How To Calculate Battery Backup Power

Deciding on which battery to use means knowing how much power it can supply. There is a way to calculate battery backup power and this post should give you an idea on how to do it.

How Do You Calculate Battery Amp Hours (Ah) for a UPS?

You will need to know the battery voltage, device load in watts, how long the UPS needs power the device in hours, the inverter efficiency, and discharge coefficient. With this information, you can determine the Amp Hours (Ah) of your UPS.

If we have:

  • W: Device Load = 100W
  • T: Time on backup power = 8 hours
  • V: Battery Voltage = 120V
  • E: Inverter Efficiency = 90%
  • C: Discharge Coefficient or Power Factor = 0.8
  • Ah: Amp Hours = ?

Amp hours of the UPS = (W × T) ÷ (V × E × C)

Ah = (100 x 8) ÷ (120 × 0.9 × 0.8)

Ah = 9.25

What Is the Backup Time of a 100 Ah Battery?

Battery backup time (T) = Ah x V/W

Using the same variables above, then if we have:

  • W: Device Load = 540W
  • V: Battery Voltage = 120V
  • Ah: Amp Hours = 100 Ah
  • T: Time on backup power = ?

T = 100 x 120/540

T = 22.2 hours

The battery backup time calculation formula

The “battery backup time calculation formula” is a way to calculate battery backup power. The formula for the calculation can be found in this video:

Here are some good universal power supplies with extended backup battery capability:

APC BE600M1 UPS Battery Backup Power Supply and Surge Protector 600VA/330W

SKE UPS Battery Backup Power Supply and Surge Protector 600VA/360W

CyberPower PR1000LCD UPS Battery Backup Power Supply 1000VA/1000W with 8 Outlets

Tripp Lite SMART1500LCD UPS Battery Backup Power Supply 1500VA/900W (Rack Mount)

As an Amazon Associate, SelectSafety earns commissions from qualifying purchases made through links in this post.

How Do I Calculate Battery Needs?

Battery need = daily load in kWh x days of electricity needed / kWh of usable capacity per battery

Here’s an illustration assuming one and a half days’ worth of electricity needed:

You expect an average daily load of 10 kWh. For 1.5 days of electricity backuip you will need 15 kWh of storage (10 kWh x 1.5 days).

The batteries you’re using each have 2.8 kWh of usable capacity available.

15-kilowatt hours divided by the battery capacity of 2.8 kWh equals 5.3 batteries.

How Do You Calculate the Power Output of a Charger?

Power output of a battery charger (in watts) = current in Amps x Volts x Efficiency

When power loss is ignored, the input and output power is the same. However, you must factor in the real-world output by multiplying it by an efficiency factor.

Here is an example for a typical laptop charger, assuming these values:

  • Amps = 2.31
  • Volts = 19.5
  • Efficiency = 75%
  • Power output (Watts) = ?

W = 2.31 x 19.5 x .75

W = 33.6

typically have output specifications of 19.5 VDC and 2.31 A.P (in watts) = 19.5 * 2.31 = 45 watts, which is the standard (approx.)

Here are some highly rated battery chargers to check out:

YONHAN Battery Charger 10-Amp 12V / 24V Smart Battery Charger

Schumacher DSR140 200/50/25/10 Amp 12V / 24V Wheeled Battery Charger

Black & Decker BC15BD 15 Amp 12V Bench Battery Charger/Maintainer with 40A Engine Start

How Much Power Does a UPS Battery Backup Use?

Most home-use UPS units consume very little power (3-10 watts per hour) to keep their batteries fully charged. They are normally rated between 92 and 95 percent in terms of energy efficiency.

Related Questions and Answers

How long will a 150 Ah battery last?

To calculate how long a backup battery with a specific amp-hours (Ah) will last, use this formula to convert its capacity to kilowatt-hours (kWh) then divide by the kW of load to ge the number of hours it will last.

Battery kWh = battery Ah × battery Volts/1000

Let’s assume we have these values:

  • Battery Ah = 150
  • Battery V = 24
  • kWh = ?

kWh = 150 x 24/1000

kWh = 3.6

Next we want to determine how many hours this battery will last with our anticipated load (in kW).

How long battery will last in hours (H) = battery kWh / 0.4 kW load from a lightbulb

H = 3.6 / 0.4

H = 90 hours

This assumes the battery is completely charged at the beginning. The battery will last longer if the load is lowered, and shorter if the load is increased.

How many watts is a 150 Ah battery?

To get the watts (W) from the amp-hours (Ah) of a battery, simply multiply the amp-hours of the battery times the battery’s voltage.

Battery W = battery Ah x battery Volts

150 amp-hour 12 volt batteries are popular for solar and backup installations. The watts available from this battery are as follows:

  • Ah = 150
  • V = 12
  • W = ?

W = 150 x 12

W = 1800 watts

How many amps is a 150 Ah battery?

The term “150 Ah” refers to a battery’s ability to provide a continuous current of 15 amperes over a 10-hour discharge period (that is, 15A x 10h = 150Ah).

How do you calculate battery watt-hours (Wh)?

Watt-hours is another unit of measurement for battery capacity (Wh). Amps and battery voltage are multiplied to arrive at Wh. In other words, the total energy stored in a 12V 100Ah (12-volt, 100-amp-hour battery) is 12 x 100, or 1200 watt-hours. 1200Wh is the maximum power that may be derived from either a 12V 100Ah or 24V 50Ah battery.

How long will a 100Ah battery run a 1000w inverter?

A fully loaded 1000-watt inverter could be operated for 34 minutes on a 12 volt 100 Ah deep-cycle battery with a consistent depth of drain of 50%. This estimate considers the 95 percent average efficiency of pure sine wave inverters.

How do you calculate how long a battery will power a device?

A battery’s lifespan may be determined by its capacity. Calculate the battery’s entire capacity and divide it by your circuit’s power to estimate how long your battery will survive.

How Many Watts Is 600VA?

To get the watts simply multiply the volt-amperes (VA) value by the power factor of your load.

If your 600VA load has a 100 percent power factor, you’ll need 600 watts to power it. In other words 600VA x 1 power factor = 600W.

If your 600VA load has a power factor of 0.8, you will need 480 watts to power that load. 600VA x 0.8 power factor = 480W.

Is VA equal to Watts?

An electrical circuit’s perceived power may be expressed in terms of volt-amperes (VA). DC circuits can’t make use of volt-ampere measurements; only AC systems can. Volt-ampere to watt (VA to w) conversion is easy. Watts are equal to VA times the power factor of the system.

What does 600VA or 1500VA mean?

VA means “volt-amperes” and is a UPS’s estimated maximum “instantaneous” output power. VA is a simple power rating that doesn’t include any reference to time or load. It does not inform you how long it will be able to provide a particular load (in watts) with power. It is just a quick indication of the maximum short-term load the UPS backup power system can handle.

VA allows different power supplies to be compared at a high level. A UPS rated at 600VA will output less than one rated at 1500VA.

The VA rating does not imply that it can output that amount of watts. A 1000VA power supply cannot output 1000 watts. Real-world wattage ratings for a power supply range from 50% to 75% of the VA rating in real-world use. This power factor (PF) of the unit must be taken into account to estimate the actual output of any UPS.

How long will a 1500VA UPS run?

A 1500VA UPS will generally provide backup power for between 10 and 60 minutes, depending on how many machines it must power and their wattage draw. You can generally operate a PC, networking equipment, and a monitor for about 10 minutes on a 1500VA UPS.

How many watts does a 40 amp battery charger use?

A battery charger that consumes 40 amps can charge 480 watts at 12 volts (actually a bit more but this is good for comparison sake).

How is LED power calculated?

If you want to know how much electricity an LED is using, multiply its voltage by its current in amps. Your LEDs’ power consumption is calculated as a wattage.


There is a way to calculate the amount of energy in a battery backup unit that is available to power your home. However, it may be better to rely on the data provided by the battery manufacturer and any information in the guidebook to help make this decision.

Related Tags

  • battery backup time calculator
  • 150ah battery backup time calculator
  • dc battery backup calculator
  • how to calculate inverter battery backup time

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Nick Klamecki, Author
About Nick Klamecki, Author

Nick Klamecki is a certified Fire and Workplace Safety expert with 15 years experience in product research and testing. He has a degree from U.C. Davis, is an active outdoorsman and spent years ensuring the safety of special needs children. Nick researches and tests workplace, industrial and safety products and provides advice on their safe use. Learn more about Nick here or connect with him on LinkedIn | Medium