diy solar

diy solar

Long time listener, first time caller from Dateland, AZ

We expect an MPPT to limit the current it draws to what it can handle.
Some have a diode which is normally reverse-biased, that will short out the PV string if hooked up backwards, to prevent damage. In that case there is a reason for maximum PV short-circuit current rating.

Some units have protection against over-voltage, while others would be killed. Some Midnight, for instance, say "Hyper-Voc", where they disconnect PV if somewhat over-voltage. That would have a limit to what it could handle.
Design your PV string so that even with the higher Voc that occurs during record cold temperature for your location, it will never exceed whatever maximum voltage is documented for the inverter/charge controller. If they document an over-voltage it can handle you can rely on that, otherwise whatever max voltage is given.
 
Sound advice. Thank you. Here in Southern Arizona, the temperatures rarely even get close to 32° F. Not sure what effect that would have on the panel voltage. Now, 120° F in the Summer, that's my problem!
 
Sound advice. Thank you. Here in Southern Arizona, the temperatures rarely even get close to 32° F. Not sure what effect that would have on the panel voltage. Now, 120° F in the Summer, that's my problem!

Look up the record low that temperatures have ever reached.
If record is 32F (zero C), then that is -25 relative to the 25 degree C temperature of nominal ratings.
If PV data sheet says temperature coefficient of Voc is -0.4%/degree C, then -25 x -0.4% = +10%, so multiply Voc by 1.10

120 degrees F will similarly decrease Vmp. Different temperature coefficient, similar math. Make sure min Vmp (edit: not Voc) is sufficient to reach the MPPT range of the charge controller.

More likely, wattage output of inverter being limited will be the problem. It may work better inside an air conditioned space; of course then the A/C will have to process it's waste heat too.
 
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Sound advice. Thank you. Here in Southern Arizona, the temperatures rarely even get close to 32° F. Not sure what effect that would have on the panel voltage. Now, 120° F in the Summer, that's my problem!

Not sure where you are but Phoenix gets 12-15 days per year of freezing temperatures. Into the 20’s.

First google hit:


Just one cold morning can over voltage a SCC and let the magic smoke out. ☹️

Edit: I see Dateland is about the same latitude as Yuma. Record low there is 13 !

 
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Correct. Phoenix is just far enough North to worry about freezing temps. Dateland is just 40 miles from the Mexican border as the crow flies, and is the second hottest area of the desert in Arizona second to Yuma. It snowed here once in January 1969 for half an hour and it never hit the ground. The next day, it was 68° F. From July to September, the temperature never drops below 100° at night.
 
Edit: I see Dateland is about the same latitude as Yuma. Record low there is 13 !
Again, correct. However, Yuma is in a "bowl" of mountains and the weather there is sharply different from Dateland outside that "bowl". We get four times more rain, slightly cooler Summers, warmer Winters and FAR more wind.
 
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