How do I calculate inverter battery backup time?

Backup time (hours) = Battery capacity (Ah) × Battery voltage (V) × Depth of discharge × Inverter efficiency ÷ Load (watts). For example, a 150 Ah, 12 V battery at 80% usable depth and 80% efficiency powering a 300 W load lasts: 150 × 12 × 0.8 × 0.8 ÷ 300 = about 3.8 hours.

How long will a 150Ah battery last on an inverter?

A 150 Ah, 12 V battery stores about 1,800 Wh, of which roughly 1,150 Wh is usable after depth of discharge and inverter losses. Running a 200 W load (a few fans and lights) it lasts about 5.7 hours; at 400 W it lasts about 2.9 hours; at 600 W about 1.9 hours.

How many fans and lights can a 150Ah inverter run?

A typical ceiling fan uses about 75 W and an LED tubelight about 20 W. A 150 Ah battery comfortably runs 3 fans and 4 LED lights (about 305 W) for roughly 3.5–4 hours. Reducing the load extends the backup proportionally.

What is depth of discharge and why does it matter?

Depth of discharge (DoD) is how much of the battery you can safely use. Draining a lead-acid inverter battery fully shortens its life, so only about 70–80% is treated as usable. A tubular battery tolerates deeper discharge than a flat-plate one. Using the correct DoD gives a realistic backup time rather than an optimistic one.

How can I increase my inverter backup time?

Reduce the connected load by using BLDC fans and LED lights, add a higher-Ah or a second battery, choose a tubular battery for deeper discharge, keep the battery healthy with topped-up water and clean terminals, and avoid running heavy loads like motors, irons or pumps on the inverter.

🔋 Backup Time

Inverter Battery Backup Time Calculator

Enter your battery and load to see exactly how many hours your inverter will run during a power cut.

🔋 Backup time depends on your load, not just the battery. The same 150 Ah battery that runs 9 hours on a couple of lights may give under 2 hours if you add a fridge or pump. Add your appliances below to see your real backup.

Battery Capacity (Ah)

Voltage (V)

12 / 24 / 48 V

No. of Batteries

Connected Load (watts)

Quick add appliances:

Usable Depth of Discharge (%)

Lead-acid ~70–80%, tubular ~80%, lithium ~90%

Inverter Efficiency (%)

Typical 75–85%

Typical Appliance Wattage

LED bulb	9 W
LED tubelight	20 W
Ceiling fan	75 W
BLDC fan	30 W
LED TV (32")	100 W
WiFi router	10 W
Laptop	65 W
Refrigerator (avg)	150–250 W
Mixer / pump	500–750 W

How This Calculator Works

It converts your battery into usable watt-hours, then divides by your load to get hours of runtime — after accounting for depth of discharge and inverter losses.

Total energy = Ah × V × number of batteries. Only the usable part (DoD × efficiency) actually reaches your appliances.

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How Inverter Backup Time Is Calculated

A battery's headline rating is in amp-hours (Ah), but appliances are rated in watts. To find backup time you convert the battery into watt-hours, keep only the part you can safely use, and divide by your load.

Total Energy (Wh) = Ah × Voltage × Number of Batteries Usable Energy (Wh) = Total Energy × DoD% × Inverter Efficiency% Backup Time (hours) = Usable Energy ÷ Load (watts)

Worked Example — 150 Ah, 300 W load

Battery: 150 Ah × 12 V × 1 = 1,800 Wh
Usable = 1,800 × 0.80 × 0.80 = 1,152 Wh
Load: 4 fans + lights ≈ 300 W
Backup = 1,152 ÷ 300 = 3.8 hours

How to Increase Backup Time

Switch to BLDC fans and LED lights — they cut the load by half or more
Add a second battery or move to a higher-Ah tubular battery
Avoid running motors, irons, pumps and heaters on the inverter
Keep the battery healthy — topped-up water, clean terminals, correct charging
Consider a lithium (LiFePO4) battery for deeper, faster, longer-life cycling

Backup Time Reference (150 Ah, 12 V Battery)

At 80% depth of discharge and 80% inverter efficiency (≈1,152 Wh usable).

Load	Example	Backup Time
100 W	2 fans + 2 lights	~11.5 hours
200 W	3 fans + 4 lights + TV	~5.8 hours
300 W	Above + router + laptop	~3.8 hours
500 W	Add a refrigerator	~2.3 hours
750 W	Heavy mixed load	~1.5 hours

* Approximate. Use the calculator above for your exact battery and load.

Frequently Asked Questions

Backup time (hours) = Battery Ah × Voltage × Depth of discharge × Inverter efficiency ÷ Load (watts). For a 150 Ah, 12 V battery at 80% usable and 80% efficiency on a 300 W load: 150 × 12 × 0.8 × 0.8 ÷ 300 ≈ 3.8 hours.

A 150 Ah, 12 V battery stores about 1,800 Wh, of which ~1,150 Wh is usable. At 200 W it lasts about 5.7 hours, at 400 W about 2.9 hours, and at 600 W about 1.9 hours.

A ceiling fan uses ~75 W and an LED tubelight ~20 W. A 150 Ah battery runs about 3 fans and 4 LED lights (~305 W) for roughly 3.5–4 hours. Less load means proportionally longer backup.

Depth of discharge (DoD) is how much of the battery you can safely use. Fully draining a lead-acid battery shortens its life, so only ~70–80% is usable. Tubular batteries tolerate deeper discharge than flat-plate ones; lithium allows ~90%. Correct DoD gives a realistic backup figure.

Use BLDC fans and LED lights to cut the load, add a higher-Ah or second battery, pick a tubular or lithium battery for deeper discharge, keep the battery maintained, and avoid heavy loads like motors, irons and pumps.

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