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What Size Solar Panel Is Needed To Charge A 100AH Battery?

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Tim Rhodes

So we have an important question asked by a lot of engineers about how to charge and what sizes of solar panels are needed to charge a 100Ah battery.

Before we head to the original question at hand, let us discuss what solar panels are and why is it more acceptable these days for more electricity generation. 

It is claimed that solar panels can generate a lot of electricity and are environmentally friendly.

If we look at other sources of electricity generation, nuclear plants are really harmful, if something goes wrong may be catastrophic.

Oil/diesel generated electricity is not a sustainable source of energy either, which makes solar energy much more acceptable since it’s abundant by nature. 

Solar electricity generation has seen rapid growth in usage, due to low carbon energy technology, and due to rapid improvements in technology, most people are shifting towards using solar panels for power generation. 

Hydro and wind are alternate renewable sources of electricity generation.

However due to technological advances, solar energy harnessing is considered much better compared to the two, and is a decent addition to power generation that is green and sustainable.

The Size Of Solar Panels Needed To Charge A 100AHBattery

What are solar panels?

Photo-voltaic modules (PV) is another term for solar panels, which contains PV cells that collect sunlight as a source of energy and pretty much store them in batteries or generators.

PV modules will use light energy as packets of energy called photons and this will be converted to electricity with a process called photo-voltaic effect.

Photo-Voltaic (PV) effect is the process where light is passed through a panel and its excited electrons will jump to another due to energy level compression.

This extra electron is the reason why there is electricity generated, as it moves around throughout the process.

A solar panel is constructed from having multiple solar cells, or PV modules in an array, and are connected in series in an electrical circuit.

Normally it is coated with glass on top to reduce reflection of sunlight, and absorb most of the light as possible for getting maximum efficiency. 

How efficient are solar panels?

Most engineers look for efficiency, and how well it performs compared to other sources of electricity.

Since they are mostly responsible for controlling the efficiency and quality of the product at hand, they want to make sure it’s highly efficient in order for them to work with it. 

The output from the solar module is rated by DC output power, and it’s range is around 100W – 360W.

Usually the efficiency of modules determines the area of rated output, for instance, a module of efficiency 10% 100W panel will have twice the area of module with 20% efficiency of 100W panel. 

Ideally, most solar panels have an efficiency of about 15% for the lower quality ones, and upto 23% for the high quality ones.

The key factors that determine the efficiency and quality of the panels are: the type of materials used in its construction, the wiring and its organization, and finally the reflection, since reflection reduces the efficiency if light gets reflected.

If you see the market, most consumers will opt for better efficiency solar panels, for its ability to conserve the energy to electric energy. 

Factors Affecting Efficiency of Solar Panels

There are few factors that affect solar panel efficiency.

The main components are panel material components, reflectance efficiency and thermodynamic efficiency. 

Panel material components

There are two types, monocrystalline and polycrystalline.

Monocrystalline components are usually efficient due to uniform composition, and tend to have better performance in unfavorable conditions such as low light and very hot conditions.

Typically most home users have monocrystalline panels.

Polycrystalline material however has fragmented composition, which is a cause for lower efficiency. But they are cheaper than monocrystalline panels.

Reflectance Efficiency

Reflectance efficiency is basically how much sunlight is reflected by the panel, the lower they are reflected, the better the efficiency of that panel.

The more sunlight is absorbed for use, the better in this case, so less reflection on solar cells means better. 

Thermodynamic efficiency is the highest efficiency that a panel can reach.

It’s the ability of the panel to use a significant amount of sunlight and convert it to electricity after excluding the loss of energy as heat. 

Risks Associated with Solar Panels

So far we have read about what a solar panel is and how efficient it can be, but did we consider the risks it poses? Let’s find out. 

There are potential risks of solar energy emissions, which means that due to rise in heat activity, there might be a risk of global thermal emissions, which can be a cause for global warming, although it’s not in large quantities, but small amounts do count! 

As the solar cells are made with silicon, the production of panels might emit hazardous or toxic chemicals into the atmosphere, due to silicon mining, or extraction of minerals, etc.

We all know that lack of care in chemical handling will cause serious injuries, or may be hazardous to the environment.

That is why manufacturers need to be extra careful not to release chemicals in the environment. 

Possibility of a short circuit, which may lead to fire, or other damaging hazards, which is why we need special care in handling the solar panel and its equipment while installing and using it. 

Few other risks such as disposing of plastic or other non-biodegradable objects such as wirings or the panel board, which should not be burnt due to release in toxins or poisonous gases harmful to the environment. 

There’s a risk involved with the workers in the solar energy industry, where they might be involved in electric shock, falls, and thermal burn hazards.

More about worker hazard in the link. 

Disadvantages of Using Solar Panels

There are several disadvantages to solar panels.

As solar panel usage is still a very new concept to the world, and with the cost of production and usage of electricity is increasing day by day, we need to look for alternate sources of energy that are renewable.

Solar energy is the alternate inexhaustible source of electricity, however, we have a few demerits of having solar panels as a source of power. 

Solar panels require sunlight to charge and store the energy to batteries, which means that location is vital to installing them. If there is not enough sunlight, the solar panel usage is not viable and useless. 

Sometimes small houses can install it on the rooftops, but large companies require larger spaces for more power generation, meaning they require more land for installing additional solar panels.

At night time, there is no power generation, due to sunlight being absent.

Due to highest efficiency being only around 23%, the energy converted is minimalistic, and therefore not a very feasible option for generating electricity. 

As we have read above, there are effects on land, water usage and environmental hazards associated with solar panels.

This is why we need to consider having trained professionals handle the installation of solar panels, so they can estimate the consequences of any hazards.

Cost of affording or installing a solar panel is very high. This is because it requires converters, inverters, DC source, such as a chargeable battery. The cost to production in electricity ratio is not feasible. 

Benefits of Using Solar Panels

Despite the fact that we have multiple disadvantages, the negative effects are minimal compared to the benefits it has for society and the people who use solar panels. The advantages of using PV panels are listed below.


Solar panel itself has no bad effects, which means that there are no emissions of gases and is pollution free.

This is why we call it environmentally-friendly. 

Panels do not depend on oil or non-renewable resources for energy conversion, unlike diesel generators that need oil to convert chemical energy to electrical energy.

Which means we do not burn any fossil, or other fuel for this purpose, as sunlight is abundant and inexhaustible. 

Power generated due to solar panels is pretty much renewable for everyday even if there is reduced sunlight.

This means that even if the weather is cloudy, it can still produce electricity. 


Solar panels can last a long time, which means no such maintenance is required, unless it has been physically damaged due to a storm or other unforeseeable events.

Since it’s not mobile, and it stays in one place, it doesn’t need much other than cleaning if you prefer. 

Conserves Energy

Using panels may help with the conservation of electricity usage, when there is a rise in demand of electricity in the modern world. 

Can be installed with batteries to store extra energy for use at night time.

This is very convenient and helps as you don’t have to pay for using the energy.

Your Own Energy Source

This also helps when there are unavoidable changes in utility bills and you don’t have to worry about it since you have your own energy source. 

Electricity produced by solar energy can be used to power almost everything at home, and is considered safe compared to other sources that generate electric current.

No need for complex connections as it can be installed close to the building or place in need. 

Having solar panels will reduce the risk of power outage as well, because there are places which have issues with load shedding, a concept where power is shared from one area to another, to cut back the usage or fulfill the demand of electricity usage. 

As we can see, there are more benefits compared to the disadvantages we have, and with time, we can improve the efficiency of solar panels everyday and prices being cheaper so that more people use renewable energy rather than non-renewable sources of electric current generation, that is why solar panel is getting a rise in demand.

How Solar Panels are Installed

Usually solar panels are very easy to install.

They can be installed almost anywhere, which includes your own rooftop, or anywhere accessible, but we need to consider the availability of sunlight in that place.

We need to assume that it gets at least 6-8 hrs of sunlight per day.

Secure or Build a Platform

Create a platform that will support the array of solar panels.

We need to make sure that the roof where the solar panels are located is strong enough to hold the weight of the panels.

Also we need to make sure there is enough space for additional panels in case of adding more in the future. 

Align the Panels

The next step is to align the panels that have been pre-fastened to create the array.

Attached sealed batteries will store the energy converted by panels. 

Installation of Inverter and Controller

And finally, a power inverter and charge controller will be installed in the garage near the standard electrical box. 

This system can be improved and expanded, and may produce a minimum of 40,000 watt hours per month – which is enough to power a lot of electrical appliances including fans, interior lights and even TVs!

Mounting Panels

Then we need to mount the panels to the platform.

Angle brackets can be used at the mounting points.

The front feet are mounted first, these are squared and centered before being secured into place.

After the front feet, we need to make sure and measure the proper angle for the array for the rear feet to be mounted. 

Connect Modules Together

You have to connect the solar modules together using wires with the junction or fuse box.

The modules should be stripped and wired in pairs before being connected to the fuse box mounted at the platform and connected to underground conduit output cables. 

The wires in the terminal box must be connected correctly, which means that red wires to the positive terminal, and black wires to the negative. 

Make Sure it is Waterproof

After the solar panels are properly installed with the wires connected, the next thing is to make sure that your wires are not only properly insulated but also waterproof.

This is to make sure when there are cases like rain or storm, it will be safe.

Use earthing wire by connecting a ground wire that runs from the mounting hardware to the ground, this helps prevent risk of electrocution. 

Run Tests

You should run tests and see how it works.

You can try turning on a few electronics and see if they are working properly.

If the components are working with no issues, attach the component covers and you are pretty much done.

Take Safety Precautions

Remember to take safety precautions when conducting electrical work, due to potential risks of getting electrocuted.

Handle with care. It’s best left to professionals since they can handle all sorts of equipment safely.

What is the Best Battery for a Solar Panel System?

Usually a battery or a set of batteries will store the solar energy captured by the panel. What type of batteries are best suited for this purpose? 

There are specific types of batteries for solar energy storage.

Lithium ion cell batteries, lead acid or saltwater cell batteries are better suited to store the solar energy.

However due to technically advanced setup, lithium ion batteries are the best for this purpose. They usually have longer life spans and are usually lighter compared to lead and saltwater cells. 

For home purpose and use only, you can opt to get lead acid batteries, because it is cheaper than lithium ion.

Though technically they have lower life span, meaning the battery may need to be replaced often. 

Saltwater cells can be a greener option out of the three with saltwater electrolytes playing a vital role in transmission of current.

It doesn’t have to be disposed of unlike the lead or lithium cells, since it can be recycled.

General lifespan of a solar panel or a PV module is around 25-30 years, and that of a battery ranges from 5-15 years.

To use them without any problems, you must change the battery at least once to match the potential of the solar panel system. 

Determining the Size of Solar Panel Needed

Now that you know a bit about how solar panels are installed and the type of batteries you need to look out for, you have to decide what size of solar panel you need.

This may vary depending on your criteria — place of installation, your budget, what items do you want to run with it and future power plans. 

It is said that the bigger solar panels have reduced costs, therefore it’s optimal to go buy bigger ones that can produce up to 5 kW. 

But the system depends on the electricity usage. Let us find out via calculations.

If you are new to measurements such as AH (Ampere-Hours), Watt (W), Voltage (V), it might be a bit confusing to start with. 

Computing Watt Hours

Let’s go into details. 

To find out the Watt-Hours (W-H), you have to multiply Battery AH with Battery Watt. After finding the W-H, you have to calculate the total Watts of the solar panel you need, which is dividing WH by the number of hours of sunlight the solar panel receives.

Finally with total Watts you multiply it with its efficiency tag (ideally around 15-20%) and add it to the total to get the actual Watts of the solar panel you need to install. 

In summary,

Watt-hours = Battery AH × Battery Watt.

Total Watt-hours of solar panel = Watt-hours ÷ Number of hours of sunlight per day.

Actual Watts of solar panel = Total Watt-hours + (Total Watt-hours of solar panel × Efficiency).

Ideal Size Needed to Charge 100AH Battery

Now that we know how to calculate the size of the panel we need, we can use it here to determine the actual size needed. 

Let us assume that the battery is of 12V, 100AH. 

Watt-hours of 100AH battery = 100 × 12 

= 1200 W-H.

Also assume we get 8 hours of sunlight for the day.

Total Watt-hours of solar panel = 1200 Watt-hours ÷ 8

= 150W-H.

Finally assuming that the solar panel is made of the best quality and is efficient up to 20%.

Therefore, Actual Watts of solar panel = 150 + (150 × 20%)

= 180 W.

Common solar panel sizes available in the market are : 

  • 120 W Panels
  • 100 W Panels
  • 50 W Panels

Hence we need a combination of 2 × 120W, 2 × 100W or 4 × 50W to cover a 180W solar panel to charge a 100 AH battery. 

How Long Will It Take to Charge a 100 AH Battery?

Now that you know the sizes you need to charge it, you might be wondering how long will it take to charge it. Let us find that out.

We know that, 

Amps delivered = Total Watts of solar Panels ÷ Battery Voltage

Time it takes to charge = Battery AH ÷ Amps delivered


Amps = 180 ÷ 12 = 15A.

Time taken to charge = 100 ÷ 15 = 6.66 hrs (which is 6 hours and 40 mins average).

The above calculation is a rough estimate of how long it might take to charge it completely. 

Remember that these calculations are roughly based on estimates, and therefore, you must consult with your manufacturer, or engineer on what size they recommend. 

Factors to Consider When Buying a Solar Panel for Home

There are some other factors you need to consider while installing panels for the first time. 

  • You may have a rooftop with damp conditions, and you need to make sure you are installing a solar panel in a dry and suitable area where it receives plenty of sunlight.
  • Avoid any shades above the panels, or it won’t be able to get enough sunlight.
  • You might need to install a lightning guard, to avoid breaking your solar panels in case of lightning or thunderstorms, so it is advised to have lightning earthing. 
  • Make sure you have a proper insulation, and it is waterproof, so that neither fire or short circuit happens. 
  • In case of an unfortunate fire breakout, you need to have a fire extinguisher in case, or call local fire service immediately. 
  • What equipment you wish to run, because you can’t run an AC out of low power generation of the solar panels. This is why you need to double check what equipment you wish to run using solar panels, and take expert guidance regarding size and power you need to generate. 

Charging A 100AH Battery With A Solar Panel

Considering how the world has evolved around technology, we require electricity no matter where we go.

It is present on all sources of gadgets around us.

What we need is a source of electricity.

And the best source of electricity, and green power comes through solar energy. 

Solar powered vehicles, devices, and even big companies having solar powered factories help reduce the usage of non-renewable power sources, resulting in more sustainable power.

This will help in development of solar energy into something even bigger, and we will hopefully cut down the usage of oil or nuclear power plants. 

We have seen how we can install solar panels by ourselves, which can be a bit tricky, but not impossible.

We have also seen the benefits of using solar panels other than traditional electric current, which outweighs the negative effects compared to the positive impacts. 

Few things to keep in mind for selecting size of panels and chargeability of it, it all depends on location, cost of installation, the items you need to power up and a good source of sunlight exposure. 

And finally we got to know the sizes and how long it might take to charge the 100AH battery. 

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