Sunlight vs Daylight

[Ekij]

I've a few small solar PV panels and I've noticed a huge drop off in open circuit voltage and short circuit current when the panel is moved from direct sunlight into just normal daylight.

Alas I live in an area of the world where sunlight isn't all that commonm, we get a lot of light white clouds (and more than a few grey ones).

If I get a large solar panel can I expect any useful power out of it when it's just in daylight? The little ones would suggest not.

[wpowokal]

Your observations are correct, solar panels rely on one of the suns particles of energy called photons hitting the solar panel. In direct sunlight the solar panel receives maximum amount when at right angles to the sun, in just daylight these particles are reflected and much less efficient, an exception here is when some clouds are present, the reflection from them together with direct sun give additional power..

Google "how solar panels work"

allan down under

[imsmooth]

They also function better in colder, and windier weather when the panels are not as hot.

[Ekij]

I'm aware that the available power drops off in sunlight.

What I'm asking for is someone with experience of how much it drops off by.

Just a rough figure, is the available power in daylight closer to 30% or 3% of the sunlight value?

[ghurd]

Depends on how thick the clouds are.

At least half.  More like 75% drop with real cloud cover.

Some extra-nasty days, I know we don't get an amp-hour out of around 200W of panels.

Worst case solar insolation month here is <0.5H/D.

G-

[Flux]

Not really possible to give you the figures you want, even in daylight there is an enormous difference between a good clear cold sunny day and a hot hazy one.

Basically the output in daylight with no hint of direct sun is very low indeed ( likely 3%). There is no real prospects of doing anything useful with solar panels unless you have direct shadow producing sunlight. If you climate doesn't have those conditions then don't waste your money. At best with a good system on a cloudy day you can hold off the battery self discharge, don't expect any useful power.

Flux

[TomW]

Just a "ferinstance":

I have 850 watts of solar panels [rated].

In good, full sun with good straight on alignment I can get from 28 to 30 or so amps into a 24 volt bank. The amount varies by battery voltage and things like high clouds, haze or whatever. Lets say 30 for easy figuring. On cloudy and overcast days I get around 5 to 10 amps. Again it really depends on how much light there is from the sky.

So, I will say my experience is that I get anywhere from 10% to 30% of full sun amps with bright cloudy skies. Again this is highly variable by many factors, including temperature, light available, battery voltage, phase of the moon or who knows what all. I get about an amp in most any light bright enough to be called "daylight", even can see the solar volts bump when cars pull in the drive at night or yard lights turn on. No current but voltage hump from the panels.

Just my experience so far. Hope that helps.

Tom

[Opera House]

Your eye is logarithmic and the solar panels are linear.  A 90% drop in brightness is barely noticed by your eye, but that is exactly what the panels see.

[wpowokal]

I have logged my 9am panel output for a number of years now and on an overcast day at best I get around 10-16% of full sun, but as Tom stated there are many variables, time of year is very significant in regards to sun angle in the sky.

allan down under

[richhagen]

I'm at about 27.5 Amps in direct sunlight, and like Tom, I see at least an amp or two in daylight with the darkest of clouds.  Usually on overcast days, the range is from about four to ten amps if the clouds are really light.  Rich

[SteveCH]

In addition to the other posts, which are good answers, the panels themselves make a difference, though it isn't enough to count on. [Unless you can find someone in your area using them and they can show you that they are doing a good job....]

I mean, I have panels made in the early 80s and I have panels made two years ago and I have some inbetween. The newer panels are much better at generating power on cloudy days, not just because they are more efficient due to improved pv cell construction but also because some of the newer ones incorporate tiny lines cut into the glass which, according to the mftr's, are able to collect more incidental photons coming from less than direct incidence, such as scattered light from clouds. I cannot say for certain how much better these panels are working, I haven't tried to set up any tests. However, I have my newest panels running through a separate controller. I have three controllers, each handling about the same total amperage-worth of panels, so I can simply go over and read the numbers from their readouts. The newer panels are better in cloudy conditions. This is not info' that one would want to use as an excuse to run out and spend thousands of bucks on pv....

[ruddycrazy]

As the type of PV panel isn't stated here I will assume poly crystaline panels pv panels are used.My 900 watt array of Kaneka thin film panels along with my AERL900B MTTP will on cloudy days output 18-20 amps and in full sun 30 amps. Since I've added another 350 watts of poly pv panels I've see the amp gauge go to 40 amps. On my shed where I have 350 watts of the same 24 volt 175 watt Sharp panels and they will in full sun output 16 amps, when its cloudy they will do 3-11 amps. With both of my farm arrays they are 24 volts. Amaphorus panels will output more power in cloudy weather so like me a mix might be best for off grid situations.

Cheers Bryan

[Madscientist267]

And sometimes, there isn't any visible loss of light at all - I live near a paper mill, and when conditions are just right, I can have a 'clear' sky, yet the output still randomly and radically drops.

Even though the moisture has dissipated to the point of invisible, if the exhaust from the stacks is being blown in the line of sight to the sun, I lose up to 50% of my normal output. After some serious thought, research, and several weeks of paying close attention, I concluded this is CO2 in the exhaust, blocking infrared; a key component in usable PV light.

This one was baffling at first, but I finally determined that I wasn't in fact losing my mind (or panel) and that there was a real cause behind it. At one point I was looking for wiring problems and even monitoring output while slightly flexing the panel to see if there was a hairline or something... all to find out the problem was in the sky, but I just couldn't directly see it. Talk about nerve racking!

Looks like all this treehugger greenhouse gas paranoia crap we've been getting our heads filled with all these years actually may have a use after all! LOL

Steve