Re: Future Solar FAQ for review

Thanks for the putting in the effort on this FAQ, very useful.

I’ve always wondered why Davis and WD uses the 1366 W/m2 number to display as that amount of sunlight energy hitting the earth’s surface is impossible (except under short term edge of cloud effect conditions) to happen. In the past I’ve asked this a number of times and everyone just seems to ignore it or try to explain the since Davis displays this number it must be right. I did some studying on solar panels when I installed a system on my RV and they use a max of 1000 W/m2 to figure solar panel output (to make it easier to calculate solar panel output) knowing that the real max is somewhere around 1060 W/m2 under ideal conditions.

People fret over their rain gauge being off 5% or wind speed being off 10% but don’t seem to care that their solar sensor readings are off 30%. Guess that is just something that has always bugged me :slight_smile:

This figure of 1366 W/m

Have a look at http://bnellis.eu/climate/ccsolar.html and the graph explains the reasons why.

Thanks Stuart, one question, i cannot find the answer on google

this is because reflections will occur for example where there are some clouds and these reflections are what increase the readings from this type of sensor, so this is far from unusual and can happen at any time of year but probably more common in summer.

Is this the solar getting through say 400w/m2 (max @ TOD)+ the solar hitting the earth reflecting back at say 200w/m2, bouncing off a cloud
and then back to the sensor at a rough total of 600w/m2

Bashy you get the max available arriving at the earth surface plus the reflections from all other things then arriving as well. Yes I suppose it is possible some reflection from the earth back up to clouds and back down again but this part is quite unlikely to account for much increase as the earth will absorb some of the energy. The bottom line is what your sensor sees is the energy from the sun plus all the reflected energy from all other sources whatever they are, not easy to define or predict although probably clouds are the main source.

Stuart

Try googling “edge of cloud effect”

I like that :slight_smile:

“Clouds can also provide brief increases in solar power. If the sun is on the edge of a white cloud, its light bounces off and passes through the cloud’s edge. When this light is combined with the direct light from the sun, this produces sudden spikes in the power output of the system.”

Maybe we should be calling it cloud refraction instead of reflection?

I have updated the text at the start (see green section) to more correctly explain this edge of cloud effect. I hope this is an improvement.

Stuart

Reads well. I had forgotten this is big issue for solar systems until hankster’s post reminded me.

Thanks guys

Yea, a lot of people that installed panels on their RV would only buy a controller that just matched their panels max output and then blew the controller when this happened. Controllers need to be sized about 25% larger than the rated output of the panels.

yes, we are off grid here, and I plot the output from my solar panels on WD via a current cost and yes we get 2 bites at the same pie when certain types are clouds are around that reflect the sun to you
(the SMA invertor we use is rated more than the nominal solar panels array but in this circumstance does go to max power point tracking mode (and handles it well)) :slight_smile:

Having done a little more research I have added a comment in the green part to suggest this could increase readings by as much as 20%, this figure might be a little conservative but still shows it can be a significant increase. I think the 25% figure used for solar energy panels probably allows a bit more for safety.

Stuart

That is good to point out the increase. I haven’t researched it myself lately but I believe the newer controllers will now shut down if there is an overvoltage instead of letting the smoke out :slight_smile:

The FAQ has now been moved here.

But I didn’t want to delete your comments :slight_smile:

Thanks to Niko for moving the text to the FAQ and I would also like to thanks those who commented which helped me fine tune it all. I hope it proves worthwhile as an FAQ.

Stuart

Thank you for putting in the effort to create it!