Formula for mixing panels

[craig110]

I've tried to find the formula to calculate the output voltage and amperage of a mixed panel combination, but haven't been able to find it.  Not that I'd do this, but I'm curious.  If you have a 12v 10A panel and a 24v 20A panel and hook them in series, what is the formula to calculate the potential output?  There are, of course, only 600 potential watts (120w + 480w) in this hypothetical, but how would this be split between voltage and amperage?  Would the amperage be limited to the rating of the smaller panel with the result being 10A at 36v?

Thanks.

Craig

[pyrocasto]

"Would the amperage be limited to the rating of the smaller panel with the result being 10A at 36v?"

Bingo.

Parellel adds the amps, takes the lower voltage.

[craig110]

Hi Chris,

Thanks for the answers.  The series result of 10A at 36v made sense as the smaller panel can only move 10 amps worth of electrons across its mechanism no matter how many electrons the larger panel can try to move.  Those that can make it through the smaller panel would have their energy increased by the second panel, thus my hunch of 10A and 36v.  The answer to the parallel configuration isn't as intuitive to me, though.  (Not that I'm saying it is wrong; I just don't "see" the result.)  Since nothing blocks the motion of any of the electrons, the panels giving 30A makes sense.  It isn't intuitive to me why the resulting voltage would be 12v, however.  Why does the presense of some 12v electrons cause the 24v electrons to drop down to only 12v of energy?  If I was pondering the parallel situation prior to your answer, I would have guessed that the resulting voltage would be the amperage-weighted average of 20v in this case.  (2x the electrons at 24v than at 12v.)  ??

Craig

[Dreadstar]

Running your 24 volt 20 amp panel in parallel would make it exactly the same as your 12 volt 10 amp panel. Causing you to loose 10 amps from the 24 volt panel.

Best thing to do in mixing panels is if you have 24 volt panels and 12 volt run them in series parallel strings. 2 x 12 volt in series 1 x 24 volt in parallel with blocking diodes on the 12 volt panels.

I did that with my sharp 208 watt panels and Shell 55 watt panels. Hope this helps.

And btw wish i had some of those 24 volt 20 amp panels. havn't seen a 480 watt single panel yet.

[craig110]

Yeah, I wish I had some of those 480 watt panels as well.  Too bad this isn't a real-life situation.  :-(

Your answer, though, was that the 24v panel would lose 10 amps.  Are you saying that the combined output would be 20A at 12v?  Chris's answer was that the combined output would be 30A at 12v.  Do we have any tie-breaker votes out there?

Craig

[Flux]

No exact answers as you are not working any case at the maximum power point but basically with panels in series you are limited to 10A at any voltage from 36 to zero.

In parallel you have 30A from 12v to zero.

Flux

[scottsAI]

Flux has it right for 0-12, 30a.

Each panel should have a blocking diode.

Voltage feedback from the 24v panel to 12v panel when the load is <20a without diodes, power loss, maybe other effects.

With diodes: loads <= 20a the output voltage is determined by the 24v panel.

If the load is >20a then the voltage will fall to the sum of the two panels current 20+10.

The voltage will be >12v considering most 12v panels output voltage is >12v.

Have fun,

Scott.

[craig110]

Thanks, Scott, that makes sense.  (And yes, I was presuming there were blocking diodes in place but I should have been explicit about that in the original question.)  The higher panel "wins" the race to satisfy the load if it alone can handle the load's request and above the larger panel's current capacity the panel's voltages get weighted by how much current they are each providing to the load.  Got it now.

Craig