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diy solar

How to select cheap solar panels for my 24V setup?

two_wheeled

New Member
Joined
Jun 26, 2024
Messages
16
Location
Phoenix
Hi, can you help a newbie out? I've searched and read a ton but I feel like I'm on the verge of buying the wrong solar panels. :)
I'm assembling a budget 24V off-grid build. I have everything but the panels. My charge controller is:
HQST 40A MPPT
Max solar input voltage = 100V
Max solar input power = 1200W (for 24V)

There are lots of used panels for sale in my area (Phoenix). My original plan was to purchase some used panels locally because they're super cheap (and please remember this is for a low budget build), but as I look around I'm finding it difficult to find panels that fit my specs easily. I'm finding lots of used panels around ~230W but when I add up multiples to find a good input voltage less than 100V, I end up with much less (or more!) than my anticipated capacity of somewhere near 1200W.
I also found that brand new 400W panels are also relatively inexpensive right now, but you have to buy at least 10 of them and I would only need three. I'm also finding other resellers with pallets of new panels for sale cheap but they don't want to sell less than 10 at a time.
Would appreciate any feedback. Thanks!
 

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You can go over in wattage just fine, it'll help in winter and cloudy weather. Most used panels are going to limit you to 2s to keep below that 100v limit on VoC, but that just means a combiner box to get more strings in parallel. Not a big deal. 2s3p on 200w panels will max out that SCC in perfect conditions so 230w panels just buy you extra. If your system gets more sunlight than it can handle, that just means you'll loose a little potential in perfect weather.

Again, not a big deal. Don't panic, grab a $100 combiner box, and throw 2s3p of the used panels on there. You'll be fine. 👍

Remember, panels don't PUSH power, the SCC just ASKS the panels for power and the panels GIVE what they can.
 
If the 230V panels are cheap enough, grab 6, tie them two in series, two, two, and grab some fused mc4 combiners, connect the output to the controller, and go for it.
It really depends on the VOC the panels output.
With 40ishV ya cant go over 2 in series.
If the VOC is 22, then you have more room.
Just keep the combined toral umder 85V and you will be safe, unless it gets VERY cold.
 
Oh! I thought the solar panel wattage was also a hard limit. I didn't know I could go over that.
So for example, there's a guy in my area with some unused Trina 255W panels. Could I connect:
two parallel strings
of
three panels in series
for a total of six panels at maximum 91.5V and 1530W?
Without blowing up the charge controller? :)
Would that be called 2p3s? I'm unfamiliar with the nomenclature.

1736308144568.png
 
Oh! I thought the solar panel wattage was also a hard limit. I didn't know I could go over that.
So for example, there's a guy in my area with some unused Trina 255W panels. Could I connect:
two parallel strings
of
three panels in series
for a total of six panels at maximum 91.5V and 1530W?
Without blowing up the charge controller? :)
Would that be called 2p3s? I'm unfamiliar with the nomenclature.

View attachment 268532
Need to see the VOC number to be sure, but I sincerely doubt 3s with those panels would be safe.
The 33V* number, whatever that is… would be RIGHT at the limit of your controller, so, ANY weather below 25C/78F, I would CERTAINLY not put 3 in series…
 
They're calling it the SmartCurve V Limit, 33v. I would stick with 2 in series with that SCC to be safe.

Yeah, the VoC or Voltage Open Circuit (that smart-v they mention, stupid marketing department! 🤨 ) is the HARD limit. Like granite hard or understanding teenage girls hard, or finding an honest career politician hard. Never Exceed!!
 
They're calling it the SmartCurve V Limit, 33v. I would stick with 2 in series with that SCC to be safe.

Yeah, the VoC or Voltage Open Circuit (that smart-v they mention, stupid marketing department! 🤨 ) is the HARD limit. Like granite hard or understanding teenage girls hard, or finding an honest career politician hard. Never Exceed!!
What you are trying to say, is the 100V limit of the charge controller is the hard limit. ABSOLUTELY never exceed.
But VOC/VLIMIT WHATEVER IS AT STANDARD TEMPS.
That voltage is only at 78F25C
As temps drop, that number increases. Usually there is a temp coefficient on the panel, but voltage calculators and formulas exist to determine voltage limits. But even without calculators, its easy to see 3s would be 99V, at 78F, and as temps drop, voltage increases, so there is no way 3S would be safe.
 
What you are trying to say, is the 100V limit of the charge controller is the hard limit. ABSOLUTELY never exceed.
But VOC/VLIMIT WHATEVER IS AT STANDARD TEMPS.
That voltage is only at 78F25C
As temps drop, that number increases. Usually there is a temp coefficient on the panel, but voltage calculators and formulas exist to determine voltage limits. But even without calculators, its easy to see 3s would be 99V, at 78F, and as temps drop, voltage increases, so there is no way 3S would be safe.

Good to know! I didn't know this, thanks.
 

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