Different Wattage Panels going to a Combiner Box

BilljustBill · November 2011

Several years ago, I ended up with a collection of assorted new and used 12v panels equal to about 1,000 w.

Two 120w American Signal (Siemens?)
Four 100w China
Two 90 w China
Four 75 w Siemens

I already have a Morningstar controller: a PWM Tri-Star 60amp. While now doing the layout on our lot for the whole solar project, I would like to use these in a third aray in the future.....If these are wired in series pairs, and each pair has its own wire run to the same combiner box, will the sum of their ratings work together without much loss?

The big picture: I'd like to use those assorted panels as a third array to follow the installation of 30 Kaneka 48v/60w panels, and 12 Kyocera 135 GX panels.

I'll have a 24v (Off Grid No sell-back) system with eight 440ah US Battery L-16
Two GVFX-3524 Outback inverters (In case Grid Tie becomes a future option)
Two M80 MPPT Outback controllers (One for Kaneka and One for Kyocera)

mike95490 · November 2011

Re: Different Wattage Panels going to a Combiner Box

Well, you must have heard the drill by now

Panels in parallel, match voltage within 10% V P max

Panels in series, match amps within 10% I P max

you will have to read the power max specs off the stickers, or post them here.

Cariboocoot · November 2011

Re: Different Wattage Panels going to a Combiner Box

The good news is you have at least two of each, meaning you can put them in series to get the Voltage up to where it should work for a 24 Volt system.

The bad news is we don't know the actual Vmp of the panels. This is where you could be in trouble on two fronts: the Vmp may not be enough to charge the system, even in series (less than 17.5 per panel for example) and/or the Vmp's of the various panels may be so different as to lead to some noticeable power loss.

So to really know how well this salmagundi will work you need the full panel specs for analysis.

BilljustBill · November 2011

Re: Different Wattage Panels going to a Combiner Box

Cariboocoot wrote: »

The good news is you have at least two of each, meaning you can put them in series to get the Voltage up to where it should work for a 24 Volt system.

The bad news is we don't know the actual Vmp of the panels. This is where you could be in trouble on two fronts: the Vmp may not be enough to charge the system, even in series (less than 17.5 per panel for example) and/or the Vmp's of the various panels may be so different as to lead to some noticeable power loss.

So to really know how well this salmagundi will work you need the full panel specs for analysis.

Thanks goes to you and Mike for your help.
Here's what the panels' list:

75 watt panel: IMP 4.4 VMP 17
90w IMP 5.29 VMP 17
100w IMP 5.71 VMP 17
120w IMP 7.1 VMP 17

Cariboocoot · November 2011

Re: Different Wattage Panels going to a Combiner Box

BilljustBill wrote: »

Thanks goes to you and Mike for your help.
Here's what the panels' list:

75 watt panel: IMP 4.4 VMP 17
90w IMP 5.29 VMP 17
100w IMP 5.71 VMP 17
120w IMP 7.1 VMP 17

All good, so long as there's not too much V-drop from long wires or hot panels. String them in two's and you should have a Vmp of 34 per string.

So you'd get:
one string of 240 Watts Vmp 34 Imp 7.1
two strings of 200 Watts Vmp 34 Imp 5.71
one string of 180 Watts Vmp 34 Imp 5.29
two strings of 150 Watts Vmp 34 Imp 4.4

Total array: 1120 Watts Vmp 34 Imp 32.61

That cumulative current could be a problem. The Imp differences between the panels limit your serial connection possibilities.

BilljustBill · November 2011

Re: Different Wattage Panels going to a Combiner Box

Cariboocoot wrote: »

All good, so long as there's not too much V-drop from long wires or hot panels. String them in two's and you should have a Vmp of 34 per string.

So you'd get:
one string of 240 Watts Vmp 34 Imp 7.1
two strings of 200 Watts Vmp 34 Imp 5.71
one string of 180 Watts Vmp 34 Imp 5.29
two strings of 150 Watts Vmp 34 Imp 4.4

Total array: 1120 Watts Vmp 34 Imp 32.61

That cumulative current could be a problem. The Imp differences between the panels limit your serial connection possibilities.

That cumulative current could be a problem. The Imp differences between the panels limit your serial connection possibilities.

I'm not sure what "Cumulative Current" means. Is it because of over currents in sunny-cold temps? Backfeeding into panels with low IMP? Any way to balance the issue by adding an equal number of 90w and 120w panels?

Cariboocoot · November 2011

Re: Different Wattage Panels going to a Combiner Box

The current of all the panels put together in that fashion adds up to 32+ Amps. This can be a problem when it comes to wiring: low Voltage and high current leads to serious V-drop between the panels and the charge controller. For instance you're looking at 6 AWG to carry that current @ 24 Volts nominal for a distance of 20 feet.

The panels with the Imp around 5 could be wired in series without much power loss, but the other two sets with Imp 7+ and Imp 4+ are too far away from these to be integrated that way. Otherwise you could put more panels in series and increase the array Voltage, lessening the current and lowering the V-drop problem.

BilljustBill · November 2011

Re: Different Wattage Panels going to a Combiner Box

Cariboocoot wrote: »

The current of all the panels put together in that fashion adds up to 32+ Amps. This can be a problem when it comes to wiring: low Voltage and high current leads to serious V-drop between the panels and the charge controller. For instance you're looking at 6 AWG to carry that current @ 24 Volts nominal for a distance of 20 feet.

The panels with the Imp around 5 could be wired in series without much power loss, but the other two sets with Imp 7+ and Imp 4+ are too far away from these to be integrated that way. Otherwise you could put more panels in series and increase the array Voltage, lessening the current and lowering the V-drop problem.

So, it's the limiting of the wire size between the array and the 24v PWM controller?

I'm locked into a 24v system, so series with more panels adds voltage, but am I right in that this controller must have 34v in and 34v out?

At a garage sale, I spent a total of $90 for two 96ft. long rolls of copper wire: One 3/000 and one 2/00, so wire isn't a problem.....

BB. · November 2011

Re: Different Wattage Panels going to a Combiner Box

Pretty much--if you have enough copper to keep the voltage drop low--then that will not be a problem.

You should also have a series protection fuse in each parallel string, at the fuse rating listed for the panel (or ~1.25x Isc rounded up).

-Bill

BilljustBill · December 2011

Re: Different Wattage Panels going to a Combiner Box

BB. wrote: »

Pretty much--if you have enough copper to keep the voltage drop low--then that will not be a problem.

You should also have a series protection fuse in each parallel string, at the fuse rating listed for the panel (or ~1.25x Isc rounded up).

-Bill

B.B.,
Thank you for your insights, too. It's good to know I can use them in a future array in the system that's going up.

A question for you, if you will. On the Kaneka panels, the amp rating is under 1 amp. If two of those 48v panels are in parallel, and the combiner box has a 3 amp breaker for each pair, what good would a fuse do when placed in series of the two panels?

BB. · December 2011

Re: Different Wattage Panels going to a Combiner Box

The idea is--You have one solar panel rated at X amps maximum current, and you have two more Strings connected in parallel (three strings total).

The two other parallel connected strings can feed 2*X into the shorted panel which is rated for 1*X of maximum current...

With the Kaneka, these are a special case... Most solar panels have a series fuse rating of ~Isc*1.25 rounded up to the next standard fuse/breaker.

The Kaneka (appear to be) rated at 7 amps series fuse (PDF sales brochure), but their Isc is only 1.22 Amps * 1.25 = 1.5 to 2 amps (next standard fuse).

So, you could put 7 amps / (1.22*1.25) = ~4 panels in parallel without any series protection fuse.

Note, the PDF brochure is a secondary source and the primary Kaneka source I found did not list a series protection fuse--So you should verify with your documentation that the series fuse rating for your K60/GSA60 panels (I assume) is really 7 amps.

Also note, that the K60 panels have a stabilize Vmp of ~67 volts which is really too low for a 48 volt bank and, probably too high, for a 150 VDC MPPT charge controller with two of those panels in series (would over volt the controller on cool days).

So, with the K60 and "typical" medium voltage MPPT controllers, you can reliably charge a 12-36 volt battery bank.

There are a couple higher voltage MPPT controllers, if needed.

-Bill

ggunn · December 2011

Re: Different Wattage Panels going to a Combiner Box

BilljustBill wrote: »

B.B.,
Thank you for your insights, too. It's good to know I can use them in a future array in the system that's going up.

A question for you, if you will. On the Kaneka panels, the amp rating is under 1 amp. If two of those 48v panels are in parallel, and the combiner box has a 3 amp breaker for each pair, what good would a fuse do when placed in series of the two panels?

For a two string array, fusing is not needed. For a three (or more) string array, the proper size fuse for each string is the next standard size up from 1.25 times the short circuit current for the string.

Different Wattage Panels going to a Combiner Box

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