Roof solar panels, go 2 50w or 100w

dwh

Tail-End Charlie
Call Renogy and verify how many bypass diodes in the panel before making any final decision. My example was based on unverified assumptions of how many diodes per panel (probably accurate, but still...double-check it).
 

dwh

Tail-End Charlie
Also, this applies to small panels. Bigger panels are different.

My 300w Astroenergy panel has 72 cells in series for a 36v Vmp (technically a "24v nominal" PV module - 36v is not enough to charge a "36v nominal" battery bank). It has 6 bypass diodes, each bypassing 12 cells.

So...

36v - normal
30v - one bypass
24v - two bypasses
18v - three bypasses
12v - four bypasses

(Actually, in Los Angeles in August (90-100 Fahrenheit ambient), I'm generally seeing 30v - 32v Vmp (running very hot) in the afternoons.)

So really, if it hits four bypasses (or three in L.A. in August), it's not going to charge my 12v battery bank. The Victron MPPT unit needs to see (battery voltage + .5v) solar input voltage or it won't even try to charge the battery.

It has exactly the same electrical characteristics as 3 x 100w 12v (24 cell) PV modules wired in series, each with two bypass diodes.








4 x 18v 50w in series is going to be 72 Vmp and 13a (on a cool sunny day) which is pushing the limit for a Victron 75/15. Still, flat mount and running hot will be a bit lower voltage and amperage, so a 75/15 would do it (barely).

They'll have 36 cells in series, and probably 2 bypass diodes, so each bypass would cut 9v.
 
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JCDriller

Adventurer
Call Renogy and verify how many bypass diodes in the panel before making any final decision. My example was based on unverified assumptions of how many diodes per panel (probably accurate, but still...double-check it).
Thanks, I found the specs on their 100w panel, it's 2 diodes. However, I haven't found the specs on their 50w panels. I sent them an email, but I'll likely call tomorrow because if they have 2 per 50w I'm going to call and change my order. I am running the Victron 100/30 unit, I wanted plenty of room for expansion in my system, maybe something to think about OP.
 

dwh

Tail-End Charlie
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dwh

Tail-End Charlie
Um..wait a minute...I think I spoke too soon...


1 x 18v 100w w/2 diodes.
Single bypass drops voltage from 18v to 9v for 100% loss.

2 x 18v 50w w/1 diode ea., in series.
Single bypass drops voltage from 36v to 18v for 50% loss.

So yea, even with only one diode each, there is more shade tolerance with 2 x 18v 50w in series over a single 18v 100w.


BUT...what about...

2 x 18v 100w w/2 diodes in series - vs. - 4 x 18v 50w w/1 diode in series?

2 x 100 in series is 36v.
Single bypass drops 9v from 36v to 27v.
Second bypass drops from 27v to 18v.
Third bypass drops to 12v - 100% loss.

4 x 50 in series is 72v.
Single bypass drops 18v from 72v to 54v.
Second bypass drops to 36v
Third bypass drops to 18v.
Fourth bypass drops to 0v - 100% loss.

So again, more shade tolerance.


But what about parallel?

1 x 18v 100w, 2 diodes.
Single bypass drops to 9v - 100% loss.

2 x 18v 50w, 1 diode ea., parallel.
Single bypass drops one panel to 0v, but the other panel is still at 18v - 50% loss.

2 x 18v 100w w/2 diodes ea., parallel.
Single bypass drops one panel to 9v - 50% array loss.
Second bypass on same panel, no difference.
Second bypass on other panel - 100% array loss.
Third bypass - 100% array loss.

4 x 18v 50w w/1 diode ea., parallel.
Single bypass drops one panel to 0v, three still at 18v - 25% array loss.
Second bypass, two at 0v, two at 18v - 50% array loss.
Third bypass, three at 0v, one at 18v - 75% array loss.
Fourth bypass, four at 0v - 100% array loss.

So yet again, more shade tolerance.



So yea, no matter how you rig it, series or parallel, two 18v 50s each with a single diode beats one 18v 100 with two diodes on shade tolerance. Same with 4 18v 1 diode 50s vs. two 18v 2 diode 100s.



If you could find 4 18v 50s with two diodes each, it wouldn't gain anything over one diode each if wired in parallel (one bypass on a panel drops the panel to 9v which is 100% loss of that panel), but in series it would double the shade tolerance: 72v, 63v, 54v, 45v, 36, 27v, 18v, 9v - 8 bypasses to get down to 100% loss.
 
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JCDriller

Adventurer
@dwh I'm tracking you, thanks for the explanation. I did call Renogy this morning and verify that their 50 W solar panel do only have one bypass diode each. However that still beats their hundred watt panels with 2. Especially since I'm running their 100/30 charge controller. So I canceled my order and placed an order for two of their hundred watt solar suitcases which have 4x 50 watt panels. I plan on wiring them in series at 72v. This will allow for smaller cable and more shade tolerance.

When discussing efficiency does the charge controller benefit from a higher differential voltage between the panels (72 or 48) vs the battery at 12v??
 
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dwh

Tail-End Charlie
When discussing efficiency does the charge controller benefit from a higher differential voltage between the panels (72 or 48) vs the battery at 12v??
Nope.

Doesn't matter if you down-convert 72v Xa to battery voltage, or 18v Ya to battery voltage - it's still going to be the same amps out at battery voltage. 200w in, 200w out.
 

sleddogsam

New member
I'm also running 2 x 100w renogy suitcases in series and they work great. If you can afford the the eclipse model they are more efficient for the size and weight. I currently run a midnight solar kid but I'm going to switch to a redarc bcdc 1240D which will automatically switch from vehicle charging to MPPT solar charging when the truck is turned off.


Sent from my iPhone using Tapatalk
 

ripperj

Explorer
So I take it that 18v 50w with two bypass diodes doesn't exist?( no research on my end to support that)
Edit I mean that I didn't attempt to find them myself

Sent from my iPhone using Tapatalk Pro
 

john61ct

Adventurer
4 x 18v 50w in series is going to be 72 Vmp and 13a (on a cool sunny day) which is pushing the limit for a Victron 75/15. Still, flat mount and running hot will be a bit lower voltage and amperage, so a 75/15 would do it (barely).
Unlike pushing up to the Isc amps limit, coming that close to the max Voc spec is risking damaging the SC.

Personally 65-67V is as high as I'd go, if there's any chance the rig will get cold clear bright conditions one day.

I know I'll be in snowy mountains for example
 

john61ct

Adventurer
When discussing efficiency does the charge controller benefit from a higher differential voltage between the panels (72 or 48) vs the battery at 12v??
wrt DC-DC buck conversion, for given hardware, each in-out pair has different efficiency levels.

In practice we rely on the mfg brand to select efficient components.

wrt MPPT definitely a **huge** difference, little point at all using MPPT technology with 12V nominal low voltage panels, much better output advantages getting up to the 40+V range
 

john61ct

Adventurer
I am running the Victron 100/30 unit, I wanted plenty of room for expansion in my system, maybe something to think about OP.
For best shade tolerance a larger number of controllers per given set of panels is better, with MPPT assuming panel voltages are up where that tech makes a significant difference.

2x the 75/15 is usually cheaper than the 100/30, so to me that's the way to go.

To get the best value per 75/15, I would get panels between 40-65V, up to 250 maybe 300w total. Technically the SC won't **output** more than ~220w to a 12V bank, but you won't often get sustained full rating wattage output out of your panels in the real world, and overpanelling a bit gives you greater **average** output per SC.
 

94Discovery

Adventurer
For best shade tolerance a larger number of controllers per given set of panels is better, with MPPT assuming panel voltages are up where that tech makes a significant difference.

2x the 75/15 is usually cheaper than the 100/30, so to me that's the way to go.

To get the best value per 75/15, I would get panels between 40-65V, up to 250 maybe 300w total. Technically the SC won't **output** more than ~220w to a 12V bank, but you won't often get sustained full rating wattage output out of your panels in the real world, and overpanelling a bit gives you greater **average** output per SC.
Sorry for the neebe question what is 100/30 or 75/15 ?
 
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