Mismatched solar panels

[Jack Rabbit Off Grid]

My buddy has 10 355w REC panels he plans to run 5s2p. He also has 5 left over REC 365w panels. What would happen if he was to run them 5s and add them as another string with the other 10?

thank you.

 

[Horsefly]

My buddy has 10 355w REC panels he plans to run 5s2p. He also has 5 left over REC 365w panels. What would happen if he was to run them 5s and add them as another string with the other 10?

thank you.

The key items to consider are the Vmp and Imp of the panels.

When put in parallel, you want the voltages to be about the same (maybe within 5%). So 5x the 355W Vmp should be within 5% 5x the 365W Vmp to avoid losses.

When put in series, you want the currents to be about the same (within 5%). (Not applicable to your question, but this would be if you wanted to put some of the 365W panels in series with the 355W panels)

The quick and dirty answer is that the 355W panels are probably not much different from the 365W panels in either voltage or current, so adding another parallel string of the 365W panels would probably be OK.

 

[Jack Rabbit Off Grid]

The key items to consider are the Vmp and Imp of the panels.

When put in parallel, you want the voltages to be about the same (maybe within 5%). So 5x the 355W Vmp should be within 5% 5x the 365W Vmp to avoid losses.

When put in series, you want the currents to be about the same (within 5%). (Not applicable to your question, but this would be if you wanted to put some of the 365W panels in series with the 355W panels)

The quick and dirty answer is that the 355W panels are probably not much different from the 365W panels in either voltage or current, so adding another parallel string of the 365W panels would probably be OK.

The 355w are 39.1v and 9.09a the 365w are 34.3v and 10.65a.

 

[niktak11]

The 355w are 39.1v and 9.09a the 365w are 34.3v and 10.65a.

Those Vmp numbers are pretty far apart

 

[Horsefly]

The 355w are 39.1v and 9.09a the 365w are 34.3v and 10.65a.

Ok, so that is not what I expected. Let's look at the math....

A string of 5 of the 355W panels will have a voltage of 5 x 39.1V = 195V, and a current of 9.09A.

A string of 5 of the 365W panels will have a voltage of 5 x 34.3V = 171.5V, and a current of 10.65A.

If your buddy puts 2 strings of 355W and one string of 365W panels in parallel, the voltage of the combined array will be the lowest of all the strings, which would be 171.5V. That is, the lower voltage string holds the voltage down on the other two. The currents would still add up across the parallel strings, so he'd get 9.09A + 9.09A + 10.65A = 28.83A. That means your total wattage 28.83A x 171.5V = 4944W. The total wattage available from all the panels before you hooked them up this way was (10 x 355W) + (5 x 365W) = 5375W. So wiring them up the way you described causes a reduction of 430W, or about 8%. It's still more power than you had with just the 2 strings of 355W panels, so maybe it is worth it. It shouldn't cause any problem (assuming your SCC can handle the voltage and total power).

Note that all this math is based on Vmp and Imp of panels, which is based on Standard Test Conditions (STC). Rule of thumb is to assume the real world production of panels is 80% or so of STC.

 

[littleharbor2]

Are the 365 watt panels 60 cell or 120 half cell Vs. the 355 watt panels possibly being 72 cell or 144 half cell models? I'm just trying to figure out why the higher wattage panels have such lower voltage.

 

[Horsefly]

Are the 365 watt panels 60 cell or 120 half cell Vs. the 355 watt panels possibly being 72 cell or 144 half cell models? I'm just trying to figure out why the higher wattage panels have such lower voltage.

Yeah, I figured that was pretty much the only way they could be that different.

 

[Jack Rabbit Off Grid]

Are the 365 watt panels 60 cell or 120 half cell Vs. the 355 watt panels possibly being 72 cell or 144 half cell models? I'm just trying to figure out why the higher wattage panels have such lower voltage.

Yes the 365w are 120 half cut and the 355’s are 72 cell.

 

[Jack Rabbit Off Grid]

Ok, so that is not what I expected. Let's look at the math....

A string of 5 of the 355W panels will have a voltage of 5 x 39.1V = 195V, and a current of 9.09A.

A string of 5 of the 365W panels will have a voltage of 5 x 34.3V = 171.5V, and a current of 10.65A.

If your buddy puts 2 strings of 355W and one string of 365W panels in parallel, the voltage of the combined array will be the lowest of all the strings, which would be 171.5V. That is, the lower voltage string holds the voltage down on the other two. The currents would still add up across the parallel strings, so he'd get 9.09A + 9.09A + 10.65A = 28.83A. That means your total wattage 28.83A x 171.5V = 4944W. The total wattage available from all the panels before you hooked them up this way was (10 x 355W) + (5 x 365W) = 5375W. So wiring them up the way you described causes a reduction of 430W, or about 8%. It's still more power than you had with just the 2 strings of 355W panels, so maybe it is worth it. It shouldn't cause any problem (assuming your SCC can handle the voltage and total power).

Note that all this math is based on Vmp and Imp of panels, which is based on Standard Test Conditions (STC). Rule of thumb is to assume the real world production of panels is 80% or so of STC.

Thank you I’ll pass this on to him.