Find out how many hours your inverter battery will run your fans, lights, TV and other appliances — or what battery size (Ah) you need for the backup you want. Includes battery type, voltage and inverter efficiency.
Formula checked 23 Oct 2026Free · no sign-up
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How to use the inverter backup calculator
Choose How long will my battery last? or What battery do I need?
Add the appliances that run during a power cut and how many of each — or type the total watts.
Enter the battery Ah, its voltage and how many batteries are connected in series (a 24 V inverter uses two 12 V batteries).
Choose the battery type and check the inverter efficiency (85% is typical).
Battery backup formula
Battery energy (Wh) = Ah × Voltage × Number of batteries in series
Backup time (h) = Battery Wh × Usable % × Inverter efficiency ÷ Load (W)
Battery needed (Ah) = Load (W) × Hours ÷ (Voltage × Usable % × Efficiency)
Worked example
Two ceiling fans (75 W each), four LED bulbs (9 W each), a TV (100 W) and a Wi-Fi router (10 W) on a 150 Ah, 12 V tubular battery:
Backup time chart (12 V, 80% usable, 85% efficiency)
Load
100 Ah
150 Ah
200 Ah
100 W
8.2 h
12.2 h
16.3 h
200 W
4.1 h
6.1 h
8.2 h
300 W
2.7 h
4.1 h
5.4 h
500 W
1.6 h
2.4 h
3.3 h
800 W
1.0 h
1.5 h
2.0 h
Real backup also falls as a battery ages: after 3–4 years a lead-acid battery may hold only 60–70% of its original capacity. If your backup has dropped, the battery — not the inverter — is usually the reason.
Tips to get more backup
Switch to BLDC fans and LED lights — they can cut your load by half and double your backup.
Keep heavy loads off the inverter: irons, geysers, heaters and ACs drain a home inverter battery in minutes.
Top up distilled water in lead-acid batteries every 2–3 months and keep terminals clean.
Don’t run a lead-acid battery flat. Deep discharges shorten its life; most inverters cut off automatically.
Size the inverter correctly: its VA rating should be at least load ÷ 0.8, with headroom for fridges and pumps that draw extra power when starting.
Frequently asked questions
How do you calculate inverter battery backup time?
Backup time (hours) = battery Ah × battery voltage × usable capacity × inverter efficiency ÷ load in watts. For a 150 Ah 12 V tubular battery (80% usable) with an 85% efficient inverter running 500 W: 150 × 12 × 0.8 × 0.85 ÷ 500 = 2.45 hours.
How long will a 150Ah battery last?
On a 12 V inverter: about 2.4 hours at 500 W, 4.1 hours at 300 W (two fans, four LED bulbs, a TV and a router), and about 12 hours at 100 W, using 80% of capacity and 85% inverter efficiency.
Why is the real backup less than Ah × V ÷ watts?
Three reasons: you should not fully empty a lead-acid battery (it shortens its life), the inverter loses 10–20% as heat, and lead-acid batteries deliver less capacity at high currents. The calculator includes the first two.
What battery do I need for 4 hours backup?
For a 500 W load on a 12 V system with a tubular battery: 500 × 4 ÷ (12 × 0.8 × 0.85) ≈ 245 Ah — for example two 150 Ah batteries on a 24 V inverter, or one large lithium battery. Use “What battery do I need?” to check your own load.
What is the difference between tubular and flat-plate batteries?
Tubular batteries handle deep, daily discharges and last longer, so you can use more of their capacity. Flat-plate batteries are cheaper but are best kept above about 50% charge. Lithium (LiFePO₄) batteries can use about 90% of their capacity and last thousands of cycles.
How many watts does a ceiling fan use?
A regular ceiling fan uses about 60–80 W at full speed and a BLDC fan about 25–35 W. LED bulbs use 7–12 W, a 32–43 inch LED TV around 60–120 W and a Wi-Fi router about 10 W. Check the label for exact values.
How we checked this calculator
We test this tool against worked examples before every release (last check: 23 Oct 2026, version 1.0.0). Results are estimates for planning — confirm critical figures with your supplier or a qualified professional.
