Work out how many hours your inverter battery will run fans, lights and a router during a power cut.
Quick answer
Backup time ≈ battery Ah × volts × usable share × inverter efficiency ÷ load in watts. A 150 Ah, 12 V tubular battery stores 1,800 Wh. Using about half of it, at 85% inverter efficiency, it runs a 300 W load (three fans, a few LED lights and a router) for about 2 hours 33 minutes.
Inverter battery backup calculator
Find out how long your inverter battery will run your fans, lights and router during a power cut.
Load guide: ceiling fan 75 W (BLDC 30 W), LED bulb 9 W, LED tube 20 W, Wi-Fi router 10 W, 43" LED TV about 80 W, laptop 60 W. Check the labels on your own appliances.
Backup = Ah × volts × number of batteries × usable share × inverter efficiency ÷ load. Lead-acid batteries last longer when you avoid discharging them below about half, so the default uses 50%. Old batteries hold less charge than their rated Ah.
How to work out your load
Add up the watts of everything that will run during the cut. Typical figures are a standard ceiling fan at 75 W, a BLDC fan at 30 W, an LED bulb at 9 W and a Wi-Fi router at 10 W. Check your own appliances' labels. The Appliance Electricity Cost Calculator lists common wattages.
Why only part of the battery counts
- Lead-acid and tubular batteries wear out faster if you regularly drain them deep. Planning on about half their capacity keeps them healthy.
- Lithium (LiFePO4) batteries handle deeper discharge, so about 80% is usable.
- The inverter itself loses energy converting DC to AC, typically 10–20%.
- Ageing matters: an old battery holds less than its rated Ah.
Choosing battery size
Work backwards. Multiply the backup hours you need by your load, then divide by (volts × usable share × efficiency) to get the Ah you need. For a 300 W load and 4 hours on a 12 V lead-acid system, that is 1,200 ÷ (12 × 0.5 × 0.85), or about 235 Ah. In practice that means a larger battery or two batteries on a 24 V inverter.
FAQs
How long will a 150 Ah battery last?
On a 12 V lead-acid system with a 300 W load, about 2.5 hours at a healthy depth of discharge. Halve the load and the time roughly doubles.
Can an inverter run an AC?
A normal home inverter can't. ACs draw 1,000–2,000 W and spike at start-up, so you need a high-capacity or solar-hybrid system sized for it.
Does a higher VA rating mean longer backup?
No. VA is how much load the inverter can carry at once. Backup time depends on battery capacity.