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

What Size Solar Panel to Charge a 12v Battery?

There is no better way to charge your battery than harvesting the free, clean energy from our beloved sun.

And when it comes to charging batteries with solar panels, one of the most frequently asked questions is: What size solar panel to charge 12v battery?

If you are looking for a detailed answer to this question, then you are just in the right place.

In this article, we will answer some of the most asked questions about charging deep cycle batteries with solar panels such as:

  1. How many watt solar panel to charge a deep cycle battery?
  2. How many solar panels to charge a 12 volt battery?
  3. How fast will a solar panel charge a battery?
  4. Can you use a 24v solar panel to charge a 12v battery?
  5. Can you charge a 24v battery with a 12v solar panel?
  6. Do I need a charge controller to charge a battery?

So without further ado, let’s jump right into it.

How many watt solar panel to charge a deep cycle battery?

Answering this question is hard, I’d love to just through my hands up and declare “it depends” and leave it at that. But if I did that you would hate me, so let’s see how to calculate the solar panel size.

Generally, the size of a solar panel to charge a battery is calculated using this simple formula:

how to calculate solar panel size to charge 12v panel
how to calculate the solar panel size

For instance, let’s assume we live in California where we get around 5.5 hours of peak sunlight per day and we have a 12 volt, 100 Ah battery that we need to charge using a solar panel.

Then how many solar panels to charge a 12 volt battery would be:

how many watts solar panel to charge a 100Ah battery
how many watts solar panel to charge a 100Ah battery

In other words, you need at least two 100-watt and one 20-watt solar panels to fully charge this 100Ah deep charge battery.

That’s all to it, right? I just need to buy those solar panels, connect them to my battery, and enjoy the off grid living.

Well, if you stopped reading and went right away to buy those three solar panels, then you will be wasting your money.

This is because batteries have something called depth of discharge, which is simply the maximum amount of energy you can safely drain from the battery without risking permanently damaging it and reducing its lifespan.

Most high quality batteries have around 80% DOD. This means that you should treat your 100Ah battery like an 80Ah one, as you don’t want the charge to go below 20%, so keep that in mind when hunting for a new battery.

So now we need to consider the battery’s depth of discharge when calculating the solar panel size.

how many watts to charge deep cycle 100Ah battery with 80% depth of discharge
how many watts to charge deep cycle 100Ah battery with 80% depth of discharge

So, to charge a 12v, 100Ah battery, you actually only need one 175-watt solar panel.

How many solar panels to charge a 12v battery?

To save your precious time and make your life a little easier, we put together a list of the most common 12v batteries and how many solar panels are needed to charge them.

Battery Capacity (12v)Solar Panel Size
50Ah Battery90-watt solar panel
80Ah Battery140-watt solar panel
100Ah Battery175-watt solar panel
120Ah Battery210-watt solar panel
200Ah Battery350-watt solar panel
how many solar panels to charge a 12v battery cheat sheet

Note: These panel sizes were calculated by assuming 5.5 hours of direct sunlight per day, so make sure to check your local sun hours for more accurate solar panel sizes.

How fast will solar panel charge a battery? 

There are a few factors that affect how long to charge a 12v battery with a solar panel such as battery capacity, panel wattage, average daily sun hours, the intensity of the sunbeam, the charge controller efficiency, weather temperature and humidity, and many more.

However, for the sake of simplicity, we will only focus on the three main parameters, which are solar panel wattage, battery capacity, and average sun peak hours.

Calculate your battery’s charging speed is pretty easy. All you need to do is to follow this simple formula:

how fast will a solar panel charge a battery
how fast will a solar panel charge a battery

For instance, if we want to charge a 100Ah battery (12v) using a 100-watt solar panel, then it would take around 12 hours of direct sunlight AKA 2-3 days.

However, this is not accurate, as we didn’t consider the battery’s depth of discharge.

Assuming 80% DOD, the time to fully charge a 100Ah deep cycle battery with a 100-watt solar panel would be around 9 and half hours.

how long to charge a 12v battery with a 100 watt solar panel
how long to charge a 12v battery with a 100 watt solar panel

We put together a list of the most common 12v battery capacities and how fast will a 100-watt solar panel charge them:

Battery Capacity (12v)Time To Fully Charge Using A 100 Watt Solar Panel
50Ah Battery4.8 hours
80Ah Battery7.68 hours
100Ah Battery9.6 hours
120Ah Battery11.52 hours
200Ah Battery19.2 hours
How fast does a 100 watt solar panel charge a 12v battery

How much power to charge 24 volt battery?

Generally, 24 volt rated batteries are not that common, as they are more expensive than their 12v counterpart and don’t have many uses.

If you need a 24v battery, you can simply build one by connecting two 12v batteries in series and it will work just like a 24 volt battery.

Calculating how much power to charge a 24v battery is very similar to its 12v counterpart.

How much power to charge 24 volt battery?
How much power to charge 24 volt battery?

So, if we have a 24v, 200Ah battery with 80% DOD, the power needed to charge this battery would be 3840 Watt.

To calculate how many watt solar panel to charge the 24 volt battery:

how many watt solar panel to charge a 24v battery
how many watt solar panel to charge a 24v battery

So to provide our 24 volt battery with enough power, we need around 700 watts of solar panels (assuming 5.5 sun hours per day). In other words, we need seven 100-watt solar panels to charge this deep charge battery.

FAQs

1. Can you use a 24v solar panel to charge a 12v battery?

Yes, but not directly.

In general, you can’t simply connect a solar panel to a battery, as this can cause the battery to overcharge, which can permanently damage the battery.

So in order to protect your battery, you should connect a charge controller between the solar panel and the battery.

This charge controller will convert the 24v solar panel output to a more suitable 12v, so you can safely charge your battery without risking permanently damaging it.

2. Can your charge a 24 volt battery with a 12 volt solar panel?

The short answer is yes, you can. However, you need to use a charge controller to convert the solar panel’s output into a more suitable current rating, so you can charge the battery without risking damaging it.

It’s important to mention that it’s not recommended to charge a deep cycle battery with a solar panel of different voltage rating, as this will result in a lot of power loss.

3. Do I need a charge controller to charge a battery using solar panels?

The problem with solar panels is that their output depends on sunlight. In other words, it fluctuates during the day. For instance, the output is at its highest during peak sun hours (noon), then it gets very low at dusk.

For this reason, a charge controller is needed to prevent the panels from overcharging the battery, which won’t only permanently damage the battery and reduce its lifespan but also can cause the battery to catch fire and explode.

It’s important to mention that you don’t need a charge controller in case of small maintenance solar panels such as trickle charge panels, as the panel output would be too low to overcharge the battery.

In general, if the panel produces less than 2 watts for each 50Ah, then you don’t need a charge controller.

4. Will a solar panel charge a dead battery?

Yes, the right size solar panel will be able to fully charge your dead battery within 5 to 7 hours.

Pro Tip: To prevent your car battery from dying, you should opt for a small 10 watt solar battery trickle kit.

This kit will provide your battery with enough charge to prevent it from dying too quickly, yet never overcharge it.

5. Why does my solar panel not charging my battery?

There are a few reasons that can cause this failure such as:

  • Broken wires
  • Faulty rectifier diode
  • Charge controller settings
  • Wrong configuration
  • The battery has gone bad

Last Words

As promised, in this article we provided you with a detailed answer to what size solar panel to charge a 12v battery in addition to some of the most frequently asked questions regarding battery charging with solar panels.

In the end, we really hope you enjoyed this article as much as we did.

Do you have any more questions? 

If so, please share your thoughts with us in the comments section below.