Victron MPPT 75/15 + 2 x 400W / 40V panels, no yield at all on an overcast day?

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rin67630 asked Nov 27, '23

Victron MPPT 75/15 + 2 x 400W / 40V panels, no yield at all on an overcast day?

Hi I have in mid Germany 2 x 420W / 40V panels on a south-west oriented balcony tilted 60°.
They were sold with a 800W Grid-Tied converter. OK on an overcasted rainy day, the yield was zero. More or less normal: the AC converter needs minimum of power to run.

I bought a Victron MPPT 75/15 hoping to get a least a little DC energy from my panels.
I was very disappointed to see that also Victron MPPT 75/15 was harvesting nothing.
Is that normal?
When the weather is clear and the winter sun shines, I can get 450W from my panels, this looks IMHO OK.
But I expected at least 30-50W on cloudy weather. Is that unrealistic?

MPPT Controllers Solar Panel power meter

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conundrum commented · Nov 27, 2023 at 05:37 PM

Hi,I have 6 100/20 mppt's,each one connected to 2 x 410w panels in series with 75 VOC.Today was mostly cloudy with grey clouds,I had 40 to 50w with dark clouds on each string.On normal cloudy day with white clouds I get even 180w with one string.But I think the difference is because I have my panels at 10 degrees angle,which in my opinion is better for cloudy days.

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frankmsc68 commented · Nov 27, 2023 at 05:57 PM

Hi rin67630,unfortunately with your layout you have got very little margin to play with:2 pannels fixed to a cirtain direction with a given/fixed tilt....If you can play a little with the tilt to see if there might be any improvement....There are many factors which influence on the final result and i'm afraid those balcony-sets are somehow ways away from beeing in a way very productive.For example: i have two strings,each about 4 kw max. power and on a cloudy day,but not completely dark i have for a few hours (after 14.30 Hrs the sun disappears behind a mountain)about 1kw to 1,8 kw maximum from both strings together.This i concider already fortunate!Best regards ,viele Gruesse ,from northern italy,Frank

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Answer 1 · 2023-11-27T18:12:08Z

Mike Dorsett answered · Nov 27, 2023 at 06:12 PM

By the look of your set up, the 2 panels are connected in parallel, and you must be operating a 24 or 12v system?

On a cloudy day, you should be able to get 10 - 25% of nominal output, depending on cloud intensity. Without more details of your system, all I can say is that the controller is the wrong type to use with these panels - you should have the panels in series, ans a controller with 100V input, 40A output for 24V and 70A output for a 12v system.

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rin67630 commented · Nov 27, 2023 at 07:04 PM

I have 2 x 36V MPP panels and a 12V battery,

IMHO that is fine to MPPT efficiently at low power / low irradiance.
I am not seeking to get the nominal 800W, my 12V 100Ah battery would never handle that.
But I expected to get 40W at least from the panels on rainy days and I am far from that.

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Answer 2 · 2023-11-27T19:01:26Z

kevgermany answered · Nov 27, 2023 at 07:01 PM

When you connected the Victronppt, did you take the converter out of the system? The MPPT needs a battery on its output side to work.

In Oberbayern I get 5-10% of nominal output when it's heavily overcast, that's with the panels horizontal. In summer 80-90%, depending on battery SOC and loads. It can get to 100%.

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rin67630 commented · Nov 28, 2023 at 08:43 AM

Yes, of course i did remove the converter.

On better days, I expect to have both, the Victron should use the power to charge the 100Ah battery and, once fully charged, switch to the grid-tie converter.
But for the moment, I get almost nothing.

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