I Tested a 100 Watt Solar Panel on a Cloudy Day (Video!)

I recently tested how much a 100 watt solar panel outputs on a cloudy day.

To do so, I rigged up a custom testing setup and monitored the solar panel’s power output throughout the day as it got cloudier and cloudier.

Here’s what I found:

A 100 watt solar panel will output around 10 to 20 watts on a typical cloudy day. But that amount can range widely depending on how cloudy it is, as well as all the other factors that affect power output, such as panel temperature and charge controller type.

Keep reading for my full test results.

Video: 100 Watt Solar Panel vs Cloudy Day

I made a short video of my power output test. Check it out:

Note: I’m going to start posting more of these sorts of short videos on my YouTube channel. If you’re interested, please consider subscribing by clicking here. Thanks! 🙏

Testing 100 Watt Solar Panel Output on a Cloudy Day

To test how much a 100 watt solar panel outputs on a cloudy day, I connected a Renogy 100 watt solar panel to a 12V 100Ah LiFePO4 battery via a Victron MPPT charge controller. The charge controller let me monitor power output with an app on my phone.

The charge controller let me monitor the 100W panel’s power output with an app on my phone.

I then took my solar panel out to my backyard and angled it towards the sun.

Full Sun

To get a baseline, I first measured the 100W solar panel’s output in full sun. I measured 90 watts.

My 100 watt solar panel output 90 watts in full sun.

Note: An output of 90 watts is actually great for a 100 watt solar panel. Often, solar panels output around 70-80% of their rated output. So a typical reading from a 100 watt solar panel in full sun is closer to 70-80 watts. I suspect the reason I got such a good reading is because I ran this test on a cool day in January. Solar panel power output drops as temperature rises, so the lower temperatures at the time of testing (around 50°F or 10°C) were helpful here.

Light Cloud Coverage

Next, I waited until the clouds started to roll in.

I took another power output reading while there was a thin layer of clouds in the sky. I got 48 watts.

My 100 watt solar panel output 48 watts in very light cloud coverage.

Partly Cloudy

But I didn’t stop there! I waited until there was medium cloud coverage — what a weather forecast might call “partly cloudy” — and took another reading: 30 watts.

The 100 watt solar panel output 30 watts in partly cloudy conditions.

Mostly Cloudy

The clouds kept on coming, so I waited until there was heavy cloud coverage, what could be classified as “mostly cloudy.” I checked the panel’s output again and got 22 watts.

The 100W solar panel’s power output dropped to 22 watts in mostly cloudy conditions.

Full Cloud Coverage

Finally, I waited until the sky was completely overcast with a thick layer of clouds. It looked like a true cloudy day, the kind where the weather forecast for the day just says “cloudy.” I took one last power output reading and got 8 watts.

My 100 watt solar panel output 8 watts in full cloud coverage.

Takeaways

Solar panels definitely work on cloudy days, just with a reduced output. A 100 watt solar panel will output around 10-20 watts on a typical cloudy day.

But!

The amount of power it outputs depends heavily on how cloudy it is. (Not to mention all the other factors — like shading, tilt angle, panel temperature, time of day, and type of charge controller — that affect solar panel output.)

With very light cloud coverage, you can get an output of around 30-50 watts with a 100 watt solar panel. On a completely overcast day, you may get around 5-10 watts, even at noon.

Tip: If your 100W solar panel is outputting significantly less power than expected, it may be worth testing your solar panel to see if it’s working properly.

What Will a 100 Watt Solar Panel Power on a Cloudy Day?

LED strip lights like these usually consume 15-20 watts at peak brightness, which is about as much power as a 100W solar panel will generate in cloudy conditions.

I crunched some numbers and found that you can charge a phone or laptop and power some LED lights with a 100 watt solar panel on a cloudy day.

To explain further, let’s assume your 100 watt solar panel is outputting an average of 15 watts on a mostly cloudy day. And let’s also assume that this average wattage goes on for a total of 4 hours over the course of the day.

With that, we can estimate that your 100 watt solar panel will output 60 watt hours in very cloudy weather (Calculation: 15W * 4 hrs = 60Wh).

Now we have an estimate of how much power a 100 watt solar panel produces on a cloudy day: 60Wh. And we can use this estimate to figure out what we can run with that much power. (For a more complete breakdown of expected energy production from a 100W solar panel, check out the test I did to find out how much energy a 100 watt solar panel can produce.)

More energy-intensive appliances like TVs and 12V fridges are mostly out of the question. Those can use 60 watts or more when on, meaning that you could only run them for 1 hour before you’ve used up all the energy your 100 watt solar panel will produce over the course of the whole day.

So we have to look at smaller devices. Think phones, laptops, and LED lights.

Nowadays, typical phone batteries have around a 20Wh capacity. Laptop batteries tend to be about a 50Wh capacity. And 2 LED bulbs, or a strip of LED strip lights, will consume about 20 watts.

This means, with those 60Wh of solar energy, you could charge your phone (20Wh) and power your LED lights for 2 hours (20W * 2 hrs = 40Wh).

Or you could charge your laptop (50Wh) and run your lights for around 30 minutes (20W * 0.5 hrs = 10Wh). Or charge both your phone and laptop most of the way.

Of course, this answer is assuming that your battery is mostly empty. If it isn’t, then what you can power varies a lot depending on its capacity and state of charge. (If you’re unsure how much remaining amp hour or watt hour capacity your battery has, check out our battery capacity calculator.)

Best 100 Watt Solar Panels

If you’re in the market for 100 watt solar panels, I recently tested 5 of the most popular options.

Here were my 3 favorites:

  1. Top Pick: Renogy 100W 12V Monocrystalline Solar Panel
  2. Budget Pick: Rich Solar 100W 12V Polycrystalline Solar Panel
  3. Honorable Mention: WindyNation 100W 12V Polycrystalline Solar Panel

Full review: Best 100 Watt Solar Panels

Almost any 100W 12V watt solar panel will work just fine. They are all rated to output 100 watts at STC, after all, so you’ll get about the same output from all of them. Beyond the ones above, there are plenty of cheap options on Amazon that will get the job done.

The Bottom Line

After testing it myself, I learned that you can expect a 100 watt solar panel to output around 10-20 watts on a typical cloudy day. But the actual power output will vary quite a bit depending on how cloudy it is, as well as other factors that affect solar panel output, such as shading and panel temperature.

For instance, over the course of my day of testing, I measured outputs of around 50 watts when there was a very thin layer of clouds covering the sun, all the way down to 8 watts when the sky was completely overcast.

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Alex Beale
Alex Beale
Alex Beale is the founder and owner of Footprint Hero. As a self-taught DIY solar enthusiast, Alex has spent 4 years producing educational solar content across YouTube, TikTok, Instagram, and the Footprint Hero blog. During that time, he's built Footprint Hero to over 7 million blog visits and 18 million YouTube views. He lives in Tennessee.