Inverter Battery Backup Calculator: Hours of Backup & VA Size
Free tool · Updated October 2, 2026 · By CouponTalk
Your result
Enter your measurements to see your result.
Quick answer
Backup time (hours) = battery Ah × battery voltage × number of batteries × efficiency ÷ load in watts. A 150 Ah, 12 V battery running a 300 W load (3 fans, 4 LED lights, a TV) at 80% efficiency gives about 4.8 hours. The inverter VA should be about load ÷ 0.7, so a 300 W load needs at least a 450–600 VA inverter.
How to use this calculator
- Add up the watts of everything you want to run: ceiling fan ~75 W, LED bulb ~9 W, LED TV ~60–100 W, router ~10 W.
- Enter your battery Ah, voltage and number of batteries.
- Choose the battery type; lithium batteries can use more of their capacity.
Reference table
| Battery | 200 W | 300 W | 500 W | 800 W |
|---|---|---|---|---|
| 100 Ah (12 V) | 4.8 h | 3.2 h | 1.9 h | 1.2 h |
| 150 Ah (12 V) | 7.2 h | 4.8 h | 2.9 h | 1.8 h |
| 200 Ah (12 V) | 9.6 h | 6.4 h | 3.8 h | 2.4 h |
These are typical figures for Indian homes. Check the product spec sheet, and ask an electrician or installer before you buy or fit anything.
Frequently asked questions
How long will a 150Ah battery last?
With a 12 V, 150 Ah battery at 80% efficiency: about 4.8 hours at 300 W, 2.9 hours at 500 W and 1.8 hours at 800 W.
What size inverter do I need?
Divide your total load in watts by 0.7 and pick the next standard size. For 500 W, choose at least a 750–900 VA inverter.
Can an inverter run an AC or fridge?
A fridge can run on a 900 VA+ inverter. A 1.5 ton AC needs a large inverter or solar system, typically 3–5 kVA with a 24 V or 48 V battery bank.
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