Derating 24v Solar Panels?

[jrnymn7]

I'm considering running two 250w/24v poly panels; tracked/pointed during spring/fall, and flat during the summer; thru the Tracer 3215BN 30a mppt controller. I can also get 200w/24v mono panels, locally, but would much prefer poly, seeing as I need the panels to perform well in low light conditions, as is common during early spring/late fall.

I have noticed when running two 140w/12v panels, tracked/pointed, thru the 30a Tracer, it is not uncommon to see 300w in and out,
this time of year. That's an increase of 7% over their rating. So, 500w (2x 250w) + 7% = 535w.

But I've also seen a 25% drop in power, at noon, going flat vs. tracked. So, 535w - 25% = 401w flat.

On a 60% soc bank, charging voltage would begin in the low 13's range, say 13.2v. 401w / 13.2v = ~ 30a... the controllers max current rating. But I would prefer not to max out the controller, especially in hot summer conditions, so I would prefer to lower wattage, and thus max/peak output amps, during the summer months.

Again, it's only during the peak summer period, say a month or so either side of the solstice, that concerns me. The rest of the year maxing out the controller will likely not be a problem, even when pointed.


So I'm wondering if de-rating the panels would be the solution, and how I should go about doing so? Would it depend on series vs. parallel wiring, for example? Or would it depend on the number of diodes and/or cells, for instance?

Your input is much appreciated!

[BFL13]

red31 wrote:

jrnymn7 wrote:
I'm just saying it would be better to run parallel/mppt, if and when the panels and controller are hot. Less bucking means higher efficiency, lower heat generation, and thus better overall performance.

Same as when not hot. 1% efficiency difference between 33v and 66v (12v system), same power. I'm sure the owner's manual graph ignores voltage drop 😉

24-24 is more efficient than 24-12 but then you have more "overhead" with 24-12 in low light that might get you more AH in a day? It is all scenario dependent it seems.

Just stick the darn thing out in the sun and you get what you get!
( Not that I would accept such advice 😞 )

[JiminDenver]

Actually I tested one 250w panel tracking and got 18 amps. That would have been 54 amps for the three panels in parallel. When I connected them all in series I only got 48 amps. The heavier wiring is worth it for the extra amps and cooler controller in my opinion.

[red31]

jrnymn7 wrote:
I'm just saying it would be better to run parallel/mppt, if and when the panels and controller are hot. Less bucking means higher efficiency, lower heat generation, and thus better overall performance.

Same as when not hot. 1% efficiency difference between 33v and 66v (12v system), same power. I'm sure the owner's manual graph ignores voltage drop 😉

[jrnymn7]

o.k., gotcha... I hadn't thought of it that way.

[JiminDenver]

jrnymn7 wrote:

Are you saying you have some loads tied directly to the controller's 'load' terminals?

No never have used the load terminals. The power just skates across the battery terminals to the inverter or any load. There are fan, pump and other systems that use no battery at all. When the sun shines they work.

[JiminDenver]

jrnymn7 wrote:
smk, I hope the Tracer qualifies as a 'good' controller 🙂 If so, a great deal at $150 (incl remote monitor)

Like the Eco-W, once you have proven it's functionality and reliability it will no longer be just a el cheapo controller. It will be a functional lower cost option and the more options the better.

[jrnymn7]

JiminDenver wrote:

jrnymn7 wrote:

Good point regarding using the solar to power loads. I don't usually think in terms of powering loads, just charging. Of course, I do realize solar is running loads all the time, in essence... just thru the bank first.

Yes, I'll try contacting ep solar, and see what they have to say. Maybe as long as I add a fan or two, it'll handle being maxed out.

The loads don't run through the bank first. The power out of the controller is there with no resistance unlike there is when the bank discharges. The only reason any power comes out of the bank is because the demand is higher than the output of the controller. Also I have started a system up, started a inverter running a light and then removed the battery. as long as the sun shown the light stayed lit.

That's why we have a A/C that pulls 450w, one of the burners on the cook top pulls 475w, a heater that pulls 400w and so on. The batteries are there for the short term big loads, over night and bad weather. Most days they are rarely out of float.

Are you saying you have some loads tied directly to the controller's 'load' terminals?

[JiminDenver]

jrnymn7 wrote:

Good point regarding using the solar to power loads. I don't usually think in terms of powering loads, just charging. Of course, I do realize solar is running loads all the time, in essence... just thru the bank first.

Yes, I'll try contacting ep solar, and see what they have to say. Maybe as long as I add a fan or two, it'll handle being maxed out.

The loads don't run through the bank first. The power out of the controller is there with no resistance unlike there is when the bank discharges. The only reason any power comes out of the bank is because the demand is higher than the output of the controller. Also I have started a system up, started a inverter running a light and then removed the battery. as long as the sun shown the light stayed lit.

That's why we have a A/C that pulls 450w, one of the burners on the cook top pulls 475w, a heater that pulls 400w and so on. The batteries are there for the short term big loads, over night and bad weather. Most days they are rarely out of float.

[jrnymn7]

smk, I hope the Tracer qualifies as a 'good' controller 🙂 If so, a great deal at $150 (incl remote monitor)

[jrnymn7]

The specs say the Tracer has current limiting protection... but that could mean a few things, so I've fired off an email.

[MrWizard]

speaking of powering loads
since i changed out three panels, i did a Solar test on Sunday
(also tweaking the C40 controller)

i ran (4) six inch desk fans, (1) ten inch fan, the fridge, and the laptop
from the inverters,
the total Solar yield for Sunday was 163.9 AmpHrs 2262.4 wattHrs
the batteries were near full when i started, and almost the same state when i finished, the peak was 22.48 amps 310.0 watts
the peaks would have been higher if starting with a lower bank and cooler weather
so that was 2.262 KW hrs of used power..
we are power hogs, especially in HOT weather

[time2roll]

The good MPPT controllers all throttle based on amps and heat.
No need to do this manually.

[jrnymn7]

JiminDenver wrote:
I would call Tracer and ask. The Eco-w doesn't say it will or wont clip amps but I have seen someone running a pair of 150ws in series and pushed it past its 20a limit. It stopped at 20.1a. It will also clip amps if it gets hot too.

I believe someone here uses a Rogue 30a MPPT controller with 490w, maybe they could chime in on how often they see it clipping amps.

I would over panel the MS in a heartbeat if I had the room. It would be great for lower light situations and let me do more earlier. It would also let me use higher watt items during the peak hours. It would be nice to see the full 60a the controller is capable of.

Good point regarding using the solar to power loads. I don't usually think in terms of powering loads, just charging. Of course, I do realize solar is running loads all the time, in essence... just thru the bank first.

Yes, I'll try contacting ep solar, and see what they have to say. Maybe as long as I add a fan or two, it'll handle being maxed out.

[jrnymn7]

fair enough, lol.