= solar, right? So, do solar panels work best when the sun is shining and the temperature is at its hottest?
If you know the answers, leave a reply below and enter our competition.
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Not necessarily - the sun always shines - it also needs to be (roughly) illuminating the part of the earth where the panels are located, and not too hot, as panel performance drops off in higher temperatures. (ok, that was really pedantic 
but I did resist mentioning that the temperature is possibly optimum when the sun is shining elsewhere …oh …)
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Depends. Yes and no.
Solar panels will generate more if the panels are under sun than under cloud (or moonnight of the werewolves amongst us), at the same location, sunlight intensity and temperature.
For example, when a panel heats up, it will become less efficient at generating. So a cooler panel will generate more than a hot panel under the same light intensity (which is why it is could be a no). However, if a cloud passes over a panel, the panels will produce less than the same panels under sunlight immediately before the cloud went over (even though the panels may cool slightly due to the cloud). The cloud reflects more sunlight back into space and and potentially absorbs some of the sunlight making the intensity of light under the cloud significantly lower (which is why it is dull when a cloud passes over).
I imagine that the panels also get to an operating temperature under sun when they cease operating (or the inverter does) to protect the electronics from heat)
Another interesting anomaly is generation will spike higher immediately before the cloud passes over the panels, as the cloud sides reflects and scatters light. This reflected and scattered light is added to that which come direct from the sun and increases the light’s intensity resulting in a increase in generation.
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The panels don’t cease operating, the output current rises as the cell temperature rises, but the voltage decreases, the overall effect being a reduction in output, at a rate typically a bit under 0.5% per degree increase. Output increases at the same rate with colder cell temperatures. Cells may reach 65-70C in very hot weather, but the panel rating is for a cell temp of 25C.
Some inverters may reduce output when the ambient temperature exceeds 40 -50C, depending on the particular model and where its temperature sensor is located, but it is unlikely they will stop operating (for good quality inverters).
Some people run fans to increase cooling of their inverters, to increase the life of the electronics and prevent the output ramping down in hot weather.
Panels are rated for a radiation intensity of 1000W/m^2, but in summer direct sunlight can be over 1200W/m^2, and with a few clouds around in the near-sun direction providing forward scattering of sunlight, plus some reflection, radiation intensity can be significantly above that. I’ve measured over 1700W/m^ with my pyranometer. This often leads to 15-20% more output than the PV array’s rating in mild weather. In the solar business, this is known as ‘cloud edge effect’. I’ve written a bit about it here: https://forums.energymatters.com.au/solar-wind-gear/topic1507.html
In cold weather with extra radiation from cloud edge effect, a PV array’s output may be over 25% more than its rating, if the inverter (or charge controller for off-grid) can handle it without clipping.
Yes PV panels do work best when the sun shines directly face-on to them, but cold temperatures and a few scattered clouds can cause them to produce significantly more output than their rating. Hot temperatures will reduce their output well below their rating.
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Panel output is rated at 25 degrees Celsius, that is the panel temperature, Output will increase at a lower ambient air temperature, as the panels are cooled. Will decrease in output as air temperature increases. I observe my 1500 watt panel reach max output an a cold winters day, very rarely in summer.
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More specifically, the cell temperature. Normal (or nominal) operating cell temperature (NOCT) for many panels under typical mild weather conditions is around 45-50C.
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Good point. Will TEST cells with my remote temp tester next time I am on the roof. Will get back to you.
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Great answer @gordon, I love the detail that you and @phb have provided for readers. Thanks to everyone else for the pointers as well 
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Sunny winter days are the best for solar PV system output.
Many studies confirm it.
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Feb and March were fairly hot - a few days with clouds …
In August we had some clouds on the 24th, unusual I know 
Year so far …
6800 Kw/h for the year, about 23.7 Kw/h per day average.
Do the longer days in summer explain it? I’m sure they do to some extent, but the data I’m pulling over modbus from the inverter suggests to me the peaks are lower in winter, with rare spikes over 4, but in summer it often pins the dial, so to speak …
Edit: 13:30 hours of sun in summer, 10:40 hours of sun in winter - winter is about 78% of the summer sunniness but very roughly my best winter days look only to be low to mid 70% of the best summer days.
Interesting … in my defence I am looking at what I think is evidence through the eyes of an IT guy not an electrical engineer …
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A number of factors come into play. It depends on how your panels are tilted- almost flat is better in summer (and any overcast weather), or tipped up to face the sun in the middle of the day in winter.
The face-on sun is a bit less intense in winter as the solar radiation has to travel through more atmosphere, but the generally cooler temps increase panel voltage and power. Summer has longer days, but hot cell temps reduce the panel voltage and power, but even so, the longer sun hours more than compensate for the lower output, so energy produced tends to be higher in summer.
In practice, when using a tracker so that the panels are facing the sun all day, the highest peak outputs happen more often in spring, on cool sunny days before midday with a few clouds around. Peak output can be similarly high in mid summer on a cool day, but you don’t get so many of those around here. Winter peaks tend to be slightly less in my experience due to the low sun angle. In winter the sun is not all that far above the horizon for much of the day, but in summer, a much smaller percentage of the day has a low sun.
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