Inverter & Battery Calculator
Find the inverter VA rating and battery Ah you need for your home load and the backup hours you want.
Result
How to use
- 1
Enter how many of each appliance you want to run during a power cut.
- 2
Choose how many hours of backup you need.
- 3
See the inverter VA, battery Ah and number of batteries to buy.
Questions
How is the inverter VA rating calculated?
VA = total load in watts ÷ power factor. Home inverters have a power factor of about 0.7–0.8, so a 400 W load needs at least a 500–600 VA inverter. This calculator uses 0.8 and picks the next standard size.
How do I calculate battery Ah?
Ah = load (W) × backup hours ÷ (battery voltage × inverter efficiency). For 400 W for 3 hours on a 12 V system at 85% efficiency: 400 × 3 ÷ (12 × 0.85) ≈ 118 Ah, so a 120–150 Ah battery.
Tall tubular or flat plate battery?
Tall tubular batteries last longer (often 4–6 years) and handle frequent long power cuts, so they suit most Indian homes. Flat plate batteries are cheaper and fine for short, occasional cuts. Top up distilled water as advised.
About this Inverter tool
Inverter & Battery Calculator — Find the inverter VA rating and battery Ah you need for your home load and the backup hours you want. It is one of 200 free tools on namasivayam.in, made by software engineer Namasivayam Vadivelan for everyday use in India.
Everything is calculated instantly in your browser: nothing you type or upload is sent to any server, there is no sign-up, and it works on mobile and desktop. Enter how many of each appliance you want to run during a power cut.
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Results are estimates to help you plan. They are not financial, tax or medical advice.