A couple of times over the last week I have seen apparently blue sky's all day, and a perfect smooth production without cloud "noise" but my peak kW of 4.5 is well below my usual 5+ maxing out my inverter. Temperatures been 30 ish with little wind, so could be temperature derating, or dust in atmosphere or deposited. First year for me, any opinions from others that have been through a few cycles ?
justpassing sounds like the temperature could be having an impact, anything over 25 degrees will start to affect performance. Also someone else on here mentioned some "Sahara dust" which could have an impact as well.
My system develops 5.1kW max when it is cold, down to 4.2kW when v hot. Quite normal.
Sun angle can also have an effect as its lower on horizon as days are getting shorter. Might be wrong, just my guess. But high temp for sure degrade performance, hence May is the best month, most of the time.
As PV Panels increase in temperature the solar pv output reduces.
Look for the "Temperature Coefficient of PMAX" quoted for your panels.
Remember that a panel on the roof may be MUCH hotter than the ambient temp. So 30'C ambient might be 60'C measured at the panel or 90'C..... (without measuring its impossible to tell exactly).
Here's an example for the Trina Vertex S panels: 400/405/410/415/420/425/430
Trina Vertex S: TSM-NE09RC.05
NOCT(Nominal Operating Cell Temperature: 43°C (±2°C)
Temperature Coefficient of PMAX (Power in Watts, W): - 0.30%/°C
Temperature Coefficient of VOC (Voltage, V): - 0.24%/°C
Temperature Coefficient of ISC (Current in Amps, I): -0.04%/°C
So a 400 Watt Panel @ 60'C would be limited to (60-43=17x-0.30%=5.1%-100%) 94.9% of 400 Watts = 379.6Watts
So a 400 Watt Panel @ 90'C would be limited to (90-43=47x-0.30%=14.1%-100%) 85.9% of 400 Watts = 343.6Watts
That also explains why blue panels perform slightly better than black panels. Black absorbs slightly more heat than blue, reducing performance.
And you will see more performance in Winter Sun as the panels get more efficient in the cold.
You could increase your efficiency of the panels by adding heatsinks to the rear or by combining Solar PV and Solar Thermal as is sometimes done.
It does make you wonder if a trickle feed water distribution system might pay for itself in cooling the panels in increasingly climate changed times 🙂
dragon2905 not sure you've used the correct starting point in your calculations. Most solar panels are rated at STC. So for instance a panel rated at 400w, which is the max a panel can output in lab conditions, generally won't produce close that amount most times. The NOCT figure on solar panels is the more realistic output and can be around 25% less than the STC figure which is the rated figure used in selling panels.
The 43 deg C you've used is the NOCT figure. For example your 400w solar panel NOCT figure is already only maybe 300w output depending on your panel spec sheet, and the higher the temperature goes the less efficient they become, but using NOCT is a useful guide as what you might average from your panels, depending on orientation (East, South, West), time of year, temperature and roof angle.
TimHutchings You are correct that I should have not only explained to the OP the difference between STC and NOTC and then used the relevant NOTC figures not STC.
So thank you for your criticism. As I am already fully aware of the difference between STC and NOTC perhaps, instead, it would have been more constructive for you to elaborate on the topic and take the time to explain to the OP and others reading the thread what the differences are and what they mean to the consumer.
In all honesty I can't even recall if what I was quoting was against the 400 or 430 watt panel (there are 8 separate STC Peak (STC) panel outputs covered on one data sheet). Never-the-less the example remains valid as a proof of "how the increased temperatures reduce power output and vice versa" especially when the OP didn't provide the Make & Model of his actual panels.
As such, in broad strokes, the illustration remains valid illustrating how the real numbers change compared to what is "sold" by installers.
It has been tried and does provide some benefit (have a search on youtube) sadly the downside of that particular method are many - water staining, pump issues, installation/maintenance logistics and freezing/pipes bursting in cold temps to name a few.
dragon2905 and thank you for your supercilious reply! I was replying to your incorrect post and did give a simple explanation of the difference between STC and NOCT.
By the way the abbreviation is NOCT (nominal operating cell temperature) not NOTC!
Don't you just love when technology lets us down. You'd imagine that when the sun shines you want to generate as much lecky as you can to maximise the sunney weather. It seems to be a similar picture with wind turbines and the need to put the brakes on when the wind gets to strong. Presumably in the wind case they design them to maximise lecky generation across a range of wind speeds including lower ones in preference to handling the much stronger conditions which are perhaps seen as less frequent events and as such don't merrit the additional expense invested into the design to handle those conditions. So, is this the same for solar panels and are they designed to get the most out of a range of light levels with the trade off being the reduced efficiency on warm sunny days ?
Maxwell I think it's more to do with the properties of solar cell construction, and the many factors that affect performance. As I put in a previous post, the pitch of the roof, the angle of the sun in the sky, the orientation (north, east, south or west), and the temperature will all affect the performance. All panels maximum output are based on an ideal set of lab conditions, which you'll never normally get in the real world. Those are Standard Test Conditions or STC on your solar spec sheet. NOCT figures in a spec sheet are more likely to be the numbers you'll get in the real world over a period of time.
Obviously the solar panels are black, and black absorbs more heat than a light colour. The efficiency of the cell deteriorates as the cells get hotter. Cold winter days will produce more from your panels, but usually the angle of the sun isn't optimal to the panels in mid winter, so output will usually entail some sort of compromise. It's true to say that Monocrystalline solar panels offer better efficiency than polycrystalline on cloudy days because they're made of pure silicon, but in reality, you're only likely to get 10-25% or the panel's STC rating when it's overcast.
It's really such a big old subject, but there's plenty of info on the internet.
Its pretty simple really. The power/m2 from sunlight decreases later in the year.
For example at my location and array angle/azimuth during June I will get an average maximum power of 870W/m2. In September that's down to 616W/m2, a reduction in peak power of 30%. That's reflected almost exactly by the peak generation figures maxing out at 2.2kW instead of 3.1kW.
NB - those figures (870W/m2 & 616W/m2) are calculated figures and assume no cloud whatsoever. PV peak panel outputs are calculated based on 1000W/m2 irradiation.
You'll get a slightly higher peak during breaks in cloud on windy days but that's to do with the ambient temperature at the PV cells being below 25C, so the voltage will rise.
tl;dr its normal....