Home » Miscellaneous19 November 2012

How to Calculate Inverter & Battery Backup Time?

Here is a quick guide on calculating inverter and battery backup time.

There are different capacity of inverters like 200VA, 250VA, 350VA, 400VA, 450VA, 500VA, 625VA, 1200VA, etc. The capacity of the inverter should be chosen based on how much watt of appliances you power at a time.

How to convert from VA to Watts?

Volts x Ampere = VA (VoltAmpere)
Volts x Ampere x Power Factor = kW (Watts)

Power Factor = 0.8 (which is the max. power factor of home standard)

If you want to run 200W of appliances then,

200 / 0.8 = 250VA

So, you should prefer inverter of 250VA or more. 300VA inverter will be the best choice.

Similarly, battery is an integral part of the inverter. The performance of the inverter depends on the connected battery and vice-versa.

The battery backup time in hours can be calculated using following formula:

Backup Time = Battery AH x 12V x N x Efficiency of Battery / Load in Watts

Battery AH = Ampere Hour Capacity of Battery
N = Number of 12 V Batteries needed
Efficiency of Battery = Generally it is 0.8, which is the max. power factor of home standard

Note: The inverter capacity is not used in backup time calculation.

Suppose, I have a 60A battery. My load is 150W. I have a single 12V battery. Generally in home standard, only single 12V battery is used.

Backup Time = ( 60 x 12 x 1 x 0.8 / 150 ) hours
= 3.84 hours

Calculate backup time by yourself

Hope this helps. Thanks.

Further Reading: How Much Power Does My Laptop Really Use?


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  • Bamidele Oluwafemi


  • Daniel

    Cheers Sir good mathematics.

  • Koomaresun

    Nice explanation!!

  • Rajeev

    I have two old but functional 100 AH batteries, my max electricity consumption would be 250 W. according to your calculation 312 to 416 va inverter would be sufficient (Power factor .8 to .6). My query is can this inverter charge my batteries? or should I go for higher capacity inverter? Provided both the batteries need to be used or any additional charger are available? I would highly appreciate your help

  • parveen arora

    As per my suggestions mineral water and boil water is benefits for long power backup in inverter and long life performance automatically you would see better performance in inverter battery.

  • alradhi

    3ph 380 volt 3000va load ,how to calculate quty. of battery if the backup time 2hrs?

  • Nepal store

    If i got the 360 AH battery and 1500va inverter how many hours to fully charge the battery

  • Pradeep

    at full load 5hrs backup.
    full load means 1200 Watts

  • Iits Pravin

    I have 27KVA load. 2 battery banks with 50 battery on each battery bank. battery is of 12v each. Battery has been used for 5 years from date of installaiton. I want to know that how much backup does this system gives?

  • Am planning for home UPS, Am looking at 6-8 hours of backup. So if i go for Microtek UPS SEBz 1100VA Sinewave and Exide Tubular Battery 150Ah (2 nos.), will i get 6-8 hours of backup? Or should i have more battery for it?

  • kamran hyder Mahar

    I cant to run Air conditioner inverter taking 3.5 amps of 220v, 770watt approximate
    I have 4 battery of 250ah 12v each
    48v inverter inverex 4kv
    How many hour can run this AC

  • Ravi

    In all such posts and articles many readers wish to have info as seen above. However numerous questions are asked but I dont see any replies! Wish there is some place where we can get answers and not merely questions

  • anil

    i have 12v 7ah battery then i used 2*10w8 then what is the backup of battery if we used countiously

  • If you have 7 AH battery and 20 Watt load then the battery backup time is 3.36 hours. I have added a form in this article. Using it, you can input battery AH and Load in Watts and calculate the battery backup time by yourself.

  • Hi Ravi,

    I have added a form in this article. Using it, you can input battery AH and Load in Watts and calculate the battery backup time by yourself.

  • 1100 VA * 0.8 = 880 Watt
    Battery Ah = 2 * 150 = 300 Ah
    So, with 1100 VA inverter and 150 Ah battery, you can use maximum of 880 Watt load for 3.27 hours. If your load is less than 880 Watt then it will give more backup. Like, if your load is half (440 Watt) then it will give double backup (around 6 hours).

  • You can use maximum of 1200 Watt with 1500 VA inverter.
    ( 1500 VA * 0.8 = 1200 Watt )

    So, if you continuously use 1200 Watt power then your 360 AH battery will be 2.88 hours backup. If your load is less then you will get more backup. Suppose, if your load is 600 Watt then you will get 5.76 hours backup.

    I have added a form in this article. Using it, you can input battery AH and Load in Watts and calculate the battery backup time by yourself.

  • Battery AH : 250 AH
    Number of Batteries : 4
    Load in Watts : 770 Watt
    Backup Time: 12.47 hours

  • Ravi

    Thanks Much appreciate

  • sweet4dreams

    Hi Mukesh, thanks for this excellent explanation, first time I understood completely. I am located in the Dominican Republic, where inverters still are VERY necessary. Just to verify: Got an inverter system with 7.5 kw, 24 Volts. Got 4 inverter lead batteries a 6 Volt with 225 AH each. So, one bank of batteries has 24 volts and total 225 AH (hope i calculated this right). There are around 2000 watts connected. If I use your calculation, this would be: Battery AH (225) x Volt (6) x number of batteries (4) x efficiency (0.8)/ used watts (2000)= 2.16 Hours, which is more or less correct in my real life usage. Now comes the controversy, for the same system, how long would be the charging time to get the batteries full again? could you provide me with this formula also?, thanks.. And another question; I have a power delivery circle around 4 hours yes and 4 hours not, i assume the batteries never get charged full. I need more than 2.16 hours, so, if i buy 4 same batteries as second bank with this short charging times, would this be helpful? and if not, what i can do in that case to get longer backup time. Thanks in advance for answering.

  • Samuel Aniefiok Patrick

    Thanks for the post

  • Kingsley Okechukwu Okpaleke

    i need to power my fridge 114w can 1.2KVA inverter handle it for me?

  • Sulaydeaai

    I have a 3 kva inverter and 150ah 2 nos battery and my power consumption is 2400w so how much time I will get a power back up ?

  • Arif Nabed

    1.2 hour

  • The way I have learnt about the backup time from a battery using an inverter I got 4 hours backup time from your calculation and from my calculation I got 1.25 hours which is correctly I am getting from UPS. So, I think there is something wrong or calculation missing in your calculation, i think your calculation could be correct with a DC voltage. In my calculation, i had to first convert the amps to AC amp to find the correct backup time. Please correct me if I am wrong

  • For finding correct backup time i think you need to first convert the watts to AC amp then convert them to DC Amps then you’ll get the correct backup time

  • Volts x Ampere x Power Factor = kW (Watts)

    Here you should remove small ‘k’ from here it will confuse other because it should have to be in watts it is looking like killowatt

  • i think above formula could correct for finding backup with DC Amps onlys

  • Naga Nathan

    Super sir

  • Solomon Alozie

    inverter is not considered when looking for back-up time. only the load and battery. in this case, you have 150ahx 2x12V x0.8=2440/2400=1.2hrs

  • Solomon Alozie

    I have 1200W load, 6nos of 4nos of 200Ah 12V batteries, how do I calculate nos of solar panel required to power my load for 8hrs?

  • Madhav Pulivendala

    Something seems amiss. Doesn’t the discharge of battery level come into picture anywhere ? Fully charged 12v battery is 14.4 volts. To what extent can we let it get discharged, obviously not zero. Can we let it go down to 10 volts ? Where does this factoring come in ?

  • Faraz

    I have 900w inverter and 200Ah battery and i m getting only 4 to 4.5 hrs only on 150w load ?????? Y what’s problem there???

  • Sudarshan Poojary

    I want to buy a 1100VA/12V Home UPS and 3 Solar Panel Each 150W and also buy a 2nos of 150ah 12v battery. So this 3 solar panels sufficient to charge this 2 batteries? Also sufficient this inverter????

  • Richard Edam

    whats the formular for converting battery from dc to AC?

  • Ochuko Sagua

    there are some factors to consider.

    1.) Depth of Discharge (DoD) (50% for Lead Acid Battery). Meaning of your 200AH only 100Ah is usable.

    2.) Efficiency of your inverter (We can assume 80%)
    3.) Power Factor(P.F) of your load (We can Assume 80% also)

    so your 150W Load is Approximately (150/0.8 VA = 187VA)
    as you are using inverter your battery supplies both the real and reactive load which drain from the battery. so to the battery it is suplying a load of 187W

    Usable energy from battery = Volt*AH*DoD = 12*200*0.5 = 1200WH

    Inverter Efficiency affects this = eff*WH = 0.8*1200 = 960WH

    960WH running 187W you have = 960/187 = 5Hours which is close to what you have. in reality your Power factor could be as bad as 0.5 sometimes.

  • Ochuko Sagua

    theoretically, these two panels are only sufficient if you have more than 6 hours of peak sunlight.
    Your usable energy from the batteries is equivalent to half your AH due to the 50% Depth of discharge limit. meaning your useable energy is 150ah x 12V = 1800WH. charging at 300W (Solar panel power available), you will need 6 hours to fill up. but this will most likely need a longer time to fill up as lead acid batteries generally need a long time (about 16 hours) to fill up.

  • Ochuko Sagua

    Using the nominal voltage of 12V covers for this voltage variation during use

  • Ochuko Sagua

    In practice you will have less than an hour of back up. Approximately 40mins. if you consider inverter efficiency(90% Used) and Battery Depth of Discharge (50%).
    [(AH x V x DoD x Efficiency)/Power] Hours = (300 x 12 x 0.5 x0.9)/2400 = 0.6Hours = 40 mins

    Load power factor wasnt considered. if the load is not a pure resistive load like heaters, the time will be lower

  • Ochuko Sagua

    Yes it can. But I will advice you turn on your fridge 1st before other devices when using the inverter as the start up power may be x5 of the rated power on the device.

  • Ochuko Sagua

    Lead acid batteries charge slow. from a full discharge, you need about 12 – 16 hours to charge to their absolute fullest. but you can get to about 80% charge in 8 – 10 hours though. this is a limitation of leads acid batteries.

    charging time is calculated thus(70% Charging absorption efficiency for Lead Acid battery);

    (Battery capacity / (Charging current * 0.7))

  • Shahzeb Malik

    Dear sagua
    I have 4 x 200 amp lead acid dry batteries.
    Inverter of 3 kva 48 volts
    1 of them is not charging properly and 3 are ok
    The batteries are almost 3 yrs old.
    The max backup my system is giving is 15 mins on load of almost 30 percent.
    What to do to get more back up time

  • Parteek .s

    Can i charge 7.5 ah ups battery with home inverter

  • akash labhe

    Hello sir, i have one solar panel of 250W and lead acid battery of 75Ah & 12V can u please tell me what rating of charge controller should i use to charge this battery?