# UPS Battery Backup Time Calculator

Source: Searchable (https://searchable.pk/tools/utilities/battery-backup-calculator)
Type: Calculator
Rates reviewed: 18 Sept 2026
Retrieved: 18 Sept 2026

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.

Version 1.0.0. Inputs: Battery capacity, Battery voltage, Number of batteries, Battery type, Load being run, Inverter efficiency.

## Worked example (default inputs)

- Battery capacity: 150 Ah
- Battery voltage: 12
- Number of batteries: 2
- Battery type: lead
- Load being run: 300 W
- Inverter efficiency: 85 %

**Backup time at 300 W: 5 h 6 min**

2 x 150 Ah 12 V lead-acid stores 3,600 Wh; using 50% of it through an inverter at 85% efficiency gives 1,530 Wh, which runs 300 W for about 5 h 6 min.

### Energy

- Stored energy: 3,600 Wh
- Usable (50% depth of discharge): 1,800 Wh
- After inverter losses (85%): 1,530 Wh

### Backup at other loads

- 100 W: 15.3 h
- 200 W: 7.7 h
- 500 W: 3.1 h
- 1000 W: 1.5 h

### Recharge

- Recharge time from a 10% (C/10) charger: about 441 h

## How it 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)

How to cite: "UPS Battery Backup Time Calculator", Searchable, rates reviewed 18 Sept 2026, https://searchable.pk/tools/utilities/battery-backup-calculator