Solar panel voltages drop significantly under load

[JWLV]

I have 6 240W panels wired in 3S2P. Each panel's VoC is 37.6 and Pmax Voltage is 30.3. The panels are laid flat on the ground in my backyard.

When I disconnect the Growatt charge controller/inverter and measure the solar panels, it measures around 95V according to my multimeter. This seems to be correct, given that the VoC is 37.6 for each panel and it's about 100F right now. Once connected to the Growatt, the voltage drop to around 63V as reported by the dashboard page from Growatt.

This doesn't seem right. Right now it's about 11:25AM, the sun is almost directly overhead and the solar panels are getting full sun with no shading or obstructions, and yet the Growatt is reporting 63.4V.

Also, the maximum charge rate I've been getting over the past several weeks has been ~ 950W, usually right around 12 noon. It's never gone over 1000W. This would seem to be consistent with the voltage at 63V (2 panels in series) and not 95V (3 panels in series).

Any ideas as to what is wrong? Is one panel bad? How could I troubleshoot it to find out which one?

 

[littleharbor2]

This being the week of the summer solstice the sun is at its highest point in the sky. if your panels are not aiming nearly vertical coupled with the extreme heat on the surface of your panels you will not get optimum output from your array. Give it a couple months and you may see output return to a more normal state.

 

[Kornbread]

I assume you have an mppt scc? It tracks maximum power point which would be at or near the Vmp noted on your panels not Voc. With 3s and a Vmp of 30.7, around noon I see max power generation hanging around the low 70v range.

Each step in output is the sun hitting another set of panels. Watch how the mppt pulls the voltage down from an early morning indirect light at ~90v to the 70v range searching for maximum power output. The batteries are full by 12:30 and the mppt controller puts on the brakes by raising the voltage to a region, 90v+, that the panels do not produce as much power. When the shop uses power, the mppt controler will adapt and produce enough power to cover the loads but not so much as to overcharge the batteries.

 

[JWLV]

I assume you have an mppt scc? It tracks maximum power point which would be at or near the Vmp noted on your panels not Voc. With 3s and a Vmp of 30.7, around noon I see max power generation hanging around the low 70v range.

Wow, I had no idea that 3 panels in series with Vmp of 30.7, it could go as low as 70V. The heat is really bad right now, at 107° F. I guess the heat is really killing the panel's power generation.

I just took a temperature reading w/ my non-contact thermometer. Check out the attached picture. The thermometer is out of focus, but it reads 164° F (73° C) !!!

Viva Las Vegas!

 

[Kornbread]

Heat affects output. Living in a very dusty environment, I recently cleaned my panels and surprisingly, got an increase of .14kw. IMO, it was the water cooling off the panels more than the actual cleaning.

Edited the previous post. you might find it interesting.

 

[rmaddy]

The Vmp of 30.3V is at STC conditions which, among other things, is 25ºC/77ºF. With a panel temperature of 73ºC/164ºF you should expect about a 25% drop in voltage. 90.9V vs 63.4V is pretty close to that (70% vs 75%).

 

[JWLV]

Thanks for everyone's help. I feel much more assured that I didn't mess anything up and the readings I'm getting are pretty close to what should be expected under current environmental conditions.

 

[Supervstech]

Thanks for everyone's help. I feel much more assured that I didn't mess anything up and the readings I'm getting are pretty close to what should be expected under current environmental conditions.

Good reason NOT to lay the panels flat on the ground. No airflow really builds up the heat on them.