# 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.

Note:

• 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).

Beware:

• 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

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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