How Long Does It Take a 100W Solar Panel to Charge a 100Ah Battery?

A solar panel is an excellent source of alternative energy, but you’ll also need a battery to store the excess energy your solar panel collects. Knowing how much power your battery will supply and how long it will take to charge can depend on a few different factors.  

It will take a 100-watt solar panel 12–14 hours of direct, peak sunlight to charge a 100-amp-hour battery on average. This calculation estimate depends on environmental variables, including the weather and direction of the sun, the angle of the solar panel, the location and time of year, and the state and age of the battery.

Continue reading to learn more about solar panel batteries, how they function, and the different variables that will determine how long it will take your battery to charge.

Solar Panel Batteries and Amp-Hours

Before we figure out how long it takes to charge a 100 amp-hour battery, we first need a basic understanding of solar panel batteries, why you need one, and how they work. Identifying how long it will take to charge your battery is more complicated than you might realize.

While solar panels are incredibly adept at collecting energy from the sun, there is another component when it comes to maintaining and accessing power when you need it. It would be best if you also had a battery that will store the energy your panels produce and sustain your energy needs when the sun is not shining. 

Any excess energy produced will be harvested and collected in the battery so that rather than sending that energy back into the grid, you can collect it for use at times when you need it, such as cloudy days, nighttime, and even during unexpected power outages (source).

Determining what size battery and whether or not your solar panel will charge the battery effectively requires some understanding of the type of batteries used for solar panels and their subsequent energy storage capacity. They’re certainly not your standard double or triple-A battery.

The batteries most often used in conjunction with solar panels are sealed lead-acid batteries referred to as “deep-cycle” batteries. A cycle battery is one that is able to be discharged and recharged, thus the term “cycle.” Deep-cycle batteries are those that can be discharged up to 80% before being recharged (source). 

Deep-cycle batteries are also the most efficient and effective in use with solar panels because they use less power over a long period of time. Ideally, though, you should not use more than 80% of your battery before charging it, even though you theoretically could. 

Letting your battery capacity drop too low will ultimately damage the battery and shorten its lifespan. Nearly all manufacturers recommend that a deep-cycle battery be recharged at 50% capacity (source).

Calculating Your Battery Power

When we refer to “amp-hours,” you are essentially looking at how much storage capacity the battery has. The sealed lead-acid batteries used with solar panel kits come in either 6 or 12-volt. If you have a 100-amp-hour battery, you’ll not only want to know how long it will take to charge but also approximately how much energy it will store.

A simple calculation will reveal the watt-hours for your battery and identify your battery’s storage capacity. You’ll need to multiply the volts by the amp-hours and then divide by 100.

VOLTS x AMP-HOURS / 100 = WATT-HOURS

So, given you have a 100-amp-hour battery, let’s assume it is also 12 volts — these are the most commonly used. This means that your battery has about 12 watt-hours, and it will be able to power a 100-watt appliance for about 12 hours on a battery with a full charge.  

Charging a 100 Amp-Hour Battery with Your Solar Panel Kit

Figuring out exactly how long it will take to charge a 100-amp-hour battery with a 100-watt solar panel is unfortunately not precise, nor straightforward. With so many variables that affect the outcome, it is only feasible to provide approximations based on the many conditions that will likely affect the outcome.

For example, if the sun is shining directly at your solar panel at the proper angle in the middle of the summer when the sun is at its highest, charging times will be faster. If it is a cloudy day or the sun is not directly facing the solar panel, this will subsequently result in longer charging times.

You also need to consider the time of year and your location. Given there are more hours of sunshine in the summer months, charging your battery in mid-August will take less time than it will in mid-January. 

But where you live matters, too. If you are charging a solar panel in the Southern states, it’ll likely take a little bit less time than if you are attempting the same in the North, but it still depends on the time of year. 

You’re probably looking at the difference between four and a half hours versus five hours of peak, direct sunlight. Still, it’s significant if you are trying to determine precisely how long it will take your battery to charge fully.  

An additional factor includes the state of your battery. The life expectancy of the battery is identified by the number of cycles the battery can withstand. The more the battery is cycled through, charged, and discharged, the lower its ability to consistently retain its original capacity.

Earlier, we mentioned that you should, ideally, charge your battery at about 50% capacity to get the most value as well as longevity out of the battery itself. 

If you have maintained your battery well, meaning you have not let it repeatedly drain, the charging times will be a bit shorter. Similarly, newer batteries will often take less time to charge than older ones.   

Unfortunately, it’s never safe to assume that you will have three consecutive days of optimal sunlight, despite weather predictions. Regardless of where you live, you’ll nearly always have to recognize that clouds can quickly replace even the best predictions for lots of sun.

So, how long will It take to charge a 100-amp-hour battery with a 100-watt panel? 

Without getting too complicated in calculations, a 100-watt panel, on average, produces about 6 amps of power at peak sunlight. The key here is that peak sunlight hours are probably around four or five hours per day on ideal, sunny days.

If you only have one 100-watt panel, it would likely take about 3 days, maybe a little bit less, given that the number of hours of peak sunlight is limited, and you’ll need about 12 to 14 hours of peak sunlight, minimally.  

Most recommendations suggest that you utilize 3×100-watt panels to charge a 100-amp-hour battery in one day with optimal conditions, but many people who use a 100-watt panel do so for the convenience factor — they’re small, not all that expensive to replace if need be, and portable.

That means they are ideal for camping or for the top of your RV.  So, we will stick with the initial calculation and assume that, for most, we are looking at a single panel. With that in mind, you’ll want to safely allow for about 3 days to fully charge your 100-amp-hour battery.  

And if you are traveling, consider the climate and region, too, and expect the charging time to change based on where you are located. 

What Can You Run with a 100-Watt Solar Panel?

When you are trying to identify what size battery you need and what you can power with a 100-watt panel, you’ll want to be sure that your battery can store about twice the daily output of your panels. In this way, you will not run out of power when you need it most.

When it comes to 100-watt panels, we’re talking about smaller devices such as laptops, cell phones, obtaining Wi-Fi connectivity with a router, and smaller lights and lamps. In general, a 100-watt solar panel can’t provide power to anything that is, itself, over 100 watts (source).

That doesn’t leave much when it comes to your solar panel, but with a 100-watt panel that is used primarily to charge a battery, you’ll be able to power your small appliances, cell phones, and laptops with the battery instead of the solar panel grid — and the battery can then provide more power than the panel itself is capable of supplying.  

Final Thoughts

A rough estimate is helpful when trying to determine how much time it will take your 100-amp-hour battery to charge from the power supply provided through one 100-watt solar panel.  But, a rough estimate is really all you can depend on.  

Once you consider the variables that will affect how long a full charge will take, you can adapt and plan accordingly for your needs. Remember to charge your battery at 50% capacity to prolong its life and avoid having to replace it sooner than you’d like. 

Tim C

An outdoor enthusiast for decades and a science and physics nerd since 1980, I have recently become very interested in using renewable energy sources for home primary power and as backup for outages, as well as for camping, boating and tailgating.

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