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HQ8T

Inverter Battery Backup Calculator

Backup hours and inverter size for your home.

What will run during a power cut?
Ceiling fan (75 W)2
BLDC fan (30 W)0
LED bulb (9 W)4
Tube light (LED) (20 W)0
LED TV (80 W)1
Wi-Fi router (10 W)1
Laptop (65 W)0
Desktop PC (200 W)0
Fridge (150 W)0
Mobile charger (10 W)0
watts

Total load: 276 W

Ah

Printed on the battery, e.g. 150Ah.

Battery type
Battery voltage
Backup time
4 h 26 min
Inverter size needed
450 VA
For 276 W load
Usable energy
1224 Wh
of 1800 Wh stored
hours

You need about 140 Ah at 12 V.

Real backup is often 10–20% lower on old batteries or in hot weather. Motors like fridges and pumps need extra surge capacity.

How to use

  1. 1Add the fans, lights and appliances you'll run in a power cut.
  2. 2Enter the battery Ah, number of batteries and type.
  3. 3See the backup time and the inverter VA you need.
  4. 4Enter the hours you want to see the battery size required.

Frequently asked questions

How long will a 150Ah battery last?

A 150Ah 12V tubular battery runs about 300W (2 fans, 4 LED bulbs, a TV and router) for roughly 4 hours.

How is backup time calculated?

Battery Ah × volts × usable depth × inverter efficiency ÷ load in watts. The tool uses 80% depth for tubular, 50% for flat plate and 90% for lithium, with 85% efficiency.

What size inverter do I need?

About load ÷ 0.8 power factor, plus 25% headroom. For a 300W load, a 500 VA inverter is enough.

Can an inverter run an AC?

Only large inverters (usually 2 kVA+ with several batteries). A 1.5 ton AC draws 1,500W or more, draining batteries quickly.

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