How Fast Does A 100w Solar Panel Charge A Battery?

How Fast Does A Solar Panel Charge -Featured Image

100 watt solar panels are lightweight and small compared to the bigger residential panels. Being smaller means less power output so how fast does a 100w solar panel charge a battery? This all depends on the size of battery you want to charge. 

Do you want to charge a small, medium or large battery?. Or a solar power station? Then the type of solar panel comes into play; Should I use a monocrystalline or polycrystalline solar panel?

There are even more factors to consider before the question can be answered. Like weather conditions, location relative to the sun, irradiance and the seasons. 

In this post I will answer the question as simply as possible without being too ‘techie’. I will use known variables and a 100Ah battery in my examples. 

How Fast Does A 100w Solar Panel Charge?

“A 12V battery capacity is measured in Amp Hours (Ah) and a 12V solar panel output is measured in watts. So the first thing we need to do is convert the solar panels 100w output to amps. This is achieved by dividing the solar panel wattage by it’s volts. 100 ÷ 12 = 8.33 Amps. 1 Amp of current charges a battery by 1 Amp-Hour. This means a 12 Volt 100w solar panel will charge a 12 Volt battery by 8.33 Amps of charge per hour.”

I really hope that has not confused you and you understand this is theory. There are many other factors involved which I explained in my introduction. But to work out how fast a 100w solar panel will charge a 100Ah capacity battery. 

You divide 100 by 8.33 that equals 12. 

Assuming it is a fully depleted battery at 0% charge to 100% fully charged. It will take 12 hours for a 100w 12 Volt solar panel to charge a 12 Volt 100Ah battery. 

The Reality Of Charging A Lead Acid Battery
The Reality Of Charging A Lead Acid Battery

In case you didn’t know, a lead acid battery should never be depleted to below 50% of it’s capacity. This is because it is easily damaged and allowing it to discharge below 50% will reduce its life cycle. So in reality with a 100Ah battery you can only use 50Ah of its capacity.

Going back to our theoretical calculation this means to charge a lead acid battery from 50% to 100%. It will take 6 hours. 

50 ÷ 8.33 = 6

So if you have 2 x 100Ah lead acid batteries depleted by 50% that would be 100Ah to charge. 

2 x 100 = 200 x 50% =100

We already know that it takes 12 hours to charge 100Ah with 1 x100 watt solar panel. This means you are going to need more than one 100w panel.

Related == >>>>>>>>>> Portable Solar Briefcase Review – ACOPower PLK 100w

Does The Type Of Solar Charge Controller Make Charging Faster Using A 100w Solar Panel?

This is a video from ACOPower showing how to expand their portable solar system . With a built-in PWM solar controller

A cheap solar charge controller can not only lengthen the charging process. But can also not protect your battery properly. Leading to permanent damage, loss of capacity or functionality over time.  Which might not be a big deal if you have cheap batteries. But batteries costing more will hurt your wallet if the controller fails to protect them.

It can be tempting to cut costs in this area. But controllers are your first line of defence when it comes to protecting your battery. Never be tempted to charge a battery with a solar panel without a controller it will overload. The result could be catastrophic for your battery and also for you personally. It is possible to cause a fire, explosion or even poison yourself.

There Are Two Types Of Solar Charge Controllers

The two types of solar charge controllers are;

Pulse-Wave Modulation (PWM)

Pulse wave modulation solar charge controllers work with matching voltages – 12 volt battery and 12V solar panel system. If the solar panel system and batteries have different voltages the controller won’t work. 

PWM charge controllers work by reducing the amount of charge going to the battery as it nears capacity. Then it trickle charges the battery to keep it topped off. 

Regarded as standard types of controllers they are simpler and less expensive than MPPT. Many manufacturers build them into their portable solar panels.


Maximum Power Point Tracking (MPPT)

Maximum Power Point Tracking charge controllers are more sophisticated and complex than PWM. They work in the same way as PWM controllers. But they also continually optimize the DC match between the solar panel and the charging 12V battery. 

This means the MPPT keeps the power transfer between solar panel and battery, at its highest efficiency. Meaning they can use the full power of the solar panels while still providing effective protection. Unlike PWM they can also be paired with different voltages between the solar panel and battery. 

So If you are planning on installing a very expensive battery bank. The extra cost of an MPPT controller will be worth it. Using an MPPT controller won’t significantly reduce the time it takes to charge a battery. It will help shave some time off and give you peace of mind

Related == >>>>>>>>>> Solar Charge Controller Review (Renogy MPPT And PWM)

What Other Factors Need To Be Taken Into Consideration When Charging With A 100w Solar Panel?

There are several other factors to take into consideration charging a battery with a 100w solar panel. 

Sunlight Time

If there are lots of clouds this will most definitely affect the charging time. This can be especially important in northerly latitudes. In southern latitudes not so much. Where you can probably expect an average of  5 hours of full sunlight per day. Which would be enough to recharge a single 100Ah battery. Providing it has been discharged by no more than 50%.

Hands making a love sign through sunshine
The Seasons

The best season for sunlight is obviously going to be summertime wherever you are on the planet. Late Spring and Autumn can also be good for sunlight but not as powerful as in the summer. In Winter the sun is weak but sunlight collection is still possible. 

Overall the seasons and cloud can reduce solar panel output by up to 60%. If we refer back to our original calculation. This means the 100w solar panel output would be reduced to 40 watts.

Dividing 40 by 12V equals 3.33

Using our 50% capacity usage of a 100Ah lead acid battery. We can now work out how long it would take in low light conditions for the 100w panel to charge it.

50 ÷ 3.33 = 15 hours

This is also the type of performance you can expect during the summertime on a cloudy day.

 ACOPower 200Ah Lead Acid Battery
ACOPower 200Ah Lead Acid Battery
Types Of Solar Panel

The best type of solar panel to use is going to be the most efficient which is currently monocrystalline. These types will draw the maximum amount of available sunlight to charge a battery. 

You might be tempted to buy polycrystalline or second hand solar panels. Because they are cheaper and you have a limited budget. I understand if you do this. But you can have few complaints if your system doesn’t work that well.


The solar irradiance is the output of light energy from the entire disk of the Sun (Source: NASA)

This can be affected by your location relative to the sun. Therefore affecting the amount of solar energy collected by the 100w panel.

Temperature Representation

Excessive heat can be a problem for solar panels. I know you’re thinking that can’t be true? Unfortunately it is, as a solar panel heats up during the day the solar cells become less efficient. This is seen in baking hot conditions and can significantly affect solar collection.


If any part of a solar panel falls into shade it can stop charging altogether. This needs to be watched out for. If it does happen the solar panel needs to be immediately moved back into the sunlight .

All these additional variables and there are more can seriously affect your solar panel charging power. 

Related == >>>>>>>>>>  How Many Batteries Can A 200 watt Solar Panel Charge?

What Types Of Battery Can A 100w Solar Panel Charge?

A 100w solar panel can charge the types of battery that would be used in a mobile or small building setting these include;

  • Starter
  • Deep Cycle
  • Flooded
  • Sealed
  • VRLA
  • AGM
  • Gel
  • Lithium Iron Phosphate (LiFePO4)

While some lead acid batteries can be very inexpensive their life cycles can be short. As low as 500 cycles, AGM batteries can be 2000 plus and LiFePO4 can be 4500 plus. But a 100w solar panel can charge them all if some a little slowly.

For instance a LiFePO4 battery can be discharged to 0% with no ill effects. With two days of good sunshine the 100w panel would charge it. But taking night time into consideration it would take 36 hours. 

Renogy Smart Lithium Iron Phosphate Battery  12V 100Ah
Renogy Smart Lithium Iron Phosphate Battery 12V 100Ah
Portable Power Stations 

Portable power stations can be charged quite effectively with 100w solar panels. This is because the lower wattage stations have relatively low Amp hour ratings.

For example a well known 1000 watt solar power station’s lithium Ion battery capacity is 46.4Ah. The company recommends a 200 watt solar input.  If you lived in the southern hemisphere do you think a 100w solar panel would do the trick?

Another example is a manufacturer’s 576w generator battery capacity is 20Ah. I believe a single 100w solar panel would keep this station charged. I can’t tell you exactly how quickly due to the built-in solar controller. The controller will  be dampening down the power to keep the generator stable. But I would guess under full sunlight it would take 3 to 4 hours.

Related == >>>>>>>>>> Renogy Smart Lithium Iron Phosphate Battery Review 12V 100Ah

My Closing Thoughts

I believe my investigation has shown that a single 100w solar panel is best used to charge 50Ah batteries or less. Even on a fine sunny day the fastest charging time you can expect is around 6 hours at best. Although you can use it to keep a 100Ah battery topped off. Or you think you will never discharge the battery by more than 20 to 50%

It is a fact that the more solar power you use to charge a battery the faster it will charge. When it comes to 100Ah batteries. I would say 300 watts of monocrystalline solar power using a MPPT controller. Would provide the quickest charging time taking all variables into consideration. 

Find out more about 100w solar panels with my Renogy 100w solar panel review.

I hope you enjoyed this post and have found it helpful. If you have any questions about 100w solar panel charging times. Or want to leave your own personal review, please feel free to leave a comment below.

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4 Comments on “How Fast Does A 100w Solar Panel Charge A Battery?”

  1. Hey Steve, thanks for writing this awesome article. I found all of this information very actionable and useful for my daily life. I didn’t even know that solar panels could be overheated so thanks for that info. So if I had solar panels that were left up on my roof for too long, they would generate less power? And what is a flooded battery? 

    1. H Gabriel I am pleased you found my article useful.

      Solar panels start losing efficiency when their temperature is over 95° F/35°C. But they do not lose very significant efficiency until they reach a temperature of 150°F/66°C and above.

      A flooded (wet cell) battery is a type of lead acid battery that uses lead plates and sulfuric acid.



  2. Thank you for answering the question about how long it takes to charge a 12 Volt 100Ah battery with a 100w 12 Volt solar panel. However, when it comes to optimizing this time, where do you think the focus should be? I believe that all designs can be improved. What aspect should be dealt with?

    1. Hi Ann

      Most certainly the solar controller you use. Charging efficiency is lost charging the battery. But it can be improved by using an MPPT solar charge controller. And of course using Monocrystalline solar panels. 

      However I think over the next few years we will see the rise of Peroskite solar panels. These types are reported to be more efficient than Mono



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