Can someone check my solar maths please

[Metamorfosis]

I have an Votronic mpp250 controller and a 180w panel left over from an unfinished upgrade to my old van which I would like to use on my new van that already has a 100w panel already in place.
The 180w panel is 20.2v and 8.91a
I believe the 100w one is a teleco and googling shows 18.5v and 5.41a

The voltage difference between the panels is fairly low whereas the amp difference is high therefore I gather that parallel wiring is my best option.
So to calculate my new size of panel array in parallel we take the lowest voltage and multiply by the combined amps to get
18.5v X (8.91+5.41)a = 264.92w so an efficiency loss by mixing missized panels.
My new van is a burstner and as such I believe the factory wiring for solar is 4mm so using this voltage drop calculator

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

I get this result
Voltage drop: 0.75
Voltage drop percentage: 4.03%
Voltage at the end: 17.75

so basically by my calculations if I add the panels in parallel and allow for voltage drop the absolute maximum in the Sahara at midday that would reach my solar votronic controller located next to batteries is 250w (17.75v*14.32a)

So by chance using the panel and 250w controller I have gives a satisfactory result at no cost to me. Does that sound about right?

 

[9526]

Sounds about right, even if you ignored all the losses, it’s highly unlikely that you’d ever get more than 250w out of 280w’s of panels

Edit

We have 300w running through a 20A VictronConnect controller (20A x 14v = 280W)

 

[autorouter]

If the panels are horizontal on the roof, you'd only expect 70% of nominal power maximum in UK, maybe 80% in the Sahara. That's well within the 250W the controller can handle.

 

[Metamorfosis]

Thanks guys.
I suspect a hell of a lot of installations have a well over specced controller.

 

[Metamorfosis]

As an aside to this I did the calculation substituting a 360w panel in place of the 180w one, so basically adding a 360w to an existing 100w installation. It's max output is 38.9v and 9.25a. In this instance series wiring produces the best outcome so the maths is combined voltage X lowest ampage and equates to (18.5+38.9)vx 5.41a=310w

It appears that by mixing these two panels we actually reduce the size of the array to below the max of the largest single panel on its own, 310w Vs 360w. Seems incredible that you could mix panels and the inefficiency is so great that the combined is less than a sngle.

So buyer beware when mixing very different size panels.

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[Raul]

You can only mix panels that have the same, or almost the same characteristics. Otherwise the smaller panel will drag the other down, creating heat and hot spot burns on the cell. Snail trail is common. The same effect is seen on installations where one is in the shade, and the other carries the load. The bypass ( shotky) diode does kick in and bypasses to a degree, some are poor quality. If you have different panels, then independent mppt is the way to go, without changing panels.

Regarding panel output on roof horizontal mounted. It depends on conditions and quality of panel. It can exceed the panel nameplate output if irradiation is over 1000w/m2, cool, and sun is relatively up; midd day summer. My 590w panels, few days ago, have exceeded 660w and sustained production over 600w for some time. The controller is the limiting factor at 50A and battery voltage, in my case.

 

[Metamorfosis]

It's interesting what you say Raul. I had considered adding a second panel independent of the first one but I would be wary of two controllers conflicting with each other. Any evidence this is or isn't the case?

 

[Raul]

No conflict at all. My house has tree different orientations on separate controllers, feeding the same battery bank. You can parallel controllers as many as you like, providing they have same, or close voltage settings. Mine act as a static tracker, and the east and west arrays has slightly higher voltage settings. It will work seamlessly, even one drops in float sooner than the other; no problems, it will feed the loads if any present.

 

[Off da Grid]

Multiple charge controllers will happily play together.
What you may find is that they may do the charge profiles slightly out of step.
One might finish bulk charging before the other.
When on hookup in October last year I noticed that one morning when the sun did come out my Votronic solar controller was producing more power than the onboard charger, usually when the charger is on the display shows a big fat zero