diy solar

diy solar

Should I use NOCT ratings in my case?

JSR

New Member
Joined
Aug 6, 2021
Messages
28
Hi guys. New to the forum, but have been watching Will's videos for some time now.
I ordered an EcoFlow Delta Pro on the Kickstarter campaign and am trying to figure out the solar panel setup to keep the Delta Pro charged.

Here's some info:
Location: I live in Southern California where summers can reach over 115F. The coldest it gets is around mid-30s in the middle of the night, but typically over 50F by the time sun comes up.

Delta Pro PV Input Specs:
- Max Voltage = 150V
- Max Current = 12A
- Max Power = 1200W

Considering the Delta Pro's input current limit, the PV setup is limited to a single string. I'd like to reach the 1200W input limit if possible. I'm aware of the recommendation to account for PV Voc at colder temps, which is why I need some help.

Ideally, 3x 400W panels would provide the perfect 1200W of solar to the Delta Pro. However, the Voc of virtually all 400W+ panels is right around 50V at STC, so 3x panels would be right at the Delta Pro input voltage limit. Adding in some voltage rise due to lower temps would bring input voltage over the Delta Pro limit (by about 15-20V).
Going down to 350W-ish panels would bring Voc well within the Delta Pro limit, but I'd be lower than the 1200W that I'd really like to hit.

However, considering STC is measured with an ambient temp of near 0C and for the most part, the temperature in my area is typically above 50F by the time the sun comes out and any light would hit the panels, it would seem the NOCT numbers is more appropriate to use. The NOCT Voc of most 400W panels is 47V, which with some colder temp rise, should fall right in to the Delta Pro's input limit.

Browsing the site, I see folks only using the STC values. If I do, then I'd either exceed the Delta Pro's limits or have less solar power than I'd like. However, using NOCT values aligns almost perfectly with the Delta Pro's limits.

So should I use STC or NOCT values for my setup design?

Thanks for the help!
 
STC is 25ºC (77ºF), NOTC is 20ºC (68ºF). And NOTC is with less sunlight than STC.

With 3 panels of 50Voc @ STC, you would have to never get below about 27ºC (81ºF) or so to avoid getting too close to 150V.

You need to find panels with no more than about 45Voc. Maybe 350W or 370W panels. You could get around 1100W total.

Or find a SCC that can handle panels in parallel.
 
Thanks rmaddy. From what I found researching, STC is 25C cell temperature (which requires an ambient temperature of around 0-3C). NOCT is 20C ambient temperature, which results in a cell temperature of 45C.
So comparing apples to apples:
STC is 25C cell temp vs. NOCT is 45C cell temp
...or...
STC is 0-3C ambient temp vs. NOCT is 20C ambient temp

My area is much closer to 20C (68F) ambient than 0C (32F).

NOCT is supposed to be a more "real life" representation for areas such as mine, which is why I'm wondering if I should use those figures instead.

Also, NOCT ratings for the panels tend to be ~72-75% of STC ratings, so a 400W panel only makes about 300W, and a 350W panel only makes 260W. If those are the "real" numbers in my warmer climate, than 3x 350W panels would only provide 780W of solar.

The SCC can't change because it's built in to the Delta Pro, so my PV input limits are hard set.

Let me know if it still makes sense to use STC values considering the above, or if using NOCT is more appropriate.

Thanks.
 
A data sheet for some panels I researched explicitly states that STC is 25ºC ambient. But a quick search does seem to indicate that in most cases, STC is cell temperature. Good to know.

BTW - just today I saw a brief output of 950W from my 990W panels while it was 90ºF. I was quite surprised. Unfortunately I don't know what the Voc was at the time so I don't know how close it was to the STC value. My point being is that you may get away with staying below 150V for a while until one time you don't and the magic smoke gets let out of your SCC.
 
Thanks again rmaddy! Yeah, that's the concern. Great for a while till we hit some weird super cold streak from global warming or something. It doesn't help that the info out there is a bit contradictory.
Great to know the panels actually can get close to their marketed ratings. I'm trying to find out from EcoFlow if there's a tolerance to their 150W max (like MPP and Growatt all-in-ones usually have a true limit that's higher than their stated one for PV voltage).
BTW, what panels do you have?
 
If you want to utilize four of those 400W panels, one option is to wire them in a 2S2P configuration, and slightly offset the direction of each panel sting. One string could be pointed SE, while the second is pointed SW.

If their individual outputs are reduced to 75% at noon, then you'd get 400W X 4 panels X 0.75 = 1200W.

One benefit is that on cloudy days when output is low, the angled panels will catch the scattered sunlight irregardless of which way they are oriented.

I have my arrays on rotating mounts, and I've utilized this strategy for years now.
 
Thanks for the idea Micheal. I was planning to mount 3 panels on the W-facing side if my garage roof and my property isn't big enough to have a ground mounted setup where I can change it's orientation.
Considering the Impp of 400W panels is around 10A, even if each series string was only outputing 75%, that'd still exceed the 12A limit of the built-in SCC. I'm not familiar with how SCCs limit the input they receive. It seems they're most sensitive to input voltage, but not sure if they're able to limit current to keep their input below the rated max power.
 
Back
Top