO4 DoD up to 85% (p. 38); A.C. power packs, 75% of lead-acid capacity between full charge and load cut-off, inverter efficiency more than 90% (pp. 58 to 60); charging electronics at least 85% efficient.

W ÷ 0.8inverter size in VA
75% / 85%usable share: lead-acid / LiFePO4
0.90minimum inverter efficiency (MNRE)
148 Ah300 W for 4 h on 12 V lead-acid

Part of the system: Inverter backup for a 2BHK, Plan rooftop solar for a 3BHK. Steps, checks and a free printable page.

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The Formula

Battery size = battery Ah = load W × backup hours ÷ 0.90 ÷ usable share (0.75 lead-acid, 0.85 LiFePO4) ÷ bank voltage; inverter VA = load W ÷ 0.8; solar Wp = (load W × hours ÷ 0.90) ÷ 0.85 ÷ units per kW per day

0.90 is the minimum inverter efficiency in MNRE's specifications for solar power packs. 0.75 and 0.85 are the shares of rated capacity that the same specifications allow between full charge and load cut-off, for lead-acid and LiFePO4. In the solar step, 0.85 is MNRE's minimum efficiency for charging electronics, and units per kW per day come from the state table on our rooftop solar planner (NASA sunshine data × 0.78).

From load to battery size: 300 W for 4 hours300 W times 4 h is 1,200 Wh; divided by 0.90 is 1,333 Wh; divided by 0.75 is 1,778 Wh; divided by 12 V is 148 Ah.1Load × backup hours300 W × 4 h = 1,200 Wh2÷ 0.90 inverter efficiency= 1,333 Wh from the battery3÷ 0.75 usable share (lead-acid)= 1,778 Wh of rated capacity4÷ 12 V bank= 148 Ah, so one 150 Ah battery
From load to battery size: 300 W for 4 hours. The same chain the calculator runs, with the quick-answer example.

Assumptions

  • The load is what runs at the same time during a cut. A fridge, pump or mixer draws a short surge when its motor starts, so leave headroom on the inverter for those.
  • Inverter VA = watts ÷ 0.8. Luminous rates its 1.5 kVA Zelio+ 1700 for a 1,260 W load, which is 0.84 W per VA, so 0.8 leaves a small margin.
  • Battery Ah is the rating printed on the battery (at the C/10 or C/20 rate). A heavy load that empties the battery faster gets less than the label.
  • Usable share: 75% of rated capacity for lead-acid, 85% for LiFePO4, as in MNRE's specifications. If the battery is charged only by solar, the IEA's lead-acid battery guide advises designing for 50% so it lasts, which means a bigger battery.
  • Batteries are counted as 12 V blocks. A 24 V bank is 2 in series, a 48 V bank 4. A 48 V lithium pack counts as 4 blocks here.
  • The battery is new. The IEA guide treats a battery at 80% of its rated capacity as worn out, so an old battery gives less backup than this.
  • The solar figure is the panel wattage that puts one full backup's energy back on an average day in your state. Cloudy and monsoon days give less, so off-grid homes need more panel or more battery.

Backup hours for common batteries

Backup hours at a steady load, worked out with the same factors as the calculator (75% usable for lead-acid, 85% for LiFePO4, 90% inverter efficiency).

Battery bank100 W200 W300 W500 W800 W
12 V 100 Ah lead-acid8.14.02.71.61.0
12 V 150 Ah lead-acid12.26.14.02.41.5
12 V 200 Ah lead-acid16.28.15.43.22.0
24 V 150 Ah lead-acid (2 batteries)24.312.28.14.93.0
12 V 100 Ah LiFePO49.24.63.11.81.1
48 V 100 Ah LiFePO436.718.412.27.34.6

Backup = volts × Ah × usable share × 0.9 ÷ load watts. Factors from MNRE's off-grid specifications, read 4 October 2026.

Backup hours at a 300 W load, by battery bankBackup hours at 300 W for each battery bank in the table.12 V 100 Ah lead-acid2.7 h12 V 150 Ah lead-acid4.0 h12 V 200 Ah lead-acid5.4 h24 V 150 Ah lead-acid (2 batteries)8.1 h12 V 100 Ah LiFePO43.1 h48 V 100 Ah LiFePO412.2 h
Backup hours at a 300 W load, by battery bank. The same numbers as the table above.

Worked Examples

A 300 W load for 4 hours on one lead-acid battery (Kolkata)

  1. Inverter: 300 W ÷ 0.8 = 375 VA, so buy the next size up
  2. Energy from the battery: 300 W × 4 h ÷ 0.9 = 1,333 Wh
  3. Battery size: 1,333 Wh ÷ 0.75 usable = 1,778 Wh, ÷ 12 V = 148.1 Ah
  4. With 150 Ah batteries that is 1 string, so 1 × 12 V 150 Ah in all, giving 4 hours of backup
  5. Solar to refill it: 1,333 Wh ÷ 0.85 = 1,569 Wh, ÷ 3.56 units per kW per day = 441 W, rounded up to 450 Wp

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An 800 W load for 3 hours on a 24 V inverter (Delhi)

  1. Inverter: 800 W ÷ 0.8 = 1,000 VA, so buy the next size up
  2. Energy from the battery: 800 W × 3 h ÷ 0.9 = 2,667 Wh
  3. Battery size: 2,667 Wh ÷ 0.75 usable = 3,556 Wh, ÷ 24 V = 148.1 Ah
  4. With 150 Ah batteries that is 1 string, so 2 × 12 V 150 Ah in all, giving 3 hours of backup
  5. Solar to refill it: 2,667 Wh ÷ 0.85 = 3,137 Wh, ÷ 3.81 units per kW per day = 823 W, rounded up to 830 Wp

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A 1,500 W lithium backup for 5 hours (Bengaluru)

  1. Inverter: 1,500 W ÷ 0.8 = 1,875 VA, so buy the next size up
  2. Energy from the battery: 1,500 W × 5 h ÷ 0.9 = 8,333 Wh
  3. Battery size: 8,333 Wh ÷ 0.85 usable = 9,804 Wh, ÷ 48 V = 204.2 Ah
  4. With 100 Ah batteries that is 3 strings, so 12 × 12 V 100 Ah in all, giving 7.3 hours of backup
  5. Solar to refill it: 8,333 Wh ÷ 0.85 = 9,804 Wh, ÷ 4.27 units per kW per day = 2,296 W, rounded up to 2,300 Wp

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How to Use This Calculator

  1. Add up the load. Write down the watts of everything you want running during a cut: fans, lights, the TV, the Wi-Fi router. The figure is on each appliance's rating plate or box.
  2. Choose the backup hours. Use your usual longest cut, not the worst one you remember. Each extra hour adds battery cost.
  3. Pick the battery type and voltage. The voltage is printed on the inverter or its manual. Lead-acid lets you use 75% of each Ah and lithium LiFePO4 85%.
  4. Enter one battery's Ah. Use the battery you plan to buy, such as 150 Ah. The calculator says how many you need.
  5. Check the solar line. If you plan to add panels, the last line gives the wattage that refills the battery on an average day in your state.

Tips and tricks

  • List only what you run during a power cut: fans, lights, router, TV.
  • Read the watts on each appliance's label instead of guessing.
  • Keep the fridge, iron, geyser and AC off the inverter unless it was sized for them.
  • For tubular lead-acid batteries, check the water level as often as the maker's manual says.
  • Have an electrician do the wiring and the changeover.

What we would do

Size for the load you need during a typical cut where you live, plus a little headroom. Pick lithium only if cuts are long and daily; for short, occasional cuts a tubular lead-acid battery costs less up front.

Frequently Asked Questions

How long will a 150 Ah battery last on an inverter?
A 12 V 150 Ah lead-acid battery holds 1,800 Wh. Using 75% of it through a 90% efficient inverter gives 1,215 Wh at the socket: about 4 hours at 300 W, 2.4 hours at 500 W and 12 hours at 100 W.
What size inverter do I need for a 1,000 W load?
1,000 ÷ 0.8 = 1,250 VA, so buy the next model up. At 12 V that load would pull 93 A from the battery. Luminous's 1.5 kVA Zelio+ 1700 runs on two batteries at 24 V, which halves the current to 46 A.
Is a lithium battery better than lead-acid for an inverter?
You get more use from each Ah. MNRE's specifications allow 85% of a LiFePO4 battery's capacity to be used against 75% for lead-acid, so 100 Ah of lithium does the work of about 113 Ah of lead-acid. The price difference is not in this calculator; compare quotes.
How many batteries does a 24 V inverter need?
Two 12 V batteries in series. Two 150 Ah batteries at 24 V are still rated 150 Ah, but they store 3,600 Wh, twice what one battery holds.
How many solar panels do I need to charge a 150 Ah battery?
To put back a full backup from a 12 V 150 Ah lead-acid battery (1,350 Wh taken out) on an average day, you need about 450 Wp of panels in West Bengal and 370 Wp in Gujarat. Cloudy days need more.

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Sources & Limitations

What this calculator does not model

  • It treats the load as steady. Real loads switch on and off, so actual backup is often longer than shown.
  • It does not size cables, fuses or the charge controller. Ask the installer for those.
  • Battery prices and the cost of solar are not included. For the cost of a grid-connected rooftop system and the PM Surya Ghar subsidy, use the rooftop solar planner.
  • The solar estimate uses one city per state and a yearly average.