How Long Will Your Inverter Battery Last? Calculate Your Backup Time in 3 Steps

This is the question “Inverter battery backup time?” every inverter buyer asks but few get a straight answer to.

How long will my inverter actually run? 3 hours? 6 hours? 8 hours?

The answer depends on three things: how much power you are drawing, how big your battery is, and how deep you can discharge it. Here is the exact formula – and how to use it for your home.

Formula: How to Measure Inverter Battery Backup Time

Backup time (hours) = Battery capacity (Ah) × Battery voltage (V) × Depth of discharge × Efficiency ÷ Load (W)

That looks complicated. Let’s break it down with a real example.

Step 1: Know Your Load

Add up the wattage of everything you run during a power cut. A typical 2BHK running fans, lights, a TV, and a WiFi router draws approximately 500–650 watts.

We will use 600W for this example.

Step 2: Know Your Battery

Most home inverter setups use one of these:

• 150Ah lead acid tubular battery – 12V
• 200Ah lead acid tubular battery – 12V
• 100Ah LiFePO4 lithium battery – 12V or 24V

The Ah (ampere-hour) rating tells you how much charge the battery holds. But you cannot use all of it.

Depth of Discharge – the number most dealers skip

Lead acid batteries should only discharge to 40–50% to protect their lifespan. Discharging them further shortens their life significantly.

LiFePO4 lithium batteries can safely discharge to 80–90%.

This means a 150Ah lead acid gives you approximately 60–75Ah of usable energy. A 100Ah LiFePO4 gives you approximately 80–90Ah – nearly the same, in a lighter, smaller battery.

Not sure which battery type suits your home? Use the Invertek UPS Finder to get a personalised recommendation based on your appliances and cut duration.

Step 3: Run the Calculation

Example	Setup	Load	Usable Energy	Backup Time
Example 1	150Ah lead acid, 12V	600W	765 Wh	~1 hr 15 min
Example 2	150Ah lead acid, 12V	300W	765 Wh	~2.5 hours
Example 3	100Ah LiFePO4, 12V	600W	969 Wh	~1 hr 35 min

Notice: a 100Ah lithium battery outperforms a 150Ah lead acid battery at the same load – despite having 33% less stated capacity. That is the difference depth of discharge makes.

How to Get More Backup Without Buying a Bigger Battery

Reduce your load. This is the most underused strategy.

• Switch all lights to LEDs – a 10W LED replaces a 60W bulb. 8 LED lights use 80W instead of 480W
• Switch off the fridge during power cuts – it holds temperature for 2+ hours if kept closed
• Turn off the TV if you can manage without it – saves 80W instantly

Cutting load from 600W to 350W extends backup time by nearly 70% – without buying a single extra battery.

Matching Battery to Power Cut Duration

Power Cut Duration	Recommended Setup
2-hour daily cuts	One 150Ah lead acid or 100Ah lithium at 400–500W load
4-hour daily cuts	Two 150Ah lead acid or 150Ah lithium at 500W load
6–8 hour cuts	Two 200Ah tubular or 200Ah LiFePO4 – or switch to a Solar Hybrid PCU

For long daily outages of 6 hours or more, a Solar Hybrid PCU is often the smarter investment -it handles the backup while also cutting your monthly electricity bill through solar generation.

Use the Invertek UPS Finder

Rather than calculating manually, use Invertek’s free UPS Finder. Enter your appliances, your average power cut duration, and it recommends the exact inverter-battery combination for your home.

All Invertek Rapid+ inverters support both lead acid and LiFePO4 lithium batteries with automatic charge profile detection – so you can start with tubular and upgrade to lithium later without changing the inverter.

👉 Find Your Ideal Inverter-Battery Setup →

Frequently Asked Questions

1. How do I calculate how long my inverter battery will last?
Use this formula: Battery capacity (Ah) × Voltage (V) × Depth of Discharge × Efficiency ÷ Load (W). For a 150Ah lead acid battery at 12V running a 600W load, this gives approximately 1 hour 15 minutes of backup. Reducing your load is the fastest way to extend that time without buying a new battery.

2. Why does a 100Ah lithium battery last longer than a 150Ah lead acid battery?
Because lithium batteries can safely discharge to 80–90% of their capacity, while lead acid batteries should only discharge to 40–50% to avoid damage. This means a 100Ah LiFePO4 battery delivers more usable energy than a 150Ah lead acid – in a lighter, smaller unit.

3. What appliances should I switch off during a power cut to extend backup time?
The biggest wins come from switching off the fridge (saves 150–200W), the TV (saves 80W), and replacing old bulbs with LEDs. Cutting your load from 600W to 350W can extend your backup time by nearly 70% without any change to your battery.

4. How many batteries do I need for a 4-hour daily power cut?
For a 500W load over 4 hours, you typically need two 150Ah lead acid batteries or one 150Ah LiFePO4 battery. The exact requirement depends on your actual load. Use the Invertek UPS Finder to get a recommendation specific to your home.

5. Can I upgrade from a lead acid battery to lithium without changing my inverter?
Yes – if your inverter supports both battery types. All Invertek Rapid+ models feature automatic charge profile detection, meaning they recognise and correctly charge both lead acid and LiFePO4 lithium batteries without any manual adjustment.

6. When should I consider a Solar Hybrid PCU instead of a regular inverter?
If your daily power cuts exceed 6 hours, or your electricity bill is high and rising, a Solar Hybrid PCU is worth serious consideration. It handles backup during outages while also running your home on solar during the day – reducing your grid consumption and your monthly bill simultaneously.