What is the output of a 100 watt solar panel on a cloudy day?

What is the output of a 100 watt solar panel on a cloudy day? - Featured Image

What is the output of a 100 watt solar panel on a cloudy day? This is a question that is often asked when people are looking for reliable off-grid power. This is because everybody knows you can’t collect sunlight when it is dark.

But what about during a cloudy day or inclement weather? Does a 100 watt solar panel still collect sunlight? In this post I will be explaining that all is not lost on a cloudy day. There are steps you can take to help mitigate that pesky cloud.

Because let’s face it unless you live in a very sunny part of the world these types of days are going to happen. 

What is the output of a 100 watt solar panel on a cloudy day?

“The output of a 100 watt solar panel is affected on a cloudy day and it all depends on the amount of cloud. Stratus, Cumulus, Cirrus, Alto and Nimbus will all prevent effective sunlight harvesting. But as a rule of thumb clouds can reduce the solar panels output by 50% to 100%. I know that you might be shocked by that figure but there are some things you can do to mitigate it.”

You probably don’t realise that a solar panel hardly ever reaches its maximum wattage. You would think that it would reach maximum output on a hot sunny day. But this is not the case because the solar cells lose efficiency the hotter they get. 

Solar panels produce their best performance in sunny cold conditions. So taking a camping trip to the north pole with your 100 watt solar panel during summer. Isn’t such an outrageous idea after all.

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What Is The Output Of A 100 Watt Solar Panel On A Sunny Day?

It’s all down to sun hours and the 100 watt solar panels orientation towards the sun. If you are fortunate enough to live in a very sunny climate you can expect 8 sun hours per day. 

By multiplying the number of sun hours = 8 by the number of watts of the solar panel = 100. This will give you the number of watts the 100 watt panel will produce per day. This would equal 800 watt hours per day. 

What About The Number Of Amps A 100W Solar Panel Produces At Full Power

It’s all well and good knowing how many watts the solar panel produces per day. But because 100-watt solar panels primary use is for portable applications charging a battery. We are more interested in how many amps it produces at full power.

To find this out the maximum power rating 100w should be divided by the maximum power voltage (VMP). This information can normally be found on the rear plate of the solar panel. Or in the manufacturer’s specifications. In this case I will use a random VMP of 18.5

So the equation for this would be 100 ÷ 18.5 = 5.41. So the maximum number of amps this 100 watt solar panel produces at full power is 5.41A. There are claims on the internet that the average 100w panel will produce 6 amps. 

Always check the panel in the way I have described for the exact amperage. Sometimes you can also find it in the manufacturer’s specifications.

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Mitigating A 100 Watt Solar Panel Output On A Cloudy Day?

YouTube Video From SolarBurrito Do Solar Panels Work When it’s Cloudy?

First of all let’s take a look at how many amps a 100 watt solar panel might push out in cloudy conditions. I have produced a table below for this purpose. For the amp output calculation I am using our 5.41A example.

Reduction In Watt/Amp Output Due To Cloud 100 Watt Solar Panel Output
0%100 Watts / 5.41 Amps
10%90 Watts / 4.87 Amps
20%80 Watts / 4.32 Amps
30%70 Watts /3.79 Amps
40%60 Watts / 3.25 Amps
50%50 Watts / 2.71 Amps
60%40 Watts / 2.16 Amps
70%30 Watts / 1.62 Amps
80%20 Watts / 1.08 Amps
90%10 Watts / 0.54 Amps
100%0 Watts / 0 Amps
What is the output of a 100 watt solar panel on a cloudy day? - Amp meter

The table shows how dramatically solar panel output can be affected by clouds. Up to a 60% reduction in power could be mitigated by carrying a second 100w solar panel. The two could be hooked up in parallel or series. This would then double the output in both watts and amps.

Combined with reduced usage the battery discharge could be kept to a reasonable level until full sun returns. This could be a solution if you are using the panels to charge a portable power station. Or charging your recreational vehicle, boat, trailer or caravan house battery with a portable panel.

But if you have installed panels you would just have to ride it out. Unless you have already factored cloudy conditions into the design of your solar array.  You can follow this link check out a full post about How Many Amps Does A 100 Watt Solar Panel Produce?

Below 60% Reduction In Power Output Due To Cloud

As you can see a below 60% reduction in power would render your charging power almost useless. A second 100 watt panel would help but at 90% reduction you would need 4 x 100w solar panels. Just to produce the 60% reduction of 2.16 amps. 

If you had a 50Ah battery that was 50% discharged. At 90% power reduction it would take 46.3 hours to bring it back to full charge. And you wouldn’t be able to use it while it was charging. 

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What Other Environmental Factors Can Affect The Output Of A 100 Watt Solar Panel?

There are quite a few environmental factors that can affect the output of a 100-watt solar panel. These need to be taken into consideration when sizing your solar array and charge controller. Some are more important than others, placement and shade being the most important.

Type of Solar Panel

As I mentioned earlier it is best to use monocrystalline solar panels to achieve maximum efficiency. However there are reports that polycrystalline panels perform better in very hot climates. 


Temperature can be a factor because as the solar panel heats up it loses efficiency. This is due to the semiconductor material the PV cell is made from. As the temperature rises the voltage between the cells drops causing a loss in efficiency. 

One way to keep the panel cooler is not to have it lying flat. It is better to stand it up to create an airflow behind it to keep the 100 watt solar panel cooler.

Placement and Orientation Towards The Sun

In the northern hemisphere solar panels should be placed in a southerly direction. In the southern hemisphere they should be placed in a northerly direction. If possible it is best to stand them up at a 45° angle.

This will ensure that the panel is set at the correct pitch toward the sun. A solar panel that is stood up will collect significantly more sunlight than one that is laid flat. This is because a panel laid flat will only collect the best sunlight when directly overhead.

Allpowers 500w Solar-Power-Station with 100w Polycrystalline Portable Solar Panel

Never let a 100 watt solar panel become shaded sunlight collection will stop altogether. This means the battery will stop charging !!!!!!!!

Dirt And Snow

Clean the panels regularly with a sponge and soapy water as dirt can hamper sun gathering. As can snow especially if completely covered in the stuff, charging will stop. 

Sunlight Time 

As a rule of thumb you are probably looking for 6 hours of continuous sunlight time per day. This should provide around 30Ah of charging per day. Which should be sufficient to charge a 50Ah battery to full capacity. 

The Seasons

The best seasons for collecting sunlight are late spring to early in the fall. During the winter months the sun is less powerful and there is less sunlight time. This doesn’t mean you can’t use your 100 watt solar panel because you can. You just have to take the extra cloud, less sunlight time and weaker sunshine into consideration.

Solar Panels Age

Over time all solar panels lose efficiency usually by about 20% at 25 years old. This means that if you think you can save money by buying second hand solar panels. You probably need to re-think it because it would be better to wait until you can afford new ones.


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.

The Albedo Effect

This is all about the sun’s rays being reflected from a light coloured surface. It really only comes into play near water or in snowy conditions.

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My Closing Thoughts

It’s pretty obvious that a 100 watt solar panel will still collect sunlight in cloudy conditions. Very heavy clouds will severely hamper the panels ability to perform. Having said that I am sure all of us have experienced gaining a sun tan when the clouds are about.

So all is not lost and remember the figures I have presented are theoretical. Real world conditions often differ from the world of hard facts and figures. You shouldn’t have too many problems collecting sunlight between late spring and early fall. 

The winter months may prove a problem but if you overestimate the amount of solar panel power. You can at least mitigate some of the effects of clouds. 

Want to know more about 100 watt solar panels follow this link below

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I hope you enjoyed this post and have found it helpful. If you have any questions about the output of a 100 watt solar panel on a cloudy day. Or want to leave your own personal review, please feel free to leave a comment below.

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    4 Comments on “What is the output of a 100 watt solar panel on a cloudy day?”

    1. Wow, this is a great article. I’m relieved to learn that not everything is lost on a cloudy day. I had no idea that a standing solar panel can collect substantially more sunlight than one that is set flat. As you mentioned, dirt can obstruct sun gathering, therefore I’ll make sure to clean my panels with a sponge and soapy water on a regular basis.
      Thank you very much.

      1. Hi Lionel 

        Yes it is important to know that a solar panel collects more sunlight if set on an angle usually 45°. However most portable solar panels can be set to the exact pitch to gather the most sunlight. Depending on what time of day it is. 

        For example the sun is lower in the sky in the mornings and evenings and higher during the rest of the day. As they say every little helps !!!



    2. Hubby and I are totally setup for off-grid living in our tiny house on wheels. But even so this whole converting watts to volts and everything else does my head in. I wish all the measurements were the same everywhere in the world. It certainly would make the whole process much easier.

      Your list of cloud coverage is very helpful. Our panels are flat on the car roof and angled on our van roof so I guess we’re good. One tip I want to add is to never discharge a battery below 60% or so. Do it once and you’re ok. Do it twice and the battery is likely dying a premature death. We had to experience this the hard (costly) way. Ouch.

      1. Hi Monika

        Yes all the different terms like watts, amps, volts and Ohms can be confusing. And sitting down and doing the necessary calculations to size a solar system can be a pest. But I am hoping that the information I provide on my site makes it easier for people to understand.

        Discharging lead-acid batteries below 50% is not a great idea because as you point out they will be damaged. I know that LiFePO4 batteries are more expensive. But they can be safely discharged to 90% without ill effect. This also means that if you use a 100Ah LiFePO4 battery 90 amp hours are available to you. 

        As opposed to only having a maximum of 50Ah available with an equivalent lead acid battery. You can take a look at my article What size solar panel and controller is needed for lithium ion batteries? This article includes a section on the advantages of Lithium batteries.

        Happy camping



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