I was thinking about solar panels for a while and a question popped into my head which i still can't answer nor test it, and i want to know if this would actually work:

If I get a 2x12v solar panels and connect them in series i should get a 24v output, that's stright forward. What in thinking though is, lets say:

I get a 15w solar panel and a 150w solar panel So: 15w = 18V 0.8A 150w= 18V 8A

If I connect them in series i will definitely get 32V, but how much Amps would I get?

At first I thought that it would divide and supply around 4.4A, but after a little thinking i thought that won't happen.. it should actually supply 8A if you follow the Ohms law

So if it puts out 34V and the resistance is nearly 1 Ohm or less then it should provide over 30A of current, but the max the panels can provide are 8A and 0.8A, but since they are in a series the Amps won't add up.. so it should stay at 8A since that is the peak? It's a generator and not a load so the current should not be divided by them?

So the final output should be 38V 8A?which would be 272w? It doesn't add up but it makes sense that it would push 8A out..

Am I right here? Searching online did not give me anything back.. and I'm not well knowledged in electricity, I only started learning it as a hobby not long ago.

If you know the answer please explain it and what are the implications here and what would the actual output be if my conclusion os wrong. would be a valuable knowledge!

Thank you in advance

  • \$\begingroup\$ No! You must have the same current rating on both of them when you connect them in series. If not protected, you may need anti parallel diodes across them too for partial shade. \$\endgroup\$ – winny Jul 24 '16 at 10:45
  • \$\begingroup\$ I'm more intrested in the mechanics of why it won't work, why won't it work? Is there an explanation to why it won't work by someone? I want to learn more about this if possible \$\endgroup\$ – bakz Jul 24 '16 at 12:40
  • \$\begingroup\$ The stronger as in more current will force feed the weaker panel. The weaker panel can't take it but the stronger will go so far as in reverse the polarity of the weaker one until it breaks or if you have a bypass diode, the diode will be forward biased and let the current pass. \$\endgroup\$ – winny Jul 24 '16 at 12:56

As pjc50 mentioned, this is a similar situation to the partial shading problem - same voltage ratings, different current ratings.

Consider two same PV panels connected in series. Each of them has a bypass diode in parallel, which is under normal operating conditions reverse biased and has high impedance, i.e. no current flows through bypass diodes.

Now let's say that incident solar irradiance on one of the PV panels is 1000 W/m2, and on the other is 500 W/m2. Assuming that the PV panel output current linearly depends on incident solar irradiance (just an approximation), the other PV panel would give half the current to the current of the first PV panel.

In general, PV panels (cells) are diodes, and when the current difference is large enough, the "weaker" PV panel starts behaving like a sink. In order to prevent overheating which could cause cracking of the glass, the bypass diodes are used, which are under these circumstances forward biased to provide a low impedance path for the current.

Ideally, each PV cell in the PV panel should be equipped with a bypass diode. Since this is too expensive, a bypass diode is used for every 10-20 PV cells in a PV panel.

  • \$\begingroup\$ But the flow of electricity is in onl 1 direction and there are diodes that prevent it from going back, why would it act like a sink? And what would be the output here? Sorry but u don't understand it fully here, I'd love to know the mechanics of why it won't work please \$\endgroup\$ – bakz Jul 24 '16 at 12:39
  • \$\begingroup\$ Watch this video for a detailed explanation: youtube.com/watch?v=4imVOP_i_LI \$\endgroup\$ – Marko Gulin Jul 24 '16 at 13:36

The current through components in series must be the same. Therefore either you're limited to 0.8A, or it ends up going through the bypass diodes and you get no benefit from the small panel.

Solar panels in series must have the same current rating and see the same level of sun. Partial shade on panels causes problems.

  • \$\begingroup\$ So it wont provide 33v 8A but actually 33v 0.8A? But why? I don't get the bybass diode thong here.. \$\endgroup\$ – bakz Jul 24 '16 at 12:36

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