How much wattage drop from heat is normal?

System

How much solar panel power reduction from summer heat is normal?

When first installed the weather was cooler and I was seeing a noon output of 2400W. Now summer heat is here and I see 1200W or maybe 1700W. I am suspecting a problem with the inverter but I don't know what is normal.

The vendor's tech sprayed the hose on the solar panels and the output increased about 200W which seems right but still not to the 2400W I was seeing originally.

The same tech told me that the inverter would shut off it it got too hot. The manual says that it will reduce its output.

BB.

Re: How much wattage drop from heat is normal?

Probably need some more details...

Where is the installation.
Size of Array (details of array how many panels, their ratings, how the array is wired in series/parallel).
GT Inverter Mfg/Model number...

I have a new 3.5kW array that will, on cool/sunny days, make 3,000 to 3,300 watts... On a hot day with less than clear sky, down towards 2,500 to 2,700 watts maximum.

Do you have the Vmp / Imp readings from the GT inverter (i.e., 300 VDC on hot day, 345 VDC on cold day, Imp upwards of 10 amps on clear sunny day for an Array rated at Imp=10 amps)?

I had an array fail (and was recently replaced after 5 years of operation)--Basically, once I was getting about 1/2 the power I normally expected, I knew there were major problems. Called the installer, they checked the Voc/Isc of the panels and ended up replacing all of them (over 20% were bad).

-Bill

Cariboocoot

Re: How much wattage drop from heat is normal?

The inverter will shut off if the inverter gets too hot; not if the panels get hot.

Panels will drop output with heat. Which panels and how much heat makes a difference, as does the angle the panels are at (could be right for Winter, but wrong for Summer - adding to the losses).

However, I have to agree that a 50% drop is outside of the norm. Like Bill said; "details?"

(Okay so I paraphrased a bit. )

ggunn

Re: How much wattage drop from heat is normal?

blinding wrote: »

How much solar panel power reduction from summer heat is normal?

When first installed the weather was cooler and I was seeing a noon output of 2400W. Now summer heat is here and I see 1200W or maybe 1700W. I am suspecting a problem with the inverter but I don't know what is normal.

The vendor's tech sprayed the hose on the solar panels and the output increased about 200W which seems right but still not to the 2400W I was seeing originally.

The same tech told me that the inverter would shut off it it got too hot. The manual says that it will reduce its output.

Did that guy check your string output circuits and fuses? Do you maybe have a shading issue that you don't have at other times a year? Is your inverter in direct sunlight? A 50% drop in production is more than I would expect solely from array heating.

niel

Re: How much wattage drop from heat is normal?

i'll add one more question. if the array is roof mounted, just how far up from the roof are the pvs mounted? a lack of airflow can increase the pv temps far in excess of what might be normal. the pvs themselves should have a derating listed in their specs for temperature excursions per degree c. do you have an infrared thermometer you can aim at the pvs in somewhat close proximity to the pvs to get a good idea of what temps we are talking about for the pvs themselves? pv temps will always be higher than ambient temps as is the case for roof temps too, but roof and pv temps can add to each other making things worse for the pvs. this is one reason why i and crewzer (a previous moderator here) had always advocated the pvs to be mounted at at least 6 inches above the roof.

ggunn

Re: How much wattage drop from heat is normal?

niel wrote: »

i'll add one more question. if the array is roof mounted, just how far up from the roof are the pvs mounted? a lack of airflow can increase the pv temps far in excess of what might be normal. the pvs themselves should have a derating listed in their specs for temperature excursions per degree c. do you have an infrared thermometer you can aim at the pvs in somewhat close proximity to the pvs to get a good idea of what temps we are talking about for the pvs themselves? pv temps will always be higher than ambient temps as is the case for roof temps too, but roof and pv temps can add to each other making things worse for the pvs. this is one reason why i and crewzer (a previous moderator here) had always advocated the pvs to be mounted at at least 6 inches above the roof.

Even so, if the voltage coefficient were, for example, -0.33% per degree C (picked off a data sheet that just happens to be lying on my desk), a 50% drop in voltage would result from a 151 degree C increase in temperature over STC which would make it 176 degrees C. Water boils at 100 degrees C. Is this system on Venus?

IMO, something else is wrong.

niel

Re: How much wattage drop from heat is normal?

ggunn wrote: »

Even so, if the voltage coefficient were, for example, -0.33% per degree C (picked off a data sheet that just happens to be lying on my desk), a 50% drop in voltage would result from a 151 degree C increase in temperature over STC which would make it 176 degrees C. Water boils at 100 degrees C. Is this system on Venus?

IMO, something else is wrong.

i agree there may be something else up with it, but we need to know exactly where it should be and a real temp reading will verify by the specs how far off spec it is. even though it is unlikely, how do you know it isn't that high without measuring?

ggunn

Re: How much wattage drop from heat is normal?

niel wrote: »

i agree there may be something else up with it, but we need to know exactly where it should be and a real temp reading will verify by the specs how far off spec it is. even though it is unlikely, how do you know it isn't that high without measuring?

Well, of course I don't know for absolute certain, but it does not seem reasonable to me that something sitting in the sun can get to 175 degrees C (about 350 degrees F) in free air without concentrating mirrors. Turning the hose on the array would likely have shattered all his modules if they had been that hot.

My money is on something else being wrong.

Cariboocoot

Re: How much wattage drop from heat is normal?

ggunn wrote: »

Well, of course I don't know for absolute certain, but it does not seem reasonable to me that something sitting in the sun can get to 175 degrees C (about 350 degrees F) in free air without concentrating mirrors. Turning the hose on the array would likely have shattered all his modules if they had been that hot.

My money is on something else being wrong.

Heat inside panels is not the same as heat outside panels. Glass is a poor thermal conductor.

But I agree with you that heat is not the one and only culprit here. It's time for "disconnect and inspect".

BB.

Re: How much wattage drop from heat is normal?

Typically the maximum rated panel temperature is ~85C or 185F... You ain't going to get hotter than that.

-Bill

ggunn

Re: How much wattage drop from heat is normal?

Cariboocoot wrote: »

Heat inside panels is not the same as heat outside panels. Glass is a poor thermal conductor.

All the more reason that the glass would break with such a steep thermal gradient across its thickness when the water hit it.

With no more info than we have, I'd guess that he has a three string array and one string is dead. Loss of 1/3 power due to the dead string plus 1/6 due to heat. That's pure conjecture, of course.

niel

Re: How much wattage drop from heat is normal?

ggunn wrote: »

All the more reason that the glass would break with such a steep thermal gradient across its thickness when the water hit it.

With no more info than we have, I'd guess that he has a three string array and one string is dead. Loss of 1/3 power due to the dead string plus 1/6 due to heat. That's pure conjecture, of course.

very possible.

we do have allot on conjecture here on the forum as most of what we do is a good educated guess because we aren't there.:roll::D

System2

Re: How much wattage drop from heat is normal?

I have collected some data. Today is about 70F, no clouds, perhaps a slight haze, and the sun is directly overhead.

The system has 14 panels. Sorry but I don't have model numbers. They are mounted on the south facing roof spaced about 6 inches above the roof. They are hot to the touch but certainly not scalding.
The slope of the roof puts the panels perpendicular to the sun. Perhaps slightly more so in the winter but pretty close.

The inverter is a Fronius IG 3000. I took the following readings from its menu.
Output 1649W (1500W by the time I finished)
AC 244V
6.3A

Panels 233V (this doesn't calculate)
5.8A

Based on what I have read I should see a drop from panel heat of 18% or 430W taking me down to ~1900W. This is much lower.

Cariboocoot

Re: How much wattage drop from heat is normal?

blinding;

I think we need a few more details, especially about the panels. Watts each or total array size?
I'll hazard the wiring is two parallel strings of seven each.
5.8 Amps from the panels sounds like only one string is active.

Still conjecture at this stage. :roll:

Blackcherry04

Re: How much wattage drop from heat is normal?

Seems like there too many variables to ever put one system against another as to output and loss.

I have Enphase Inverters on 235 W panels that would clip at 6950 W for maybe 30 minutes a day in the spring. Today they were at 6450 W and a total output for the day of 49.9 kwh so far. The number of sun hours more that offset any loss from heat on my system. Today was 88 deg, thats maybe 5 deg lower than a normal summer day here in Kentucky.