In this article you will learn what size solar panel you need to charge a 12V battery.
It’s very common to charge a solar panel with a deep cycle battery. In fact, you’ll now see plenty of RVs and vans using this method to keep themselves powered up.
Choosing the right size solar panel is crucial to ensure you are able to deliver enough energy in a given period of time. Using underpowered equipment will leave you frustrated while going too large will simply be a waste of money.
So let’s take a look at the correct size solar panel for you, factors to consider before buying, and the best solar panels for 12V batteries.
The Importance of Batteries for Off-Grid Solar Panels
Solar panels do not just complement 12V batteries, they’re actually best friends.
In order to run an off-grid solar panel, you will need a deep cycle battery. So while a battery can work without solar panels, a solar panel can’t work properly without a battery.
This is because in an off-grid system, like on an RV or boat, you must connect the solar panel to the battery, then draw electricity from the battery via a power inverter. If you try to draw power directly from the solar panel the supply is erratic. Plus it enables you to store energy to use later.
What does this all mean for you? Well, this means that you’ll have ample choice when it comes to picking out a solar array for your 12V battery.
What Size Solar Panel to Charge a 12V Battery?
A 100 Watt solar panel could fully charge a 50Ah 12V battery in 8 hours and a 100Ah 12V battery in 16 hours.
Now we know the two work well together, you just need to know what size panel will do the job.
Technically, any size solar panel can charge a 12V battery. For example, you can get 5W trickle chargers to 200W portable panels.
Of course, a 200W panel is going to charge a hell of a lot faster than 5W.
The size you require comes down to two things – battery capacity and recharge time.
The larger the solar panel the quicker it will charge. And the bigger the battery the longer it will take to charge.
I’m not going to thrust complicated equations in your face because that just gets confusing. Keep reading to find real work experience and tips to help you decide.
In my experience, a 100 Watt solar panel delivers 6 Amps per hour (Ah) or 30Ah per day. This is less than the ‘maximum’ power because they never work at 100% performance due to weather, climate, and other factors.
We can use these average numbers to work out charging times compared to battery size.
Happily, battery size is marked in Ah. For example, you can get 50Ah, 100Ah, 200Ah 12V batteries.
Example 12V Charging Times
Let’s say you have a 50Ah 12V battery and 100 Watt solar panel. Knowing that you get 6Ah each hour of energy, it would take 8.3 hours to fully charge – (50 ÷ 6 = 8.3).
If you have a 100Ah battery with a 100W solar panel it takes over 16 hours to fully charge – (100 ÷ 6 = 16.6).
Knowing these numbers, you can use them to make your own calculations.
Here is a table with solar panel power and average Ah energy production. Remember, the final Ah number is lower than if the solar panel runs at 100% performance as it takes into account real world factors which reduce performance.
You can use the table below to calculate charge time with your 12V battery. For a full charge, just divide the total battery Ah capacity by solar panel Ah output for charging time in hours.
| Solar Panel Watts | Average Ah Output (Amps per hour) | Average Ah per day* |
| 50 | 3 | 15 |
| 100 | 6 | 30 |
| 150 | 9 | 45 |
| 200 | 12 | 60 |
| 250 | 15 | 75 |
| 300 | 18 | 90 |
*Remember a solar panel only works well during peak sunlight hours, it won’t charge 24 hours a day. More like 4 or 5 hours a day of good performance.
What are Amp-Hours (Ah)?
We’ve talked a lot about the Ah which is short for Amp-Hours. An Amp-Hours is simply a unit of electrical charge. It tells you how much electric energy a battery or solar panel can store or deliver in an hour.
For example, if you have a 50Ah battery that is the total storage capacity. So the total energy you can draw is 50Ah. If you draw a current of 50 Amps it would last 1 hour or if you draw a current of 1 Amp it would last 50 hours.
Solar panel size is measured in Watts so we need to do a conversion to find out how many Amps it can produce.
To do this you simply use the formula Watts ÷ Volts = Amps. Then knowing the solar panel Amps you can easily work out how long it takes to charge your 12V battery.
Other Factors to Consider when Charging a 12V Battery
There’s many unique parameters to take into account when charging deep cycle batteries with solar. To be honest, it always depends on your equipment, weather conditions, and energy requirements. Let’s look at the main things to keep in mind.
Discharge Level
Your battery won’t always be completely empty, in fact, you should avoid completely depleting a deep cycle battery. Running them flat can cause damage. Quite often you’ll only need to charge it 50% or less, especially if your solar panels are continually keeping it topped up.
You can use a battery monitor to view discharge level.
Many people consider how much energy they use over the course of a day rather than just charging the battery. This is because solar panels will constantly provide energy during daylight hours. So you might look at total energy usage to match with your solar panel size.
Solar Charge Controller
You should always connect a solar charge controller between your solar panels and battery. This helps to regulate the charge cycle and avoid dangers like overcharging. In addition, solar chargers deliver different efficiency of performance. An MPPT controller will help you charge 30% more efficiently compared to a cheaper PWM controller.
Peak Sun Hours
Don’t forget that solar panels only work best in direct sunlight. You won’t get any electricity during the night and performance will drop dramatically in early morning, evening, and adverse weather like cloud cover. This means efficient usage only occurs during peak sun hours. Usually, this is between the hours of 11am and 3pm. So you will only get 4 to 5 hours of good battery charging time each day. To give you an example, if it takes 8 hours to charge your battery this equates to about 2 days.
FAQ about Solar Panels to Charge a 12V Battery
Time to answer your most asked questions.
Yes, a 100 watt solar panel will charge a 12V battery. This is a very popular size and there are plenty of products to choose from. You can expect a 100W panel to charge a 50Ah battery in 8 hours and a 100Ah battery in 16 hours.
Yes, 80 watts will charge a 12V battery. As it is a mid powered solar panel you should not try to use it on large capacity batteries as it will take a very long time, days in fact. It would work well with 50Ah batteries or smaller.
50 watts will charge a 12V battery and will work best with smaller deep cycle batteries. With one 50 watt solar panel you shouldn’t try to charge batteries larger than 35Ah as it will take a very long time. This size will work best with smaller applications like electric gates.
What Size Solar Panel to Charge a 12V Battery Summary
To be honest, it’s up to you to work out what size solar panel to charge a 12V battery.
You need to compare the size of the battery and required speed of charging to get your final decision. If you have a large battery of over 50Ah then you’ll need a bigger solar panel of 100W or more. If you just want to trickle charge a vehicle or smaller deep cycle battery then much smaller solar panels will do the trick.
I hope this article gives you an insight into the decision making process. And of course, if you need it’s easy to add more solar panels to speed up charging as you go!
