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SJ-Chris

San Jose, Ca

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Posted: 04/08/21 02:32am Link  |  Quote  |  Print  |  Notify Moderator

wanderingaimlessly wrote:

I'm not apparently making my question clear.
To take a different tack,
To charge a 300 Amp hr, 12 volt battery bank
Would 3, 100 watt ,solar panels such as
renogy 100 wired in parellel,
provide a substantial upgrade, or downgrade as compared to a single REC 325

The gain or loss due to voltage difference is my question.
Or is this a case of 300 watts is 300 watts,,voltage be damned?,,


To answer the question I think you keep asking.... The higher voltage (in and of itself) of the 325w panel doesn't magically get you something extra (i.e 2x).

I think what you are missing is this simple equation...
Panel Voltage x Panel current(amps) = Watts

So on the 100w panel, it is ~17.9v x ~5.72amps = ~100w
And on the 325w panel, it is ~34v x ~9.56amps - ~325w
It is the combination of voltage and amps that gives you watts.

The 325w higher voltage panel with the MPPT solar charge controller will be inherently slightly more efficient because it will utilize ALL of the available voltage (via converting from higher input voltage to the exact voltage, and higher amps, needed by the battery at whatever charging stage it is at). The PWM solar charge controller (on the Renogy 12v panels for example) will not (it will simply pass the non-scaled current through to the batteries).

Under an ideal scenario (which never really happens as outlined in some of the other posts talking about inefficiencies, etc), you could see...

On the 325w MPPT system: 34v / 13.6v = 2.5 x 9.56amps = 24.125 amps.
Whereas on the 100w PWM system: 17.9v simply gets chopped to 13.6v and the charge controller delivers 5.72 amps. By comparison, if you could somehow magically put together 3.25 of the 100w panels it would deliver 3.25 x 5.72amps = 18.59 amps (compared to the 24.125amps with MPPT).
Again...this is just for illustrative purposes to make a point (MPPT is more efficient).

My strong suggestion would be for you to string 2 of the 325w panels together (...and even 1 more if you have the space). A 325w panel will likely only be able to add ~80-90AH per day back into the batteries (on a sunny day). Adding just one more 325w panel gives you 160-180AH generated per sunny day (much better). If you want to save some $$, here is a link discussing used panels:
https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/30175150.cfm
Most panels should last 20-30 years which is a pretty long time, so buying used panels as old as 5-10 years old is not an issue.

Good luck!
Chris


San Jose, CA
Own two 2015 Thor Majestic 28a Class C RVs

BFL13

Victoria, BC

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Posted: 04/08/21 07:33am Link  |  Quote  |  Print  |  Notify Moderator

A note on PWM. Vmp and Imp have no meaning except for the panel's rating in watts. Your Voc and Isc are what matters.

If the panel is 22 Voc and 6.3 Isc, you set it out in the sun and the panel heats up, losing voltage, so you might see 20 Voc if you took a reading. If you took the Isc reading that will be the amps you see out of the controller given decent wiring. That amps figure will vary all day as the sun rises and falls. You can see 6.3 amps at high noon, eg.

The lower voltage in the heat does not matter with PWM so it is fairly meaningless too. However, the lower voltage in the heat does matter with MPPT, where it is all about Watts.

Your 325w panel will lose about 10% of its watts with the panel about 25C above 25C at 50C. See the spec showing about 44C panel temp. You can't use the 325w rating for anything in most cases, because that is for 25C panel temp, which means it must be near freezing ambient.

If you have an IR gun and the panel is tilted up, you can check its temp on the back side of it. Use the spec they give for how much voltage it loses per heat, and you can then compare that with the input watts displayed on the controller. ( but only if you get the urge [emoticon] )


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CA Traveler

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Posted: 04/08/21 08:22am Link  |  Quote  |  Print  |  Notify Moderator

Reality check for PWM usage: About 10% of the panel watts are also lost because the higher voltage is NOT used. The linked panel above has Isc of 5.72A and with a battery charging voltage of 14.4V the power loss is also about 10%.

Below 25C panel power increases while Isc declines favoring MPPT.

Truthfully nothing beats installing an extra panel and just enjoying the camping experience without stressing over these power and voltage variations.

3 other factors that I considered and still apply today:

1. Panel cost per watt is much less for 24V panels (both 60 and 72 cell) than 12V panels.

2. For equivalent panel watts the square footage of 24V panels is less that 12V panels.

3. A larger panel array of 12V panels didn't fit as well on my roof.


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Bob


BFL13

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Posted: 04/08/21 08:53am Link  |  Quote  |  Print  |  Notify Moderator

"Below 25C panel power increases while Isc declines favoring MPPT."

That 25C is panel temp, not ambient though.

The knock that PWM wastes power has no meaning because the panel goes to battery voltage, well below whatever panel voltage is at the time. Watts are of no interest--it is all about the amps.

You do get more amps using MPPT, especially in the shoulder hours, but by high noon the MPPT is only a little ahead on amps. I agree with over-panelling a bit with MPPT to get a bigger AH haul over the whole day

red31

Dallas

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Posted: 04/08/21 09:09am Link  |  Quote  |  Print  |  Notify Moderator

controller eff of mppt is a real loss of power via heat sink, esp cheaper models!
mismatched or in low power conditions.

2oldman

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Posted: 04/08/21 09:10am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

I agree with over-panelling a bit with MPPT to get a bigger AH haul over the whole day
There's very little downside to over-panelling.

FWC

The Wilderness

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Posted: 04/08/21 09:23am Link  |  Quote  |  Print  |  Notify Moderator

red31 wrote:

controller eff of mppt is a real loss of power via heat sink, esp cheaper models!
mismatched or in low power conditions.


While I can't speak to some of the off brand/cheaper models of MPPT controllers, for the reputable ones (think Victron, Morningstar) the conversion efficiency is really pretty good. In the figure below, the orange line is a Morningstar MPPT controller, they don't say which controller the blue line is. In either case the power lost to heat is much lower than the power gained by MPPT and reduced wire losses.

[image]

BFL13

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Posted: 04/08/21 09:52am Link  |  Quote  |  Print  |  Notify Moderator

My Tracer MPPT shows efficiency for 12v RV system with 17v, 34v, and 68v panels.

The curves vary up and down as the panel watts goes up with higher watt charging power being less efficient. Anyway, very roughly:

-17v (what we usually call "12-12") = 95%
-34v ("24-12") = 93.5%
-68v = 90%

CA Traveler

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Posted: 04/08/21 09:58am Link  |  Quote  |  Print  |  Notify Moderator

BFL13 wrote:

The knock that PWM wastes power has no meaning because the panel goes to battery voltage, well below whatever panel voltage is at the time. Watts are of no interest--it is all about the amps.
It's certaintly relevant when comparing 3x100W vs 300W. Both panels panels lose about the same amount of power.

Power loss is power loss REGARDLESS of how the power is used.

PWM power loss is even more relevant at lower battery voltages. For example at 13.4V float PWM can only supply Isc. So when the house load exceeds Isc the extra amps come from the battery while MPPT can supply more than Isc amps, reducing the battery draw down.

CA Traveler

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Posted: 04/08/21 10:06am Link  |  Quote  |  Print  |  Notify Moderator

When I was considering solar my 750W MPPT setup was only $100 more than PWM. Considering the larger PWM panel footprint, installation complexity, increased daily usable power it was a no brainer. Plus shade tolerance of my 3x serial panels is much better than parallel 12V panels.

It's much the same 7 years later with cheaper panels and more MPPT selection.

There are of course options and installation that favor a variety of equipment.

* This post was edited 04/08/21 10:23am by CA Traveler *

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