What Size Solar Panel Do I Need To Charge A 12V Battery?

Should you want to take advantage of the benefits solar energy has to offer, but don’t know where to begin, read on. Whilst solar panels are the better option for you and the planet, understanding the terminology and everything involved is far from easy. You need to know about amp-hours, amp-rating, wattage, voltage, batteries and solar panel types.

Being able to calculate the solar panel size needed to charge the electrical item in mind is also helpful, In fact, the more you read, the more complex things get. This is pretty annoying when all you want to do is charge your 12v battery and do your bit for the planet.

Don’t despair. This guide tells you everything you need to know about charging a 12v battery with solar panels. This article gives you the must-know facts without the confusion. Learn the necessary aspects to effectively charge your battery and which size of solar panel you need to use. Don’t forget to check out our final thoughts section for additional information too.

What You Need To Know

What You Need To Know

There are several steps you need to follow In order to get the right solar setup to charge your 12v battery,  These include

Voltage – the voltage is your battery’s capability. A 12v battery requires a minimum of 13.6 volts to fully charge. You should know the voltage of both the solar panels and battery

Wattage – Find out the wattage of your solar panel. A 12v battery generally needs a solar panel with a wattage of between 100 – 120 watts.

Amp-hours – knowing the amp hours (Ah) is important. Amp hours is the amp rating of how many amps a battery gives. Basically it lets you know how much energy the battery will provide in an hour. Once you know your battery’s amp-rating you can determine the amp-rating required by your solar panel.

Weather conditions – The weather should always be factored in. Estimating the available hours of sunlight and allow for the fact that conditions will not always be optimal.

Calculating The Solar Panel Size Needed 

Solar panel power is provided in watts. In order to determine the energy they deliver to a battery (amp-hours), you must divide the Watts by the voltage of the panel.

1. Calculate Amp-rating

To find out the amp-rating of your solar panel, divide the wattage by the voltage. A 12v solar panel with a wattage of 100 delivers 18 volts. From this you can deduce the amp rating by dividing the wattage by volts. In this instance it would be: 100w / 18 v = 5.5 amps per hour.

2. Convert Amp Hours To Watt-hours 

This is done by multiplying the Ah number by the voltage. Once you have the figure you must factor in the amount of available sunlight.

3. Divide Watt-hours By Approximate Hours Of Sunlight

This is fairly simple to do. However, the weather is unpredictable and therefore results may vary slightly. 

 Once you have divided the watt-hours by hours of available sunlight, you will have the total number of watts your solar panel should provide.


Lead-acid batteries are by far the most popular batteries used. However, lithium batteries are also used.

12 Volt Batteries

12V batteries can range between 5 Ah and 200 Ah. To calculate the size of the solar panel needed for your battery you have to look at the following aspects:

Battery capacity (Ah)

How much is used on average

How quickly you wish to charge your battery

Solar panel sizes range from 10 – 100 watts. 

How Long Does It Take To Charge A 12v Battery?

Charging a completely drained battery can usually be achieved in 5 hours. However, it depends on the battery and the weather.

A 100W solar panel can produce up to 400 watt-hours of energy each day, if the total output does not surpass 6 amp.

Basically, the larger the solar panel, the more quickly your battery will be charged.

Other Components Needed

Other Components Needed

Solar charge controller – A solar charge controller prevents the battery from being harmed through over charging. The size of your charge controller should suit the amps of the panel.

How To Charge Your Battery Using Solar Panels

You will be pleased to hear that The process of charging your battery is far easier than trying to calculate the solar panel size. To set up your solar system all you need to do is connect the battery to the solar panel. Next, set up the panel as you would normally do.

Final Thoughts

It is recommended that to get the optimal efficacy of your solar panel, it should be placed where it can obtain the most sunlight. Experts suggest the panels should be placed at a 45-degree angle, facing directly towards the sun. A flat surface is also highly recommended.

When choosing the size of your solar panel it is advised to go slightly above the calculated solar panel watts. This allows wiggle room in the event of the weather turning sour and reducing your energy production. 

It is possible to charge a 12v battery with a 5 Watt solar panel. However, this is not particularly efficient for most needs. Most people prefer to use a larger size solar panel to give them the best solar setup. Also, selecting larger sized solar panels means you don’t need so many to achieve the desired amount of energy.

As mentioned previously, larger panels produce energy at a faster rate. However, picking the size to correctly match your battery capacity is essential. Failure to do this will result in inevitable problems, potential over charging and harm the battery.

Solar generators are a great alternative for those who don’t want the headache of working out the sums for factoring batteries. Solar generators remove the need for batteries and separate components. Thus, life becomes instantly easier.