Solar Panels maximum output compared to actual output

djmattc

Hey all
I've had a 3.5kWp solar PV installation up for about 2 weeks now. The maximum output I've seen at any one time is 2.7kW. Bearing in mind the sun is at full strength this week and I'm 99% sure that it can't get any brighter!
So my question, is that normal for a solar system to have an actual output which is over 20% less than it's maximum or should I be expecting more?

Merlin139

What is the capacity of your Inverter?

What direction is your system facing?

What angle are they at?

Has it been very hot where you are?

I have a 3.975 kWp system and a 4.2kWp inverter. The last 2 days when it has been hot my peak has been 3.22, 3.17 and today 3.29.

Last month I had a couple of days when it hit 3.7 or 3.9 Onthese days we had a nice cool breeze. When it gets much above 20 degrees peak production will drop off.

djmattc

Thanks for your reply.

Inverter is 3.6kW.
Roof is south facing.
I believe the roof is about 40 degrees - steep but not overkill.
It's been close to 30C here today air temperature so I guess the panels are baking.
1PM seems to be peak output @ 2.7kW.

So your output values seem pretty much in line with what I'm seeing. Would be interesting to see what the output would be on a cooler day. Do you know why they are less efficient when they're too hot?

Martyn1981

djmattc wrote: »

Hey all
I've had a 3.5kWp solar PV installation up for about 2 weeks now. The maximum output I've seen at any one time is 2.7kW. Bearing in mind the sun is at full strength this week and I'm 99% sure that it can't get any brighter!
So my question, is that normal for a solar system to have an actual output which is over 20% less than it's maximum or should I be expecting more?

Hiya. Probably down to the high temps and still air. 80% output in sustained sunshine on hot days is probably about the limit.

If you look up the spec sheet for your panels, you'll probably find the peak rating (the p in Wp) is based on 25C. Most panels lose around 1% efficiency per 2.5C rise. So hot panels in the sun, no cloud, no cold air blowing might hit 75C (+50C) (for example) and lose 20% efficiency.

It does work the other way too, so excellent clear skies in April or October can give you more than 100% of rating, but after a while those panels will heat up in the direct sun too. That's why the UK does OK(ish) with PV, and the best locations only get about 2x the generation of the UK, when you might think they'd do much better than that.

If the heat wave stays and air temps continue to rise, expect to see kW's continue to fall, but remember that long and lovely weather gives you lots of kWh's in exchange.

djmattc

Great, thank you for your detailed reply.

ajbell

I have a 4kw system and am south facing, in cooler conditions mine max at 3.75kw but the heat knocks around 20% off peak.

Dave_Fowler

A 40 degree pitch at this time of the year is far from optimal. My 28 degree pitch roof produced 1.7kWh more than the 38 degree roof.

Peak panel temperature was 60C.

Some days ago, when there was a mixture of sun and cloud, one of the 250Wp panels was recorded as producing 294W when the sun came out from behind the clouds and the panels were cool. So in the Spring and Autumn you will find the output from the system may well be limited by the inverter. You'll also benefit from the steeper angle because the sun will be much lower in the sky.

Dave F

djmattc

Thank you, some very interesting comments on here. It would be very interesting to see how they perform on cooler days.

pinnks

While we are talking comparisons I thought the comparison of today's production with yesterday's is interesting.

In theory both days looked as though they would develop along similar lines given the current heatwave - similar temperatures, cloudless sky from morn till night and so on but at about 09:00 I glanced out of the window to complete cloud cover and sun trying to penetrate through. Oh no, what has happened to those blue skies and darn it, my production will tank today even if the clouds clear!

About 2 hours later the sky was a blanket of blue again and I noticed a massive hit on production on my small system but an increase in expected production on the larger west-facing system resulting from cloud-reflected radiation, as production before the sun comes round at midday is pants.

Surprisingly, what was lost on the one system was compensated by that unexpected increase on the other. For the rest of the day the graphs are almost identical, including the impact of afternoon shading from the chimney on the small system :mad:

Small system yesterday 9.725; today 8.848
Large system yesterday 20.674; today 21.753
Total generated yesterday 30.399; today 30.601

The 6 graphs are behind the link below, with yesterday on the left and today on the right; total at the top with the two systems below. (ignore the lack of blue on the total graph from yesterday - just a graphics glitch)

https://www.dropbox.com/s/6igylwtwqv3gijk/Comparison.pdf?dl=0 :T :beer:

Martyn1981

pinnks wrote: »

About 2 hours later the sky was a blanket of blue again and I noticed a massive hit on production on my small system but an increase in expected production on the larger west-facing system resulting from cloud-reflected radiation, as production before the sun comes round at midday is pants.

Surprisingly, what was lost on the one system was compensated by that unexpected increase on the other. For the rest of the day the graphs are almost identical, including the impact of afternoon shading from the chimney on the small system :mad:

This is something that fascinated me too. Pure blue skies in the morning or evening leave generation on the system not in the sun extremely low. Some years back I had one system shutdown as gen was so low.

This week I've seen (for example) 1.4kW on west system and 90W on the east system in pure blue afternoon sky, or 1.2kW + 400W with a bit of white in the sky.

Fun!

ASavvyBuyer

Martyn1981 wrote: »

This is something that fascinated me too. Pure blue skies in the morning or evening leave generation on the system not in the sun extremely low. Some years back I had one system shutdown as gen was so low.

This week I've seen (for example) 1.4kW on west system and 90W on the east system in pure blue afternoon sky, or 1.2kW + 400W with a bit of white in the sky.

Fun!

We find the same with only having a West facing system. A bit of light cloud in the mornings helps, as the diffused reflected sunlight can boost our morning production to 1kWh by 9 or 10AM.

The highest day we had this month was like that, light cloud in the morning followed by brilliant blue sky in the afternoon, with a cooling breeze.