How long will a 150 Ah battery run a fan?

One 12 V 150 Ah lead-acid battery gives about 765 usable Wh through an inverter. A 75 W ceiling fan runs about 10 hours; two fans and a few LED lights (220 W) about 3.5 hours.

How many batteries do I need for 8 hours of load shedding?

Work out your load in watts and multiply by 8 for the watt-hours, then divide by 0.5 x 0.85 x 12 V x Ah. For 300 W over 8 hours you need about 470 Ah of 12 V lead-acid, three 150 Ah or two 200 Ah batteries and a bigger inverter to charge them.

Is lithium worth the extra price?

A lithium pack gives 80% usable capacity, 3,000 or more cycles and weighs a third as much; lead-acid gives 50% usable and 500 to 1,200 cycles. Over five years lithium usually costs less per usable kilowatt-hour.

Why is my backup shorter than calculated?

Battery age, heat, undercharging before the outage, and loads that draw more than their label (old fans, refrigerators starting) all cut real backup time.

Utilitiesv1.0.0Last reviewed 18 Sept 2026

UPS Battery Backup Time Calculator

How many hours your UPS, inverter or solar battery bank will run a given load, from the battery's amp-hours, voltage, count and type, with the load in watts.

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How this is calculated

A battery bank stores Ah x V x number of batteries watt-hours. Only part of that is usable: a lead-acid or tubular battery should not be taken below 50% state of charge on a daily cycle, while lithium iron phosphate packs are rated for 80 to 90%.

The inverter then loses 10 to 15% turning DC into 220 V AC. So:

Backup hours = Ah x V x batteries x depth of discharge x inverter efficiency / load in watts.

Two 150 Ah 12 V lead-acid batteries with a 300 W load: 3,600 Wh x 0.5 x 0.85 / 300 = about 5 hours.

Frequently asked questions

How long will a 150 Ah battery run a fan?: One 12 V 150 Ah lead-acid battery gives about 765 usable Wh through an inverter. A 75 W ceiling fan runs about 10 hours; two fans and a few LED lights (220 W) about 3.5 hours.
How many batteries do I need for 8 hours of load shedding?: Work out your load in watts and multiply by 8 for the watt-hours, then divide by 0.5 x 0.85 x 12 V x Ah. For 300 W over 8 hours you need about 470 Ah of 12 V lead-acid, three 150 Ah or two 200 Ah batteries and a bigger inverter to charge them.
Is lithium worth the extra price?: A lithium pack gives 80% usable capacity, 3,000 or more cycles and weighs a third as much; lead-acid gives 50% usable and 500 to 1,200 cycles. Over five years lithium usually costs less per usable kilowatt-hour.
Why is my backup shorter than calculated?: Battery age, heat, undercharging before the outage, and loads that draw more than their label (old fans, refrigerators starting) all cut real backup time.

Sources

Battery manufacturers' depth-of-discharge guidance (AGS, Exide, Phoenix, Osaka; lithium iron phosphate packs): Searchable

Version 1.0.0 · reviewed 18 Sept 2026. Rates change with the Federal Budget and regulator notifications; we update this tool when they do. Confirm with the primary source before making financial decisions.

Topics:Solar Energy