Is a 100-Watt Solar Panel Enough to Charge a 100 Ah Battery

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The sun is a great source of clean energy. What’s even better is that you can store this energy in batteries – making it available anytime, anywhere. Depending on the size of the system can be used for anything from lighting up your backyard to providing reliable electricity in off-grid homes. 

Solar batteries are the heart and soul of a solar setup since they provide the storage necessary for you to have access to power whenever you need it. The 12v solar battery is one of the most important components in a solar system.

It’s the powerhouse behind your energy. 12v batteries have been used for decades to store energy for everything from cars, to boats, to homes. 

In this article, we take a look at charging them using solar panels.

Solar Battery

What Size Solar Panel to Charge a 12v Battery 

A 12v battery will need a 12v Solar Panel, right? Actually No!. The truth is even the so-called 12v solar panels are not actually 12v. In order for a current to flow from the solar panel to the battery, there needs to be a potential difference. The potential difference creates a current that flows from the solar panel to the battery.

A 12v solar panel will more likely be 18v (sometimes higher)  when exposed to sunlight this voltage can fully charge a 12v battery which when fully charged is around 14.4V.


How many Watts Solar panels for a 12v battery

The wattage of the solar panel will depend on the size of the battery and how fast you want to charge the battery. There can also be budget considerations as the higher the wattage of a solar panel the higher the price. 

The simplest way to match a solar panel to a battery is to calculate the watt-hours. A 100-watt solar panel will produce approximately 500 Watt-hours in 6 hours of full sunlight. This is enough to fully charge a 40 Ah battery. 

To calculate the watt-hours capacity of a battery you multiply the voltage by the Ah rating. So a 100 Ah 12v battery will have a 1200 Wh.  For a 100Ah  battery, you will need a 200-watt solar panel to fully charge the battery in around 7 hours. 

Any Wattage solar panel can charge a 12v battery however the amps will dictate the time and whether the charge is just for maintaining the battery, partially charging, or fully charging it. 

How Many Amps to Charge a Battery from Solar

The number of amps to charge a battery will depend on the size of the battery. Batteries will have a c rating that indicates at what percentage of their capacity that it should be charged. 

For example, a 50 Ah car battery can be trickled charged at 2 amps and fast-charged at a maximum of 10 amps.

So for a trickle charge, you can choose a solar panel from 5 – 30 watts, and for fast charging that’s anything from 120 – 200 watts. 

When using solar panels it is a must to have a charge controller to regulate the charge from the solar panel so as not to overcharge the battery. 

Picking the Right Charge Controller 

A charge controller is a device that is used to regulate the charging of batteries. It is used to prevent overcharging and to protect the battery from damage. The charge controller is also used to regulate the voltage and current of the battery. 

Custom settings may be required for AGM and Lithium batteries as the default settings are usually for lead-acid batteries. 

A battery charge cycle consists of four main stages: charging, constant current, constant voltage, and floating. The cycle begins when the charger is first connected to the battery. This starts the charging stage, during which the charger supplies a constant current to the battery. 

The constant current stage lasts until the battery reaches a specific voltage, at which point the charger switches to supplying a constant voltage. The constant voltage stage continues until the current flowing into the battery drops to a very low level. 

This signals that the battery is nearly full, and the charger switches to the floating stage. In this final stage, the charger maintains a lower voltage on the battery to prevent overcharging. 

Some controllers conduct routine battery equalization (at least once a month). A battery’s equalization is the process of maintaining all cells within the battery at the same state of charge. 

This is done through controlled overcharging, where each cell is raised to a voltage above its normal operating voltage. This helps to break down any stratification that may have built up within the cells and allows for the equalization of charge levels.  

How to Set Up a Solar Panel to Charge Your Batteries

  1. In order to start determining the location to install the charge controller. This should be as close to the battery. Make sure that this area is water-tight and allows for as much airflow as possible.
  2. Ensure you choose the right wire gauge for your cables. At the end of the cables use alligator clips or ring terminals that are to be hooked up to the battery terminals. The clips should be labeled positive or negative depending on where the cable is hooked up on the panel.
  3. Access the battery and identify the negative and positive terminals. This is easy as the negative terminal which is the ground cable will have a negative (-) and the power or positive cable will have plus (+) sign.
  4. When using a Charge Controller the battery should be the first thing you connect to the Controller. You should NEVER connect a Solar Panel to the Controller without a Battery. The charge controller should be clearly labeled for ‘BATTERY + and -‘to make SURE the polarity is correct.
  5. Once you have connected the battery now connect the charge controller to the solar panels. The charge controller should also be labeled for a ‘PV INPUT + and -‘. This is where you hook up the panels. Make sure not to mix up the positive and negative Positive (+) is usually red, while a negative (-) is usually black. If you are not sure you can check polarity using a multimeter.
  6. Make sure to select the correct profile for your battery on the charge controller whether Lithium, Gel or AGM batteries. More on this next.

Factors that Affect Solar Panel Output

Solar Panel Type 

There are two main types of solar panels you will find in the market; Monocrystalline and Polycrystalline solar panels. Monocrystalline solar panels are more efficient than other types of solar panels because they are made from a single crystal of silicon. 

This means that the electrons have a direct path to flow from the sun to the panel, which results in more energy being converted into electricity. In addition, monocrystalline solar panels have a long lifespan and are less likely to degrade over time. 

Battery Type 

The battery type can also influence how fast a battery bank is charged. AGM batteries are much more efficient at charging than old-school Lead-acid batteries. On the other hand, Lithium-ion batteries are typically easier to charge than AGM batteries.

This is due to the low resistance of lithium-ion batteries. With AGM batteries tend to slow down as they charge however for lithium-ion batteries there is a constant flow of charge throughout, which further reduces the time it takes to get them back up to full power. 


High temperatures are actually good for batteries as it reduces the resistance within batteries. However, they should not get too hot as this can cause gassing. 

Low temperatures can also be an issue, especially with lithium-ion batteries that cannot be charged under freezing temperatures. 

Solar panels are actually more efficient in lower temperatures so high temperatures and intense sunlight are not the most ideal. But low temperatures and a sunny day are just right. 

Efficiency Losses

Through the system, they will be small losses due to efficiency or inefficiency losses. First of all, not all the power from the sun gets converted to power, then again the are losses to the current as it flows through the wires to the battery. And again if you use an inverter in your system, not all the DC power will get through. This is why it is always important to oversize your system. 


Following that point of efficiency is the weather. Cloudy, rainy days are going to affect the output from your solar panels. Snow covering your solar panels will hinder power production. 


With an All powers 100-watt solar panel, you can charge your devices, and power up equipment from afar. Easily connect the car-charger connector to your cigarette lighter for maintaining your car battery, and the USB connector to charge your smartphone, tablet, or other electronic devices. The panels fold easily for storage and have multiple connectors to directly charge solar generators and even laptops. 

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