Battery size calculator

To calculate approximately how big a battery and inverter you require for your power backup needs in Amp-hours (Ah), enter the power of each appliance and the average length of loadshedding time. The first couple of lines are given as an example; just click in the boxes to replace the values with an estimate of your own power consumption.

To calculate approximately how big a battery and inverter you require for your power backup needs. in Amp-hours (Ah), enter the power of each appliance and the average
length of time loadshedding time. The first couple of lines are given as an example; just click in the boxes to replace the values with
an estimate of your own power consumption.

Battery size needed calculator

Appliance Watts   Amp hour
Hours downtime required
We’re also catering for a 90% effecency in inverter, baterry and charger
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Amp Hour battery

20 Responses to “Battery size calculator”

  1. Fabian says:

    can I get a 2KVA inveter with a sealed 12v 200A battery for the calculations shown above to back up for 10hours
    tv 120
    lights 120
    fan 240
    radio 220

    i need help on this ASAP

  2. edy says:

    if those calculation need 3x200w batt and can be used to power 700w for 10 hr…
    is it already 50% usage only ?
    how long the batts took for re-charge ?

  3. Da B says:

    Hi Admin.
    Thanks for the calculation on battery charger and duration for full charge.
    Please I would like to know how you came about the formular for charging time. Particularly is the 0.7 value you use to multiply the AH before dividing by the charger Amp output.

    I am impressed with all your explanation as this page solved a lot of pressing issues I had.

    My question is about using a solar panel to charge batteries. Please kindly illustrate charge time using a case study of a 40W solar board ( you choose the Amp) and a 12V/200AH deep cycle battery.

    I wait your response

  4. admin says:

    You will need 3x 200ah batteries to maintain that load for 10 hours.

    Your total load is 700w / 12v battery = you need a 58.3 aH battery to keep your system running for 1 hour, to keep it running for 10hour is 583ah, and that is basically 3×200W batteries connected in parallel.

  5. admin says:

    I have not catered for % usage in the calculation, and the charge time of the battery. The % usage will depend will depend on how much your specific inverter drains the battery. I will post the calculation shortly

    How long a battery takes to recharge depends on the charger used. The formula for working out how long a charger will take to recharge a battery from 100% flat state is. The batteries Amp Hours x 0.7 divided by the chargers charging current in amps. If we use a 8AMP battery -

    200ah battery x 0.7 = 140
    divided by 8AMP: 140/8 = 17.5 Hours

  6. admin says:


    so now if already charge the batt in 17.5 hours for a 200ah 12v batt.

    now i want to use that batt. how to calculate the wattage i am able to use for that same hour ?

    and what kind of inverter should i choose to achieve the result from the above case ?

    thank you

  7. admin says:

    Edy. The formula you are looking for is POWER divided by VOLTS x CURRENT, so this is equal to WATTS divided by VOLTS x AMPS. Using this calculation I can work out the following. Watts = volts x amps, so 12V x 200 amps = 2400Watts. So for 1 hour I could get 2400Watts out of a fully charged 200Ah battery. You would then need at least a 3000W inverter.

  8. admin says:

    The calculation used is the accepted norm between Battery and Battery Charger Manufactures Globally. The 0.7 calculation is the accepted level, basically 70% of the AH rating. A standard needs to be used as there are many factors like battery resistance and different charging currents that effect the actual charging time. I am not an expert on Solar but I would think that each Solar system must stipulate what current (Amps) the panel will supply. Once we have that information then the same formula should work as the basics are still the same – a charger and a battery

  9. admin says:

    Hi Admin,

    I am back again with a troubling question. I have a 2500w/12v inverter with 10Amp charger internally. I connected 4units of a 12v/200AH battery.
    These batteries are all in parallel.

    I am wondering, how does the charger charge these batteries?? do we still use the same calc used earlier.

  10. Jason Galea says:

    What kind of a battery do i need for a 1100 watt watering pump that is 4.6 amps

  11. admin says:

    A deep cycle battery, the size will depend on how long you want to keep the pump running from the battery.

  12. Sujai says:

    I have 75 X 2 -150 AH exide tubular batteries with me.I intend to buy APC 800 VA inverter (12 volt). As per their brochure, it is standardised to use only upto 120 AH battery. So will this be a problem if i buy that one or should i go for a 1000 VA inverter which recommends only 100 X 2 AH batteries. Pls advise.

  13. the-tech-guy says:

    Dum fuck, where do you find these idiots An inverter can have 1000 batteries connected if you want it to run for a VERY long time. Overheating is only linked to how much watts it uses. If the Inverter is an 800W and you are drawing 800 watts then it will run hotter than if you draw 500 watts.Phone the supplier of the shit APC 800 VA Inverter and not to waste my fucking time.

  14. tayo says:

    I intend to run 1HP split unit airconditioner + one computer system using 2 flat screen monitors on a 5KVA/96V pure sine wave inverter. how long will it run?

    My other choice is 2 nos 1HP split airconditioner + 0ne computer system using 2 flat screen monitors on 7.5kva/120V inverter,how long will my batteries run this load?

  15. Dewole10 says:

    Take it easy on the guy…

    Sujai, your inverter (APC) advises not to use more than 120Ah batteries because they know the charging current of the device cannot handle the bigger batteries in good time and would probably never charge them to completion.

    If you have a large number of batteries, a small inverter will not get them all to charge completely in reasonable time so you will need to buy an additional charger.

    For example, 800watt inverter charges at 8amps; if you have a 200Ah battery connected and it’s fully discharged, it will take almost 20 hours to charge fully. If your increase your number of batteries to 3 (in parallel), then you would need thrice that amount of time for charging. Instead of waiting 60 hours for the 3 batteries to charge fully, you could buy a 15amp charger to increase your total charging current to 23amp and thus reduce the amount of time required to charge.

  16. admin says:

    lol its all “tung in the cheek”. Totally agree, I always use an external charger, the battery is always the systems weak spot. Use a decent charger and you’ll extend battery life and system up time.

  17. admin says:

    Use the calculator on this page

  18. Rathina Balaji says:

    I recommened the following calculation to find your battery size so that you can have bettery life with its maximum capacity.

    1) Calaculated the Load(in Watts) that has to be operated.
    2) Apply the following Rule of thumb
    100 Watts(Load) = 10 A (of Battery Capacity)

    3) Fix your time needed for operation of the appliance(your load) on the battery.
    4) c = ( T * I^1.1 )/0.8

    T – Time of discharge
    C – Battery size or Battery Capacity
    I – Discharge current.
    I have to operate a load of 500w for 3 hours.So the calculation goes this way

    Load = 500 Watts
    By Rule of thumb 100 Watts = 10A
    => 500 Watts = 50A

    Dishcarge time = 3 hours

    Thus battery size C = ( 3 * 50^1.1 ) / 0.8
    = ( 3 * 73.93 )/0.8
    = 277.23 AH battery capacity

    With this size of battery bank your life of the battery will be maximum.

  19. Ndy says:

    pls i have an inverter that uses 24v battery and is 2.4kw inverter.i have 4 batteries of 12V 200Ah battery, can i use this batteries on the inverter if i connect two batteries in series then in parallel with the second batteries. will it blow my inverter?
    Thank you.

  20. admin says:

    Yes , that will work fine

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