Home & Power
How Long Will My Inverter Battery Last? Backup Time Explained
Add up your load, apply one formula, and see backup times for 100 to 220 Ah batteries, plus how to size the battery and inverter.
By Team Vind · Updated · 5 min read
The shop says your 150 Ah battery gives "up to 8 hours", but it dies after three. Backup time depends on how much you run on it, not just the battery's size. Here's how to calculate it yourself, and how to choose the right battery and inverter for your home.
Step 1: add up your load
List what you'll run during a power cut and add the watts. A typical 2BHK:
| Appliance | Watts each | How many | Total |
|---|---|---|---|
| Ceiling fan | 75 W | 3 | 225 W |
| LED bulb / tube | 10 W | 4 | 40 W |
| Wi-Fi router | 10 W | 1 | 10 W |
| LED TV (43 inch) | 80 W | 1 | 80 W |
| Total load | 355 W |
Step 2: the backup formula
Backup (hours) = battery Ah × voltage × usable share × inverter efficiency ÷ load (W)
A battery stores Ah × V watt-hours: a 150 Ah, 12 V battery holds 1,800 Wh. You shouldn't drain a tubular battery fully, so count about 80%, and the inverter loses about 15% converting it to AC.
For our 355 W load: 150 × 12 × 0.8 × 0.85 ÷ 355 = 3 h 27 min.
Try it: Inverter Battery Backup Calculator
Your Backup Time
Backup at different loads
| Load | Backup |
|---|---|
| 100 W | 12 h 14 min |
| 200 W | 6 h 7 min |
| 300 W | 4 h 5 min |
| 500 W | 2 h 27 min |
| 750 W | 1 h 38 min |
| 1000 W | 1 h 13 min |
Backup by battery size
| Battery (12 V) | 100 W load | 200 W load | 355 W load | 500 W load |
|---|---|---|---|---|
| 100 Ah | 8 h 10 min | 4 h 5 min | 2 h 18 min | 1 h 38 min |
| 150 Ah | 12 h 14 min | 6 h 7 min | 3 h 27 min | 2 h 27 min |
| 200 Ah | 16 h 19 min | 8 h 10 min | 4 h 36 min | 3 h 16 min |
| 220 Ah | 17 h 57 min | 8 h 59 min | 5 h 3 min | 3 h 35 min |
Choosing a battery for the backup you want
Battery Ah = hours × load (W) ÷ (voltage × 0.8 × 0.85)
For 4 hours on 355 W: 4 × 355 ÷ (12 × 0.8 × 0.85) = 174 Ah. Round up to the next size you can buy, here a 200 Ah battery.
Choosing the inverter
Inverters are rated in VA. Pick one whose VA is about 1.25 to 1.5 times your load in watts, to allow for power factor and motor start-up. For 355 W, that's 444–533 VA, so a 600 VA inverter, or a size up if you plan to add more appliances. Read kVA vs kW for why VA and watts differ.
Thinking about solar to charge it? See the solar panel calculator and the PM Surya Ghar subsidy guide.
Frequently asked questions
How long will a 150Ah battery last?
About 6 h 7 min on a 200 W load, or 2 h 27 min on 500 W, for a new tubular battery.
Is a 200Ah battery better than 150Ah?
It stores a third more energy, so it gives about a third more backup on the same load, but it costs more and takes longer to charge.
Can I connect two batteries?
Yes, if your inverter supports 24 V. Two 12 V batteries in series double the stored energy, and the backup roughly doubles.
Why does my battery give less backup than calculated?
Age, low water level, heavy loads and loose terminals all reduce backup. Lead-acid batteries also give less of their capacity at high loads.
Sources
- Bureau of Energy Efficiency: appliance star ratings and typical power use
- Manufacturer data sheets for tubular batteries (rated capacity at the C10/C20 rate)
Last reviewed: 10 October 2026