Home Battery & Inverter Backup Sizing Calculator
Size an inverter battery bank correctly for your real backup load, including depth of discharge and conversion losses.
Result
- Energy delivered per cycle
- 2.00kWh
- Gross battery bank required
- 4.55kWh
- Battery capacity needed
- 189Ah
- Minimum inverter rating
- 781VA
- Effective cost per backup unit
- ₹10.10
- Grid units to recharge fully
- 2.53kWh
Estimates only. Nothing you type is stored or sent anywhere — the calculation runs entirely in your browser.
How this works
Battery banks are almost always undersized because people size them on nameplate capacity instead of usable capacity. A 150 Ah, 12 V lead-acid battery stores 1.8 kWh on paper, but you can only draw about 50% of it repeatedly without destroying cycle life, and the inverter itself loses 10–15% converting DC to AC. Real usable output is closer to 0.8 kWh.
This calculator works backwards from the load you actually want to run. It converts your watt-hours of demand into the gross DC storage required by dividing by both the usable depth of discharge and the inverter efficiency, then expresses that as amp-hours at your chosen bank voltage. Lithium (LFP) banks tolerate 80–90% depth of discharge, which is why an LFP bank of half the nameplate size often outperforms lead-acid.
The inverter VA rating is sized on instantaneous demand, not energy. It applies a power factor of 0.8 and a 25% headroom margin so that motor loads — fridge compressors, water pumps, mixers — can draw their starting surge without tripping the inverter.
Common questions
- How many batteries do I need for a 150 Ah requirement at 24 V?
- Two 12 V, 150 Ah batteries wired in series give you 24 V at 150 Ah. Wiring them in parallel instead gives 12 V at 300 Ah. Series raises voltage, parallel raises amp-hours — the stored energy is the same either way.
- Is lithium worth the extra cost over tubular lead-acid?
- Usually yes over a 10-year horizon. LFP delivers roughly 3,000–5,000 cycles against 1,000–1,500 for tubular lead-acid, needs no water topping, and gives you far more usable energy per nameplate kWh. The upfront cost is roughly double but the cost per delivered unit is lower.
- Why is my running cost per unit higher than my tariff?
- Because every unit stored is a unit lost twice — once charging and once discharging. A round-trip efficiency of 80% means you buy 1.25 units from the grid for every unit your appliances actually consume from the battery.