The Solar Panel Size to Charge Your 12V Battery (FAST)

You can charge a 12V battery with any size solar panel, as long as it’s a 12V solar panel.

In fact:

I recently charged my own 12V battery with a 5W 12V solar panel — the smallest 12V solar panel I’ve ever seen:

And with a 100W 12V solar panel — one of the most common solar panel sizes:

As long as you’re using a 12V solar panel — and connecting it properly — it can charge a 12V battery.

But!

Some solar panel sizes will take MONTHS to charge a 12V battery, while others will take just hours or days.

This begs the question:

What size solar panel do I need to charge a 12V battery quickly?

Let’s find out.

What Size Solar Panel Will Charge a 12V Battery Quickly?

TLDR: A 100W solar panel is a good size for quickly charging many of the most common 12V battery capacities, such as 50Ah, 100Ah, and 200Ah. For example, a 100W solar panel will charge a 50Ah deep cycle lead acid battery in around 8 hours of direct sunlight. And it will charge a 100Ah lithium battery in around 22 hours of direct sunlight.

Charge time depends mainly on 2 things:

  1. Battery capacity in amp hours (Ah)
  2. Solar panel power output in watts (W)

A small solar panel will take a long time to charge a big battery. Like how a small hose will take a long time to fill up a big bucket.

Alternatively, a big solar panel will charge a small battery quickly. A big hose quickly fills a small bucket.

Got a big battery and a big solar panel? Or a small battery and a small solar panel?

Your charge time will land somewhere in between.

A 100W solar panel (left) next to a 5W solar panel. Both are 12V solar panels and both can charge a 12V battery. But the 100W panel can output up to 20 times the power of the 5W panel, so it will charge a 12V battery much faster.

Because there are so many different sizes of 12V batteries, there is no single solar panel size that will charge every one of them quickly.

Instead:

Let’s look at some examples to see what size solar panel you’d need to charge some common 12V battery capacities.

(At the end I’ll also give a tip on how to estimate the charge time for your specific setup.)

Example 1: 12V 100Ah Lithium Battery

Let’s say you have the following solar charging setup:

  • 12V 100Ah lithium iron phosphate battery (100% discharged)
  • MPPT charge controller

Here are estimates of how long it’ll take to charge the battery with various solar panel sizes:

Conclusion: For a 12V 100Ah lithium battery paired with an MPPT charge controller, use an 80W solar panel or bigger to fully charge the battery in a week or less of sunny weather. Use a 500W solar array or bigger to fully charge the battery in a day or less of sunny weather. (Assuming 5 hours of usable sunlight per day.)

Example 2: 12V 50Ah Car Battery

Let’s say you have the following solar charging setup:

  • 12V 50Ah lead acid car battery (50% discharged)
  • MPPT charge controller

Here are estimates of how long it’ll take to charge the battery with various solar panel sizes:

Conclusion: For a 12V 50Ah car battery paired with an MPPT charge controller, use a 5W or 10W solar panel to trickle charge the battery (like I did with my solar 12V car battery charger). Use a 20W solar panel or bigger to fully charge the battery in a week or less of sunny weather. Use a 200W solar array or bigger to fully charge the battery in a day or less of sunny weather. (Assuming 5 hours of usable sunlight per day.)

Example 3: 12V 200Ah Deep Cycle Battery

Let’s say you have the following solar charging setup:

  • 12V 200Ah deep cycle lead acid battery (50% discharged)
  • MPPT charge controller

Here are estimates of how long it’ll take to charge the battery with various solar panel sizes:

Conclusion: For a 12V 200Ah deep cycle lead acid battery paired with an MPPT charge controller, use a 5W or 10W solar panel to trickle charge the battery. Use an 80W solar panel or bigger to fully charge the battery in a week or less of sunny weather. Use a 800W solar array or bigger to fully charge the battery in a day or less of sunny weather. (Assuming 5 hours of usable sunlight per day.)

Tip: How to Find the Right Solar Panel Size to Charge Your 12V Battery

Is your 12V battery a different size than the ones in my examples?

Good news:

You can find the right size solar panel for your battery with our solar panel charge time calculator.

Use our solar panel charge time calculator to find the right size solar panel for your 12V battery.

In fact, this calculator is what I used to estimate the charge times in the examples above.

Input the specifics of your battery capacity, battery type (lead acid or lithium), and charge controller type (PWM or MPPT). It even works for other battery voltages, such as 24V and 48V.

Then you can plug in different solar panel sizes until you find the one that charges your battery at your desired speed.

Try it out:

How to Charge a 12V Battery with a Solar Panel

You’ve got your 12V battery. ✔

You’ve got your perfectly sized solar panel. ✔

Now you need to connect them together to start solar charging the battery.

Here’s a quick tutorial:

Note: For more in-depth instructions, check out my full tutorial on how to connect a solar panel to a battery.

Parts

  • 12V solar panel
  • 12V battery
  • Solar charge controller (one that’s compatible with 12V batteries)
  • Battery cables
  • Solar panel to charge controller adaptor cables

Step 1: Connect the 12V Battery to the Charge Controller

Insert the stripped end of the negative battery cable into the negative battery terminal on the charge controller. Screw the terminal shut with a screwdriver. Repeat the process for the positive battery cable.

Connect the ring terminal on the negative battery cable to the negative terminal on the 12V battery. Repeat the process for the positive battery cable.

Your charge controller should turn on to indicate the battery and charge controller are properly connected. Mine has a battery indicator light that turned on.

Now you need to program your charge controller for your battery type. This lets the charge controller know which voltage to charge the battery to.

Consult your instruction manual for how to do this. In this example, I’m using a lead acid battery so I programmed my charge controller for that battery type.

Check!

Step 2: Connect the Solar Panel to the Charge Controller

Connect the positive solar panel adaptor cable to the positive solar panel cable. Repeat the process for the negative solar panel cable.

Insert the negative solar cable into the negative solar PV terminal on the charge controller and screw the terminal shut. Repeat the process for the positive solar cable.

Your charge controller should indicate that the solar panel is properly connected. Mine has a PV indicator light that turned on.

Done!

Step 3: Put the Solar Panel Outside in Direct Sunlight

Your solar panel and battery are connected…

…but the solar panel isn’t charging the battery yet because the panel isn’t in direct sunlight.

That’s an easy fix.

Take your solar panel outside and angle it towards the sun.

Once you do, your charge controller should indicate that the solar panel is charging the 12V battery. For example, the PV indicator light on my charge controller starts blinking green when the battery is charging.

There ya go!

You’re charging a 12V battery with a solar panel.

(Simple, no?)

Now all you have to do is wait for the solar panel to charge the battery. The charge controller will automatically stop the charging once the battery is fully charged.

Solar Charging 12V Batteries FAQ

How Long Does It Take to Charge a 12V Battery with a Solar Panel?

Hopefully I’ve driven home the point by now:

Charging time depends mainly on the battery capacity (Ah) and solar panel power (W).

But…

…these aren’t the only factors affecting charging speed.

Here are 3 more:

1. Solar Charge Controller Type

A solar charge controller sits between the battery and solar panel and regulates the voltage and current output by the solar panel to safely charge the battery.

Charge controllers sit between the battery and solar panel. They regulate the voltage and current output by the solar panel to safely charge the battery.

There are two types of solar charge controllers: PWM and MPPT.

An MPPT will charge your batter faster because MPPT charge controllers are much more efficient at converting the incoming solar energy to the right charging parameters for the battery — around 95-97% efficient for most models.

A PWM will charge your battery slower because PWM charge controllers are much less efficient — around 75%.

I know what you’re thinking…

Efficiency, schmefficiency. How much of a difference does charge controller type actually make?

To that I say:

Let’s look at an example. (Yep, another one. 😄)

Here are estimates of how long it’ll take to charge a 12V 50Ah lead acid battery with both a PWM and MPPT charge controller:

The difference in charge time is substantial for small solar panels. For instance, for a 5W panel, an MPPT will charge the battery around 30 hours faster than a PWM. That’s roughly a week of sunny weather.

But when you increase the size of the solar panel or solar array, the difference becomes negligible. For a 100W panel, an MPPT will charge the battery only around 1.5 hours faster than a PWM.

However, the bigger the solar panel, the more energy — and thus money — a PWM charge controller wastes.

2. Battery Chemistry

A 12V 33Ah sealed lead acid battery

The two main battery chemistries used in solar power systems are lead acid and lithium iron phosphate (LiFePO4).

Lead acid batteries have a recommended depth of discharge of only 50%. Discharge a lead acid battery more and you can shorten its lifespan or permanently damage it. That means, for a fully charged 100Ah lead acid battery, you can only use around 50Ah (leaving C-rates aside). That also means that — come time to charge the battery — you’ll at most have to recharge 50Ah.

Lithium iron phosphate (LiFePO4) batteries from many manufacturers can be safely discharged 100% and still last for thousands of cycles. So a 100Ah lithium battery can actually supply 100Ah, and needs a full 100Ah recharge.

So:

A “discharged” lead acid battery (50% discharged) charges faster than a “discharged” lithium battery (100% discharged) of the same capacity only because the lead acid battery has 50% less capacity to recharge.

Because both batteries are starting from different depths of discharge, it’s like comparing apples to oranges.

If we were to compare apples to apples — i.e., start from the same depth of discharge with both batteries — the lithium battery would charge faster because it loses less energy as waste during the charging process. It can also accept a greater maximum charge current.

3. Environmental Factors

A solar panel produces much less power in shade or cloudy weather.

Weather, shade, time of year — all have an effect on solar panel output. A big effect.

It’ll take longer for a solar panel to charge a 12V battery in cloudy weather than it will in sunny weather, in shade than in direct sunlight, in winter than in summer.

Consider these environmental factors when sizing your solar 12V battery charger. Consider oversizing your system a bit to account for seasonal changes in power output or long streaks of cloudy weather.

How Many Solar Panels Do I Need to Charge a 12V Battery?

Me happily charging a 12V battery with a single 100W solar panel

You only need one 12V solar panel to charge a 12V battery. A 100W solar panel is a good size for quickly charging some of the most common 12V battery capacities, such as 50Ah, 100Ah, and 200Ah.

You can always add more solar panels later if you want to charge your 12V battery faster.

If you just want to trickle charge a 12V battery, you can get away with a small 12V solar panel, such as a 5W or 10W panel. That’s right — a 5W 12V panel will charge a 12V battery.

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Alex Beale
Alex Beale
Hi, I'm Alex. I started Footprint Hero to help people reduce their environmental impact. My current obsession is DIY solar power projects, which I've been building since 2020.
Alex Beale

Alex Beale

Hi, I'm Alex. I started Footprint Hero to help people reduce their environmental impact. My current obsession is DIY solar power projects, which I've been building since 2020.

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2 thoughts on “The Solar Panel Size to Charge Your 12V Battery (FAST)”

  1. What good are Solar panels if they take so much time to charge a 12v battery to full capacity…. just how many spare batteries do I have to carry to just use my trolling motor for the day on my inflatable pontoon boat? Now I understand why the roofs of RV and Home are completely covered in solar panels…. solar panels are not a very effective way to produce power! Good To Know…. I won`t bother buying any! Rod

    1. There are people who have built solar powered boats before. I’d recommend looking on YouTube for inspiration. True that it might not make sense in your case (in some cases it just doesn’t), but it can be done.

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