What Size Solar Panel To Charge 100ah Battery? (Explained)

Looking for the solar panel size to charge a 100ah battery?

A 100ah battery is more than enough to provide two people with enough power to get them through the evening in an RV with moderate power usage. As a result, it is one of the most common types of battery to have installed in an RV.

Because most people will need that 100ah battery to provide them with power throughout the evening, they need to know what size of solar panel they need to keep it fully topped up.

On this page, we are going to talk about choosing the right size of solar panel, the factors that can influence that charge rate, and even how long it would take the solar panel to charge the battery.

So, what size solar panel to charge 100ah battery? On average a 300-watt solar panel will be more than enough to charge a 100ah battery fully for 5-hours per day. This will help to account for any drop-offs in power throughout the course of the day.

What Does The AH Mean?

Before we go any further, we do want to talk about what the AH means in a battery. This is because a lot of people are somewhat confused about the time, many of whom are used to dealing with watts and voltage.

👉 The AH stands for ampere-hours. It is one of the most important numbers to know when you are buying a battery. It essentially means the amount of power that can be stored in the battery

👉 Every electrical appliance in your RV will consume a certain amount of power per hour. This power consumption will be listed as amps or AH.

So, if you had something that consumed 2AH, then you would expect it to last roughly 50-hours if that were the only thing being powered by a 100ah battery.

If it consumed 4AH, then the battery would only be able to provide it with 12-hours of battery life.

👉 It is vital to know how many AH you are using in your vehicle throughout the day. This is because it will tell you the size of the battery that you need, as well as the size of the solar panel that you need to keep up with all that power consumption.

We will talk a little bit about this shortly. 

Some Assumptions

We are going to be making a couple of assumptions on this page. Mostly, because we do not want the information to be too needlessly complex.

👉 Firstly, we are going to be assuming that all you are doing with your solar panel is charging your battery. This means that you are not going to be trying to power anything else inside of the RV at the same time. No fridge.

Nothing. After all, if the solar panel is having to divert power elsewhere in the RV, then it is not going to be able to charge the battery at the same time.

Well, it can, but you would likely need a much larger solar panel than what we talk about here. We will discuss that in a bit of depth shortly.

👉 Secondly, we are also not going to be taking into account the type of battery that you have. This means the battery tech, battery manufacturer, age of the battery, etc.

There are too many factors at play there. What we will do, however, is give you a general overview of how long you can expect the battery to take to charge. Over time, you should be observing how long your battery takes to charge.

You will find that this may be higher or lower than the information that we share here. That is fine. As we said, these are just averages rather than a firm number.

👉 It is worth bearing in mind that there are some battery types that should not be drained below 50%, otherwise you will be damaging the battery. This means that, in many cases, you won’t actually be charging the full 100ah.

Although, as we said before, we are not going to be going into depth on that here. It is all about general numbers here. We are making the assumption that you want to charge a full 100ah, as opposed to a fraction of a battery.

👉 Finally, we are going to assume that you have a 12-Volt battery. Most of the batteries that are 100ah will be 12-volts.

In theory, you could use this information to work out the charge time of a 6V and 24V battery. However, because there are several factors that could impact the charge time here, as well as cause damage to your battery, we won’t go into that. 

Read also: How Many Amps Does a 250-watt Solar Panel Produce? (Explained)

How to Work Out the Size of Solar Panel You Need 

It is actually pretty simple. You just need to know three things:

• The average amount of sunlight in the day
• The voltage of your battery
• The AH of the battery

👉 You already know the second two pieces of information. You likely have a 12v battery, and it has a capacity of 100ah.

This means that you only need the third piece of information. This is the average amount of sunlight per day.

This will be the hours where your solar panel should be receiving the full amount of power. On average, this should be about 5-hours per day.

👉 This means that we only have 5 hours a day where we are able to give the battery all of the juice that it needs.

This means that we need to be getting 100-amps over the course of 5-hours. Sure, there will be times outside of this where the battery will be receiving a trickle charge, but we can’t rely on that.

👉 We divide 100 into 5. This means that we need to supply 20-amps per hour to the battery.

👉 We now take this 20 and multiply it by the voltage. This means that, at minimum, we need a 240-watt solar panel to provide all the power that the battery requires to full charge.

We suggest that, at minimum, you get a 300-watt solar panel. This will help to account for any drop-offs in power throughout the course of the day.

We suggest that, at minimum, you get a 300-watt solar panel. This will help to account for any drop-offs in power throughout the course of the day.

How to Size your Solar Power System >> Check out the video below:

What Happens If I am Using Power Throughout The Day?

Then this is going to completely change the amount of power that you need the solar panel to be able to provide. It does make your calculation a little bit more complicated, and we are afraid that we cannot be terribly accurate when it comes to calculating things here. Although, we can give you a rough calculation.

The first thing that you will need to do is monitor your battery consumption throughout the day i.e. those 5-hours of the day where the sun is probably at the strongest, and thus your solar panel is providing the most amount of power into the batteries and your power system.

We suggest that you actively monitor your battery consumption here as opposed to walking around and measuring everything.

Read also: This Is What Happens to Solar Power When Batteries Are Full? – (FACTS)

You will get a much more accurate figure. Remember, it will likely be less than the amount of power that you will use in the evening. This is because the lights will probably be switched on when it gets dark at night.

Once you know your power consumption for those 5-hours, we can add it to the calculation that we used in the previous section.

By now, you should probably know that you will need around 300-watts to charge a 100ah battery fully for 5-hours per day. So, let’s assume that you are using 20-amps of additional power during the 5-hours of charging. Although, this is an example.

The figure may be higher or lower than this for you. The calculation now will be:

100amps + power consumption over 5-hours (in this case 20ah). This will give us 120ah of power that we need to generate over 5-hours. 

This means that we now need to produce 120-amps of power during those 5-hours. The calculation is now to divide that 120 into 5. This means that we now need to generate 24-amps of power an hour. 

Now, multiply that 24-amps by 12v (assuming this is the voltage of your battery) and this gives us 288-watts minimum to charge that battery and provide our power needs.

However, as before, we are going to want to bump that number up a little bit to account for any charges that may influence the amount of power that the battery receives from the solar panel.

This means that, at minimum, you would be using a 350-watt solar panel to provide you with all of the power that you need.

We know. We are going very heavy on the math here. However, we can assure you, when it comes to dealing with electricity, math is everything. It isn’t especially complicated, though.

We are sure that the bulk of you can work out these calculations in your head!

Can Anything Impact the Speed at Which the Solar Panel Charges At?

Yes. The amount of sunlight is going to be a huge factor here. Our calculations work only if your solar panel is exposed to the unobstructed sun.

This means that it should not be cloudy, and your solar panel should not be in the shade. The charge time will be a lot slower if this happens.

You will also need to account for the quality of the solar panel. The better the quality of the solar panel, the more stable the power that it produces will be. It may even be able to produce more amps per hour.

The latest tech always can. Although, do bear in mind that the best solar panels on the market can be incredibly expensive!

How to Buy a Solar Panel to Charge a 100ah Battery

Finding a solar panel to keep that 100ah battery topped up is dead simple. By now, you will already know that you need a minimum of 300-watts of power. You will need more than this if you are planning on providing power to your RV in addition to the battery. This will narrow down your search drastically.

As a side note here, you can either purchase one 300-watt solar panel, or multiple solar panels that add up to 300-watts. The charge time doesn’t really change here. So, you can go for whatever option is going to be more cost-effective for you.

Make sure that you look at the amperage per hour that the solar panel is able to produce. As we said before, you need a solar panel that is capable of producing at least 20-amps per hour. This is the only way that you can be sure that your battery will be fully topped up throughout the day.

Finally, make sure that you read any reviews for a solar panel before you part with your cash. This way, you will know if people have run into issues with the solar panel before.

If they have, then try and work out whether these are issues that are going to be impacting you.

Do You Need to Buy a Charge Controller?

In theory, you don’t need a charge controller. In reality, you should not be without one.

A charge controller will help to ensure that the battery is receiving the right amount of power. It will also help to ensure that the battery is not overcharged by the solar panel.

Basically, the charge controller will help to preserve the lifespan of your battery.

The charge controller is also going to be playing a huge role in diverting power to the battery. Anything that is not being directly consumed by the appliances in your RV will be sent straight to the battery.

Therefore, you will almost certainly need one if you are expecting your solar panel set-up to cover both battery and genera RV usage.

Conclusion

You will need at least 300-watts of solar power in order to ensure that you fully charge a 100ah battery With that amount of power, you should be able to charge the battery fully within about 5-hours.

This is the average amount of full sunlight that you are going to be getting each day.

Do bear in mind that you will need a higher wattage of solar panel if you are planning on using RV power in addition to battery power.

References

https://www.renogy.com/blog/what-size-solar-panel-do-i-need-to-charge-a-12v-battery/