Solar Panels not generating enough power

[JWLV]

I have 18 solar panels that I bought from Santan Solar. These are used panels which Santan says are 240W. I believe they are relabeled Trina panels.
I have these panels tilted at 10-degrees, which is ideal for my location at this time of year (July 23) to get maximum sunlight.
So far, the most power I'm seeing is around 2700 watts. With 18 of these 240 watt panels, the theoretical maximum power is 4320 watts. The difference is 1620 watts. I'm getting almost 38% less than the maximum. That seems to be rather a large difference. Does this loss seem correct given the environmental details below? What do you guys think?

Environmental details:
The panels are not in any shade or near any trees.
Each string is 3-panels in series. Total of 6 strings. 3 strings connected to one combiner box to a Growatt SPF 3000. The other 3 strings connected to another combiner box to a second Growatt SPF 3000.
Both Growatts are connected in parallel to 22KHW of LiFePo4 batteris.
The time of the measurement readings is between 11am-1pm.
The weather at the time of the measurement readings is extremely hot, around 110 F air temperature. The panels are much too hot to touch. I estimate the surface of the panels are at least 150 F.

 

[wattmatters]

Do you have enough load? Or enough MPPT capacity?

MPPT controllers will only draw from PV based on available load. If there is only 2700 W of load (battery charge capacity + loads) then that's all the system will generate.

 

[JWLV]

Do you have enough load? Or enough MPPT capacity?

MPPT controllers will only draw from PV based on available load. If there is only 2700 W of load (battery charge capacity + loads) then that's all the system will generate.

I have two Growatt SPF 3000TL LVM connected to 22KHW of batteries. So yes, there is sufficient load.

 

[Tomthumb62]

Expect 75-80% output under optimal conditions. Your estimate of 150F on the panel surface would certainly degrade output, so you’re probably right on track, all things considered. More panels would help, if your MPPT can accept more. Or add a second MPPT with more panels.

 

[Tomthumb62]

I have two Growatt SPF 3000TL LVM connected to 22KHW of batteries. So yes, there is sufficient load.

Those aren’t loads, those can deliver power, if loads are available. What do they power? Air conditioning?

 

[JWLV]

Those aren’t loads, those can deliver power, if loads are available. What do they power? Air conditioning?

Mini split, electric vehicle charger, 2 chest freezers, one refrigerator, and a few other small electronics. Trust me when I say there is more than sufficient load.

 

[v_green57]

Disconnect your panels for a day. Let the battery bank discharge lower than it normally would.

The next day, turn them back on during the "peak" hours. Then you will (should) see the max. power output the array is capable of unless something else is wrong.

All the stuff you list above is intermittent, not on all at once. It won't create a big a$$ continuous load, which is what you need to see max power from your array. The over-discharged battery bank should do that.

 

[JWLV]

Expect 75-80% output under optimal conditions. Your estimate of 150F on the panel surface would certainly degrade output, so you’re probably right on track, all things considered. More panels would help, if your MPPT can accept more. Or add a second MPPT with more panels.

I’m curious as to what is an acceptable degradation based on the panel surface temperature.

 

[400bird]

If you have combiner boxes, measure the amperage from each string. See if you have any outliers that are low.

What's the angle (compass heading) you have them facing? That can decrease output.

I’m curious as to what is an acceptable degradation based on the panel surface temperature.

That's included on the panel spec sheet, normally used to calculate increased power when cold. Just calculate for 150+ panel temp, not freezing temps. I'd recommend measuring the temp, not guessing.

 

[wattmatters]

I have two Growatt SPF 3000TL LVM connected to 22KHW of batteries. So yes, there is sufficient load.

Not if the batteries are fully charged.

 

[Rafagus]

I have 16 used Trina 240w panels mounted on a movable platform, trailer, and angle is adjustable. Rig allows me to experiment with different locations on my property and with Solar South. Panels are clean and I've tested each one for voltage and short circuit amps. 2500 watts is my best output with the normal sunny day of about 2,200 watts. I'm reading my output on 2 EG4 3K controllers. I just got a meter that will test actual watts and I'll post if I find anything unusual. I suspect though that it is what it is.

 

[wattmatters]

I’m curious as to what is an acceptable degradation based on the panel surface temperature.

It should be listed on the panel specs, usually somewhere between -0.3%/K to 0.5%/K relative to STC (25°C).

So, e.g. say it's rated at -0.45%/K and the temperature is 65°C (150°F).

60°C - 25°C = delta of +35°C = delta of +35 K

So the power loss will be -0.45%/K x +35 K = -15.75%, so a panel rated at 240 W at STC would be expected, at best, to generate 202 W at 65°C.

 

[Tomthumb62]

I’m curious as to what is an acceptable degradation based on the panel surface temperature.

Edited. What @wattmatters said. Use one of the online calculators if you don’t want to do the math.

 

[JWLV]

I have 16 used Trina 240w panels mounted on a movable platform, trailer, and angle is adjustable. Rig allows me to experiment with different locations on my property and with Solar South. Panels are clean and I've tested each one for voltage and short circuit amps. 2500 watts is my best output with the normal sunny day of about 2,200 watts. I'm reading my output on 2 EG4 3K controllers. I just got a meter that will test actual watts and I'll post if I find anything unusual. I suspect though that it is what it is.

That's very close to what I am getting on my panels. Did you get your Trina 240w panels from Santan Solar?

 

[JWLV]

It should be listed on the panel specs, usually somewhere between -0.3%/K to 0.5%/K relative to STC (25°C).

So, e.g. say it's rated at -0.45%/K and the temperature is 65°C (150°F).

60°C - 25°C = delta of +35°C = delta of +35 K

So the power loss will be -0.45%/K x +35 K = -15.75%, so a panel rated at 240 W at STC would be expected, at best, to generate 202 W at 65°C.

Perfect! That is the formula I was looking for.

 

[Tomthumb62]

Perfect! That is the formula I was looking for.

Here's a good calculator:

Solar Panel Maximum Voltage Calculator - Footprint Hero

Calculate the maximum open circuit voltage of your solar array. Find your max solar panel voltage to correctly size your solar charge controller.

footprinthero.com

It asks for "coldest temperature" but if you just put in the highest instead, it will tell you how much less voltage it will produce due to the ambient heat. The Voc is usually based upon what it will produce in open circuit at 77F (or 70F, I can't remember). Lower than that and the panel will produce more voltage. Higher than that and it will produce less voltage. The voltage a panel can produce has a direct effect on how much (or little) watts it can produce at that given moment.

 

[Mattb4]

62% is a bit low but you do have used panels. 70-80% would be more inline. One thing to check is if you are getting a accurate measurement of PV watts. I am finding that of the 3 AIO's I have used so far a great difference in their measurement accuracy. If you have a DC clamp on meter and a separate DMM to where you can measure PV amps and voltage at peak time, and do the math to get watts, you can double check them.

 

[Quattrohead]

You might have a string or 2 not producing due to a bad panel. Isolate each string and test again.

 

[Rafagus]

That's very close to what I am getting on my panels. Did you get your Trina 240w panels from Santan Solar?

No I got them from a local guy. He gets them from roofers when the roof needs replacing but the panels are good.

 

[Rafagus]

I have 2 strings of the used Trina 240w panels. Each string goes into a separate controller. I got an Elejoy meter that runs MPPT and reports watts. 7 of my used panels were all very close at:
Watts: 150
Inp: 7.1
UMP 21.5

One panel was:
Watts: 75
Inp: 0.03
UMP 25.15

While testing I used contact cleaner and carburetor brushes on the connectors. It didn't make a difference. My panels are mounted on a movable platform so I could figure out a location that would work. The panels are at the correct angle and at solar south. Prior to commissioning I gave them a good cleaning, this made a big difference.

I'm assuming the reason the string with the underperforming panel wasn't dead is because the voltage on the bad panel is okay?