I must be so unobservant but just noticed the average for my 6 years of data says June generated less power on average than May, and July is better than June as well. This dispite June having the longest day. I looked at my estimates (from the official figures for my panel orientation) and - yes - the estimates show June as a “poor” month as well. I just can’t see the logic of this. Looking at other peoples systems it’s usually the case for any panel orientation. Anyone got an explanation?
well June only has 30 days the other too have 31 days
Aside from one less day of production, in some areas the best production doesn’t coincide with the month of solstice because of local weather conditions. e.g. where I am December is a rainier month than January.
Two other reasons; 1. Solar PV is less efficient in the heat although you would then think July and August would be less efficient and 2. Azimuth - typical roof angles are probably too shallow for face-on sun in June and May/July and slightly lower in the sky and hence a better angle “on average” over the day.
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Very interesting. I have a similar situation with my system. I looked up your weather means for Northern New Hampshire and see that you precipitation is pretty even over the year by months. It may be that your cloud cover during June may be higher in your area during June and your position near mountains may also play a part in determining how much fog and low clouds you may experience at your PV site. On the Southern California coastal areas May and June tend to be gray and gloomy with cool nights and mornings and hazy sunshine in the hours around noon with temperatures in the mid 70’s. As a result we have less PV output in those months than other Spring and Summer months. Here, April has particularly excellent PV output as does July and August. That being said every place is different for different reasons.
Here is what my records show on a monthly basis over several years.
Thanks to all for your ideas. Agreed June a day shorter than May and July so that’s something. I also note the issue of panel heat - May has a lower ambient temperature c/f June so that could also explain lower June figures but as for july being better than june - it must be that extra day and may be slightly lower panel temperatures as sun has passed it’s best. I have looked at a few UK systems and it seems quite general so it’s not “local” conditions. Eg Bournemouth south facing has the same effect as my (Birmingham E facing).
You could also use PVWatts to see what your expected output is predicted to be by month for your location:
https://pvwatts.nrel.gov/pvwatts.php
It probably has to do with the location of the Polar Front - I’m not a met man but I had to study some global met for the commercial pilots licence. From memory, the polar front, which is not, I think, a front in the same terms as the ones we see on synoptic charts, is part of the transatlantic weather pattern. If it shifts south we get lots of arctic frontal weather, if north we get equatorial weather influence, hence heat waves. The polar front is associated with the jetstream, producing strong winds, generally westerlies, at high altitude. They run parallel to the polar front and are the reason why it’s usually quicker to fly from the USA to Europe than the other way round. It’s a long time since I looked at this so some of what I’ve said may be rubbish, but I think it’s generally what happens.
I believe we sometime equate “heat” to solar productivity. That isn’t exactly true. It can be hot out at mid Summer and the solar production may be lower than expected due to high humidity which blocks a large portions of solar radiation from reaching the PV panels. On the other hand on a cold clear mid Winter day the solar radiation will be higher than expected due to low humidity allowing a larger percentage of the total solar radiation to reach your PV panels. In general low humidity and clear atmospheric conditions produce higher than expected PV production and high humidity and hazy atmospheric conditions produce lower than expected PV production. See dark blue areas in curve below. Also note the very small absorption due to CO2…very interesting.
Solar panel efficiency drops with heat. 0.3% to 0.5% reduction in output per degree C is typical.
On a hot roof the panels can easily be 30-40C above their rated temperature for peak performance (usually 25C), meaning they’ll produce 10% - 20% less power than when they are cool.
Fortunate for me my heating is minimal due to the persistent westerly winds from the Pacific Ocean. My panels face that direction and benefit from the cool air flowing over them keeping the temperature quite moderate most of the Summer. I don’t have a thermometer on them but my weather station is nearby and it seldom exceeds 80 degrees throughout the Summer months. I included the graph in the previous post to show how water vapor absorbs large portions of the sun’s infrared radiation. Our humidity right on the coast seldom dips below 60% and this year in particular has been significantly higher than that all of the month of June. However, this may not be the case for you and many others.
Solar panels don’t generate much electrical energy from insolation with wavelengths longer than ~1100nm or shorter than ~400nm. Photons at longer wavelengths don’t have enough energy for the photoelectric effect and the shorter wavelengths are too strong. So most of the energy comes from the visible light and near infrared parts of the spectrum. In your chart most of the H₂O absorption occurs outside this wavelength range.