backchannel: Renolgy MPPT clues

OP wants to know if s/he needs 20A or 30A controller for 260w of panel. I suggested 20A, as the maximal/theoretical output would be 20.4A.

A thread participant objected to my analysis of why 30A is unwarranted, and why MPPT output current is higher than incoming panel current.

what I wrote

The 30A will be larger, heavier, more $$, and very likely have higher parasitic losses. For no practical advantage.

One of the definiing characteristics of MPPT controllers is DC-DC downconverting; 99% of the time controller output current will be higher than what’s coming in from the panels

the response

I disagree. If you look at the Renolgy product line

If you look at the product line you will see the name of the company is Renogy.

“Renolgy” is a shibboleth useful for detecting low-information posters. It’s a cognitive error, not a typo: “I can see the two words in front of me and don’t see the difference”.

the 30a controller is not dramatically different than the 20a.

OP didn’t mention Renogy but this participant did. So let’s fact-check my claims against the specs given in the Rover manual.

  • size: 20A = 132.37 cubic inches. 30A = 193.97 cubic inches. 30A is 46.5% larger.
  • weight: 20A = 3.08lbs. 30A = 4.41lbs. 30A is 43.2% heavier.
  • parasitic losses: no way to know from this document. Both are given “≤100mA @ 12V”. We’d need to hook them both up on meters to see what they require to self-power.
  • cost (MSRP listed at Amazon): 20A = $109.99. 30A $139.99. 30A cost 27.3% more.

As for parasitic loss the 30a will run cooler and reduce loss.


The two controllers will make the same amount of power and will generate roughly the same amount of heat while doing it.

And finally it offers an upgrade path if desired.

Upgrade to what? The only reasonable upgrade path on the same controller would be another identical panel.

  • 260w on 20A = 8% overpaneled, using the most conservative math (260w / 12v = 21.67A) <- what I am suggesting
  • 520w on 30A = 44% overpaneled (!!) <- the result of what the participant is suggesting

If OP tossed the panel and installed 300w I’d still say 20A. If they went to 400w then 30A would be appropriate.

Your MPPT statement makes no sense. My guess is you copied that from somewhere.

Paste it into Google and see if I copied it from somewhere. No? Hmm.

If one is accustomed to bullshitting on topics one does not understand then one might assume everyone else is bullshitting, too. And yet there really are people with expertise on a given subject. Related: hierarchy of bullshit

What you are describing is not a “characteristic of MPPT” but all controllers.

No, it isn’t.

It is simply what happens when you convert from a high voltage to the 12v of a house voltage. All controllers do that!

They do not, as 10 minutes watching the controller display (or 10 minutes reading on the topic) would demonstrate. There is nothing for non-MPPT controllers to convert because they are running the panels at ~Vbatt.

OP’s panels are Rated 30.7v Vmp, 37.8v Voc, 8.50A Imp, 9.01A Isc. Let’s look at what various controller types do with that at an average charging voltage of 13v:

  • MPPT 19A (247w), assuming 5% bucking losses. 30.7v x 8.5A x 0.95 efficiency
  • PWM ~9.01A (117.1w) 13v ~9.01A
  • shunt ~9.01A (117.1w) 13v ~9.01A
  • series ~9.01A (117.1w) 13v ~9.01A

But you are missing my point. People are acting as if you put a 30a controller you will need lower gauge because of the higher amperage.

They are incorrectly assuming a larger controller requires heavier wiring for the same current. 20A of power running through a 20A, or 30A, or 1000A controller requires the same wire gauge.

The amperage is going to be determined by the solar panels not the controller.

With MPPT controllers both incoming and outgoing current is determined by the controller. Input current is controlled by selecting the appropriate Vpanel along the power curve. Output current is controlled by buck conversion.


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