What Size Solar Panel do you Need to Charge a 12v Battery?

What Size Solar Panel do you Need to Charge a 12v Battery?

Twelve-volt batteries have become a popular choice when you need power for an RV, boat or car. Hooking a battery up to solar panels has also become a widely used option to charge the battery. 

But how do you know what solar panel wattage is necessary and how much time it would take to charge a 12-volt battery?

Answering this question can be found by investigating the type of solar panel setup being used and calculating from there the number of solar panels required to charge the battery adequately.

Using Solar Panels to Charge a 12-Volt Battery

One of the main advantages of using solar panels to charge a 12-volt battery is the ability to harness power from the sun when a power grid is unavailable. 

Determining the type of solar panel to use for this setup can differ based on the kind of panel utilized. Understanding more about the various options and examining their specs can help answer this question.

Flexible Solar Panels

Many individuals prefer utilizing flexible solar panels due to the easy portability and mobility they offer. This type of solar panel is also an excellent choice to use in areas with irregular surfaces as they will typically provide a 30° arc. 

Using a flexible solar panel rated at 100 W and up to 18 V is one way to charge a 12 V battery.

Fixed Solar Panels

Having an adequate amount of flat surface should make it easy to use fixed solar panels. These can be mounted on the top of an RV or roof or anywhere containing a flat surface. 

Using a 100 W mounted solar panel should also provide enough voltage for a single 12 V battery.

Folding Solar Panels

Depending on the requirements needed, folding solar panels can also be used. Large folding solar panels will offer about 20 to 80 W and are held up by a steel stand. 

Using the large-sized panels should be adequate to charge a 12 battery. However, small-sized folding solar panels will be tougher to use as they have specifications near 6 to 30 W per panel.

Finding the Correct Solar Panel

Rigid, fixed and flexible solar panels are available in several different sizes and have wattage outputs varying from 20, 50, 100, 120 and 200 W. Charging a 12 vote battery requires a solar panel with the capability to charge this type of battery.

Determining the solar panel wattage and hours needed to charge a 12 V battery entirely depends on two factors. The first factor is related to the number of ampere hours (or “”) amp hours (symbol:Ah) to be replaced daily to charge the battery fully. 

Here are some optimum operating current ratings for panels with various wattages:

  • 20 W solar panel: 1.15 Ah
  • 50 W solar panel: 2.75 Ah
  • 100 W solar panel: 5.89 Ah
  • 120 W solar panel: 6.67 Ah
  • 200 W solar panel: 11.76 Ah

If a battery is rated at 100 Ah, it takes 10 hours to draw 10 A from the battery. With the same specs, it would take 20 hours to draw 5 A from the battery. Determining the number of hours to fully charge a battery can be completed by using the following formula:

(Optimum operating current) x (X hours) = 100 Ah

Using a 100 W solar panel takes roughly 18 hours to fully charge according to the formula: Hours = 100 ÷ 5.56 = 17.98. If a person wants to fully charge their battery in a day, it will take a minimum of three 100 W solar panels to complete this task.

The second factor to consider when charging a battery to full capacity is the number of hours of available sunlight each day. Determining the number of solar power needed to charge a battery requires the use of the following formula:

12 V x 100Ah = 1200 W hours

Knowing the number of solar panel watts needed can be found by dividing the 1200 W hours by the available sunlight each day. If the best sunlight is shining for six hours each day, 200 W would be needed, based on the formula (1200 divided by 6 = 200 solar panel watts). Even then, it’s usually best to utilize a wattage slightly higher than 200 W as most solar panels don’t generate the maximum output indicated by their specifications. In this example, it would probably be best to use 220 to 240 W in total to make up for this difference.

Using two 120 W solar panels would likely be the most beneficial setup in this scenario to reach the 240 W needed for a full battery charge. Otherwise, it will take five 50 W solar panels to get the required number of watts required to charge the battery in a day. Flexible solar panels are available with 50, 100 and 120 W specifications and can be used together to achieve a full charge. Using a 200 W fixed solar panel will work but may not fully charge the battery by itself. This factor depends on available sunlight, which changes throughout the year.

Other Factors Affecting the Ability To Charge a 12 V Battery

Using a solar charger controller is also required to safeguard the battery from receiving an excessive number of watts. It acts as an on-off switch and turns off when the battery has a full charge. It also stops the battery from releasing watts when the sun goes down, which will occur if a solar charger controller isn’t used.

Finding an adequate amount of sunlight daily is essential to help ensure the battery can get charged before the available amount of sunshine disappears. During the summer, when the sun is out for most of the day, it should be easier to fully charge a 12 V battery without worrying about it disappearing too soon.

Looking at the table below shows the number of hours it takes to charge a 12 V battery fully for different solar panels and their respective wattages. Using a 200 W solar panel is ideal in the right conditions. Otherwise, using two 120 W solar panels may be an excellent option.

Solar Panel WattageHours To Fully Charge
50 Watts36 Hours
100 Watts18 Hours
120 Watts16.2 Hours
200 Watts9 Hours
What Size Solar Panel To Charge 12v Battery?