Battery size calculator for a solar panel size (watts)

Use this calculator for an indicative battery capacity (Ah) suited to for charging with solar panel sizes in the 10 – 370W.

This is useful when for example when  you want to know what size of battery to connect to a  100W solar panel or any other solar panel size.


  • You need to specify the average sunshine hours for your location
  • Battery size is approximate; calculator is based on 12V battery size. For a 12V solar panel, number of batteries will be 1. For 24V battery, number of batteries will be 2.
  • Use the closest available battery size equal or more than the calculated battery size.
  • Number of batteries shows the number of batteries that need to be combined to match a solar panel voltage

Worked example

Using the calculator above, 100 W solar panel will charge an estimated 43Ah deep-cycle AGM battery or 39Ah Lithium in  about 5 hours (average daily sunshine hours for a location).

battery size calculator 100W solar


  • If the battery is much smaller, the excess power from the solar panels will have no storage space and will therefore  be wasted.
  • On the other hand, if the battery is much bigger, the solar panel cannot fully charge the battery which shortens its life in the long term!

Formula for calculating an indicative battery size suited to a solar panel capacity

The battery size (Ah) is calculated from, (I x h) ÷ DoD where:

  • I is the solar panel charging current
  • h is the  average hours of sunshine for your location
  • DoD, Depth of Discharge is the usable capacity of the battery – 50% for Flooded or AGM and 90% for Lithium.

Calculation assumes 15% of power generated by the solar panels is wasted during the process of transmitting and storage into battery energy.

Table showing approximate battery sizes for selected  solar panel size (W); based on average 5 sunshine hours  per day

No.Solar panel (W)*Estimated Battery Size (Ah) Deep-cycle flooded or AGM*Estimated Battery Size (Ah) Lithium

Note: Battery capacity is approximate! These are not standard battery sizes – you need to find a standard battery size close to indicated value

Tips to improve solar panel and battery performance

  • Use  a charge regulator to properly charge your battery and prevent damage  from over-discharging  below the recommended capacity
  • Position your solar panels to avoid shadows being cast on it. These can significantly reduce the power produced by your solar panel.

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