# Solar panels in series

Discussion in 'General Electronics Discussion' started by flippineck, Aug 27, 2014.

1. ### flippineck

292
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Sep 8, 2013
Let's say I have 2 identical solar panels, both illuminated the same, and connected in series.

Let's say each panel is generating 17 Volts, and the + & - ends of the whole array are connected together through a 10 ohm resistor.

Let's say at the same level of illumination, one single panel connected to a 10 ohm resistor would drive 4 Amps through that resistor..

What would I measure for current through and voltage across the resistor, with both panels connected in series?

Do the currents add together i.e. 8 Amps with the voltage being 34V?

Would it be the same current, but twice the voltage i.e. 4 Amps @ 34V?

Something else?

Looking for just a basic analysis given the assumptions, avoiding wider discussion of MPPT etc

2. ### KrisBlueNZSadly passed away in 2015

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Nov 28, 2011
Assuming they have exactly the same characteristics in all respects:

Supplies in series: voltages add together; current is the same
Supplies in parallel: currents add together; voltage is the same.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
if the 4A you get from one panel is near the maximum current (i.e. short circuit current) for that panel, then the current you get from two in series will only be slightly higher (it will ever exceed the short circuit current).

However, you should be able to get 4A through a 20 ohm resistor.

4. ### flippineck

292
9
Sep 8, 2013
ok so, if I had six of these panels in series, and their individual short circuit max current was listed in the specs as 5.3A, I could safely assume that the current generated by the whole array on a good sunny day would not exceed 5.3A (give or take manufacturing tolerance etc)?

I would not need to design the storage battery charging controller to handle input currents of (six panels x 5 and a bit amps each) 32A?

If I fused the array at 10A to give a bit of headroom that would be ok?

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Yeah, you would likely never even see 5.3A unless the panels were moved from shade suddenly into bright sunlight (they are more efficient when cool) This happens when there is cloud and it moves causing sunlight to hit the panel. The power is initially quite high, and then drops by anything up to 20% as the panel heats up.

6. ### flippineck

292
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Sep 8, 2013
Thanks chaps.

Since the panels open circuit max voltage is around 20V, I'm figuring I can use P=IV =5ish x (20x6) = roughly 600 Watts as the maximum output power?

Should I add 20% on top of that or is it more the case, when the clouds depart I'll maybe get the 600W briefly if I'm lucky then it'll fall down to what, around 480?

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
With an open circuit voltage of around 20V and a short circuit current of 5.3A, I would expect that you'd see the max power at around 16V and 4.5A (Are they 70W panels?)

So 6 in series would give you 96V at 4.5A, or 432W.

Generally speaking, max power will occur at a point where you get around 80 to 90% of the short circuit current and about 80% of the open circuit voltage. However that varies a little. And of course it varies with illumination (so does open circuit voltage and short circuit current for that matter)

It's very hard to do calculations when you are giving figures of what you read from the panels. We don't know what the level of solar irradiation is, the angle of the panels, their temperature, etc. All we can do is take your single point and extrapolate for the same conditions.

Interestingly enough, I've re-read your first post. You sat 4A through a 10 ohm resistor. Was this with all 6 panels connected in series (or at least more than 1 or 2)? That would require 40V, way more than the 17 you report.

The best thing to do (short of reading the datasheet) is to measure the open circuit voltage and short circuit current of a single panel under some set of conditions and then calculate the 80% voltage and 80% to 90% current thing to determine where the sweet spot is likely to be. Then (optionally) select an appropriate load to verify that. Once you know that you can multiply the voltage by the number of panels in series and multiply the current by the number of strings in parallel.

As an example, if you found the figures were 15V at 4A, then
• 6 panels in series would give you 90V at 4A,
• 6 in parallel would give you 15V at 24A,
• two strings of three would give you 45V at 8A,
• three strings of two would give 30V at 12A
But these would be at the current level of illumination.  