The altE Solar store has some good info regarding this question as well as a demonstration of the effect of mixing panels and I tend to believe their testing a lot more than other resources (and they have the weather to verify this kind of stuff
)
I have copy/pasted some of the
points in their explanation which can be found here -
https://www.altestore.com/blog/2012/08/adding-capacity-to-your-existing-pv-system/ and embolded the key points, as well as a few extra things to note.
1) IN SERIES
Panel voltage is additive when panels are wired in series. The only way to wire in additional panels and guarantee that the voltage of all strings would remain equal is to add the exact same number of new panels to each string. The drawback to doing this however is that within each string all panels will produce the same current as the current of the lowest panel. So if you buy a panel that has a higher current it won’t produce the amount of current that is listed on its nameplate (it would perform at the lowest performing panel’s current), and you’d essentially be paying for a panel that will produce less power than its rating.
Note that this means if you bought a panel with a LOWER current spec of the existing panel/s you would be downgrading all the existing panels.
You would only have multiple panels in series if you had an MPPT controller (see the third bullet). PWM Controllers essentially truncate the 'excess' voltage so add a second panel in series with PWM ... you will get no extra power and could in fact reduce the arrays output.
2) IN PARALLEL
Panel current is additive when panels, or strings of panels are wired in parallel. Say you were to try to just add the new panels all in one or more strings composed of only the new panel types. While each of these strings would be able to produce the maximum amount of current they are rated for, the voltage difference between the old strings and new strings becomes a problem. If the panels in the new strings had a higher voltage rating than the panels in the old strings, the new string, and each panel within it, will only be limited to operating at the same voltage as the old strings. So again, you’re essentially paying for a panel that will produce less power than its rating.
If you are using a PWM controller, then high voltages are wasted anyway. If you are using an MPPT controller then the above bold point applies. But also note the third bullet below.
3) USING MPPT CONTROLLERS
If you are using an MPPT charge controller, mixing panels of different voltages and/or currents within an array will have an adverse effect on the entire system. Different module outputs make it impossible for the controller to determine the optimal operating voltage and current because they will be different between the different types of panels.
What this point is basically saying is that the main feature of MPPT - that is the Maximum Power Point Tracking algorithm - gets confused and can no longer work optimally. So adding a random 100W panel for example may not only not give you the full 100W due to points 1) and 2) above, but also stop the whole original setup working as well as before.
So my own summary would be:
Try and get the same panel specs (matching Voc and Isc at the basic level) (Here is another link from altE with a good guide -
https://www.altestore.com/blog/2016/04/how-do-i-read-specifications-of-my-solar-panel/ )
If you have a PWM controller, add in parallel but don't waste money on getting anything other than "12V" (so ~20V Voc) Panels
If you have MPPT, avoid mixing and if you do so, be prepared for a level of disappointment
If you do have to fit mismatched panels, consider multiple controllers - one for each standard.
Finally, to get round the problem of mismatching, you can get cable them a little 'creatively' to make them match more
Last link, again with the altEstore which talks about this ....
