What Size Solar Panel Do I Need to Charge a 12V Battery

Road trips are becoming more fun and memorable than ever. Every year, we witness new improvements in the electrical devices that can be fitted inside RVs.

When it comes to the power source, it’s possible to utilize shore cables, generators, or solar panels. I don’t know about you, but I’ve always favored the latest because of its better potential and unmatched portability.

Today, I’ll dive deeper into the solar setup to help you put up one for yourself. I’ll mainly talk about what size solar panel do you need to charge a 12v battery, and I’ll also discuss some related topics that every RV user should know.

The Short Answer

In the summertime, with a 100Ah 12V battery, I’d recommend a 160W panel that measures 26” by 58”. You can also opt for two 100W panels with each one measuring 21” by 47” to generously cover your power needs.

In the wintertime, since you’ll get fewer hours of sunlight, you may need to change that setup. You can choose from two 160W panels, three 100W panels, or five 50W panels.

How did I calculate this? Continue reading to find out!

First Off, It’s Not Only About the Volts!

Before we get to the important details, you have to understand that the battery voltage isn’t the only factor to consider. To clear things up, let’s quickly summarize some important technical jargon behind electricity and batteries.

The intensity of the electrical current is measured by amperage (A). The force that drives that current in wires is denoted by voltage (V). When we multiply amps by volts, we get the wattage (W), which describes the overall power.

As you already know, any battery has a certain capacity dictated by the chemicals inside it. This capacity is typically measured by amperage per hour (Ah). For example, a 100Ah battery is capable of supplying 20 amps over 5 hours, or 25 amps over 4 hours.

How Does a Solar Panel Connect to an RV System?

To know the suitable solar panel size, we need to take a quick look at the overall solar setup. Generally speaking, any solar setup has 3 parts: the solar panels, charge controller, and the battery.

The charge controller is probably the most crucial part of the setup. When your batteries are charged at 80%, they send the solar power directly to your RV circuit to prevent overcharging. Some manufacturers usually push this percentage up to 90%, but it rarely approaches 100%.

As the battery loses power, the charge controllers will shut down the system with any connected devices before the battery gets fully depleted. And again, this is supposed to drive the battery’s lifetime to the max.

How to Choose the Size of the Solar Panel?

Now that we’ve covered the basics, we can start discussing the actual details. To choose a suitable solar panel, you should follow these steps:

Step 1: Calculate Your Maximum Battery Capacity

As I said earlier, the capacity of a battery is measured by Ah (amperage per hour). But to carry on with the rest of the steps, we’ll need to calculate this value in wattage.

To do that, multiply the capacity by your battery voltage. Assuming that you have a 100Ah battery operating at 12V, your maximum wattage should be 1200 watts.

However, with a charge controller installed, you won’t be able to use that full capacity. Instead, you may be restricted to anywhere between 80% and 90%.

For the sake of this how-to, let’s calculate based on 80%. Now your usable battery capacity should be 960 watts.

Step 2: Calculate the Sun Output

It’s important to figure out the average sunlight exposure in the area that you’ll travel to. In sunny environments, you can dial back on the number of panels since each one will work on full potential. In cloudy areas, it’s better to add extra panels to collect as much sunlight as possible.

In summer, we can expect an average of 6 hours of sunlight per day. Winter usually has 4 hours of useful sun, on the other hand.

So on average, when you divide the maximum battery wattage over the average hours of sunlight, you’ll find that you’ll need 160 watts/hour in summer, and 240 watts/hour in winter.

Step 3: Calculating the Number of Panels

. In the following table, you’ll find the approximate values of the commercially famous panels.

Wattage Width Length  Thickness
50 watts 21 inches 26 inches 1 inch
100 watts 21 inches 47 inches 1.5 inches
160 watts 26 inches 58 inches 1.5 inches

Now you can carry on with the calculation process based on the values we got in the previous steps. In summer, you can suffice by a 160W panel, two 100W panels, or three 50W panels. In winter, you’ll need two 160W panels, three 100W panels, or five 50W panels.

To make sure I’m perfectly clear, these numbers are merely provided for the sake of an easier explanation. You’ll have to calculate your own values based on your battery, sunlight hours, available solar panels, etc.

Before You Go, Think About the Battery Type

Contrary to common belief, the battery type can make a world of difference in your solar setup.

Commercially, you should be able to find three battery types: flooded lead-acid, sealed lead-acid, and lithium iron phosphate.

Between the three, lithium iron phosphate (LiFePO4) batteries provide the highest efficiency. Since they can handle higher amperage, they’ll recharge much faster, which will be super helpful in days with few sunlight hours.

Lead-acid batteries, both flooded and sealed, will probably overheat if you charge them too fast. Not only that, but their charge rate will also slow down as they approach the full capacity.

By the nature of the beast, the superior performance of lithium iron phosphate batteries comes at a tremendously expensive cost. But I think they’d be a worthy investment since they tend to live longer.

The Final Word

What size solar panel do you need to charge a 12V battery? It depends.

For a 12V 100Ah battery, one 160W solar panel or two 100W panels should be enough. Remember, this will probably change in the wintertime. You may need to add an additional panel in each setup.