Solar Charging Car Battery

How Many Solar Watts Will Charge a Car Battery?

You can use solar panels in various ways, such as providing your home with low-cost power and giving you access to electricity if you’re looking to go live “off the grid.” One consideration for people living in RVs and vans is using solar panels to charge their car batteries. While it is possible to charge a car battery with solar panels, how many solar watts—and solar panels—do you need to charge your car battery efficiently?

The number of solar watts you need to charge your car battery will depend on your battery’s amp-hour capacity and how many volts you charge it at. You will need to multiply the voltage by the amps to determine the right solar wattage.

In this article, I’ll help you understand how you can use your solar panels to charge your car battery in further detail. I’ll give you some examples you can use as a reference to determine how many solar watts you need to recharge your car batteries and how long your car battery will take to charge.

How To Determine How Many Solar Watts You Need To Recharge Your Car Batteries

As mentioned above, to determine how many solar watts you need to recharge your car batteries, there are two numbers you will need to know first: the amp rating of your batteries and the voltage rating of your solar panels.

Solar panels are generally 12V, so, for this example, we’ll consider a solar panel with a 12V rating. If your car batteries have a 100 amp-hour rating, you can now multiply these two numbers together: 12 x 100. That means your batteries will take 1200 watts to charge fully.

How Long Will It Take Your Car Batteries To Charge Completely?

Once you’ve determined how many watts you need to charge your batteries, you should calculate how long it will take them to charge completely.

The first step is to look at the rating of your solar panels. RV solar panels tend to be between 100-400 watts. Let’s consider a solar panel that is 200 watts for this example.

The true power of a solar panel is generally 75%-80% of the advertised wattage. Let us consider that your solar panel has a true power of 75%. If the advertised wattage is 200 watts, this means the true power is 150 watts.

In this case, your solar panel generates 150 watts per hour of sunlight it receives. The United States gets 3-5 peak hours of sunlight on an average day. If your panel receives 4 hours of sunlight in a day, it will produce 600 watts of power per day.

Since your car batteries require 1200 watts to charge completely, they will need 2 days to charge completely (or 8 hours of sunlight).

However, it should be noted that most people travel with more than one solar panel. Your car batteries may charge much quicker depending on how many panels you have and what their wattage is.

For example, let’s consider 3 solar panels of 400 watts each. Each panel will have a true power output of 300 watts (75% of 400). Since there are 3 panels, your panels will generate 900 watts per hour of sunlight.

Since your car batteries need 1200 watts to charge completely, it will take 1 hour and 20 minutes to charge fully.

Is It Possible to Overcharge Car Batteries With Solar Panels?

It’s possible to overcharge a car battery with solar panels. Overcharging can cause car batteries to get overheated, reduce their lifespan, and, in severe cases, destroy them altogether.

However, overcharging via solar panels isn’t a significant concern. This is because most people use a solar charge controller when using their solar panels.

Solar charge controllers regulate the voltage produced by solar panels. When used in concert with charging batteries, it ensures there’s no worry about your car batteries overcharging.

How Many Solar Panels Should You Carry When Traveling in an RV?

If you’re traveling in an RV, you’ll want to be confident that you have enough solar panels to provide you with the power you need. After all, unlike residential solar panels, you cannot simply order additional panels when you’re on the road.

In general, your solar panels should provide you with at least 1800 watts of power if you plan on running your air conditioner on solar power for at least 8 hours a day. Keep in mind that this power consumption is simply for your air conditioner—if you’re living the van life and have other appliances you need to run, you will have additional power requirements.

Similarly, you’ll need additional power if you expect to use your solar panels a lot to recharge appliances, including your car batteries and laptop. Most RV owners will carry 3-4 200-watt solar panels with them if they aren’t running their AC on solar power.

Assuming 4 hours of sunlight a day, this will give them 1800-2400 watts of energy per hour of sunlight, assuming their panels give them a 75% true power output. Keep in mind that this requirement will go up depending on how many appliances you plan on using and for how long you plan to use them.

Additionally, if you’re running your air conditioner on solar power rather than generator power, you’ll need additional panels, as discussed above. In that case, you should carry about 6-8 200-watt solar panels rather than 3-4.

Final Thoughts

Determining how many solar watts you’ll need to charge your car battery will require some math. You will have to know the voltage of your solar panels and the amp hours rating of your car battery. You then need to multiply the two numbers to determine how many solar watts will be required to charge the battery fully.

Now you can determine how long it will take to fully charge your battery with your current solar set-up by, considering the true power output of your panels and the number of peak sunlight hours you receive.


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