Wiring for Solar Panels of Different Amperages 1

[soiset]

I am installing solar panels on my home, and trying to make it a bit more economically feasible, I have bought all used panels. These panels are in a variety of power ratings: 50, 64, 75, 100, and 180 watts, but the peak voltage ratings are similar, all in the 16-17 volt range. This will be a grid-tied system, so I am planning to wire most of the panels in series to get the optimal voltage range for my inverter.

My question is this: How does the math work for series/parallel wiring in a case like this? If I wire in series, and the voltages are simply cumulative, what happens to the amperage? What happens when you wire a 100 watt, 16.8 volt panel, in series with a 50 watt, 16.8 volt panel? Are there any disadvantages, electrically, to wiring differently rated panels together?

Thanks, and feel free to ask me about shear walls and PT beams,

Chris

 

[itsmoked]

I think the rule would be; "if you are going to mix panels - ESPECIALLY power ratings - then you need to wire them ALL in parallel", as then each will provide what-it-can to the whole.

The same would definitely go for batteries.

If you mix them, the internal wiring for a 50W panel is not designed to carry twice the current that the 100W panel was designed to carry, and in series all the panels must carry the full current.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[davidbeach]

In series you have the same current through the whole string. Depending on the current, you will give capacity on the higher output cells or run too much current through the lower output cells.

 

[soiset]

I believe that the internal wiring for the panels is all the same (at least the ones from the same manufacturer), but what if I ordered the series in the succession of their power ratings, with the lowest rated first, on the way to the inverter? If the wiring is the same in each, how would the math work?

The cells are all identical. Some panels just have more than others. I assume the cells are wired in parallel/series in order to yield the optimum voltage for charging a 12-volt battery, which is about 16.8 volts, I guess.

 

[davidbeach]

Sounds like you need a lot more information, guessing is how thing wind up with the magic smoke getting out.

 

[LionelHutz]

If you just put them in series then you'll really only be able to get the current output of the smallest panel - In this case you'd be limited to the current capability of the 50W panel.

If you can, do a series/parallel arrangement. For example, put a 50W and 100W in parallel which makes the equivalent of a 150W panel @ 16.8V. Then, put 2 x 75W panels in parallel which makes another 150W panel @ 16.8V. Then put both of these sets in series giving you the equivalent of a 300W panel with double the voltage - 33.6V. You could then put this all in series with a 180W panel giving you a 450W panel at 3x the voltage - 50.4V.

See how the powers add up except that in series you have to de-rate each step to the same rating as the smallest step. In the above case you have to call the 180W panel only 150W.

As another example, if you put all of the above in series with a 50W panel then you get 200W at 4x the voltage. This is going the wrong way...you added a panel but went from 450W to 200W of power.

I hope that's enough to make sense of the math...

 

[itsmoked]

Nice summation Mr. Hutz.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

To expand on Lionel's suggestion.
Decide how many series connections you need to get the desired voltage.
Divede your panels into that many groups with the same total wattage in each group.
For instance, if you want 4 groups in series, divide your panels into 4 groups such that each group has the same wattage.
Connect the groups in parallel.
There may be some panels left over. Leave them left over until you aquire more panels.
It may be well to permanently connect a voltmeter across each group to monitor the voltage of each group independently.
If the voltage rises across one group, look for open-circuited panels.
If the voltage falls across a group look for a panel that is starting to short out.
respectfully

 

[soiset]

Thanks. I'm getting a better idea of how it works now.