Panel/String Size Concerns with Tesla Inverter

[Rapidrezults]

I recently had my 8.16kW system installed with two Powerwalls. I've been running across threads that talk about minimum string size with different inverters. From what I've found, the minimum string size with the new Tesla inverter is 5 panels. I have a 4 panel cluster running on it's own string, and I'm wondering if this was a design flaw by Tesla. I am pre-PTO and have been monitoring the system - on sunny days it appears to be producing well, however, the last few days we have had a few random cloudy/rainy days here in N. Cal and I'm not sure if the behavior(graph chart)seen in the Tesla app is normal for cloudy days. Yesterday had very sporadic production as you can see in the chart. Layout and screenshots posted below. Any input is greatly appreciated.

4 strings running 4 seperate MPPT inputs with new Tesla Inverter. 2 7 panel strings in front of house and 2 clusters in rear on separate strings:

Normal Day(mostly sunny):

Cloudy Days:

 

[wjgjr]

While I can't answer the technical question about the impact of 4 panels on the effectiveness of the MPPT, I can say that your graph looks completely reasonable for a cloudy day, both in the many spikes and the ratio of production to your full-sun day. (And, on days with heavy, rainy clouds, relative production will be even worse.) And, overall, the production number for the sunny day seems reasonable for the size and orientation of your system.

 

[jjrandorin]

No idea on the technical question about strings and whatnot, but the production looks fine, both for a regular sunny day and for a rainy day.

 

[Rapidrezults]

While I can't answer the technical question about the impact of 4 panels on the effectiveness of the MPPT, I can say that your graph looks completely reasonable for a cloudy day, both in the many spikes and the ratio of production to your full-sun day. (And, on days with heavy, rainy clouds, relative production will be even worse.) And, overall, the production number for the sunny day seems reasonable for the size and orientation of your system.

Yes, I was thinking the same thing for the normal days. My highest day of production has been 52kWh which I did not think I would get considering my horrible NE facing array.

 

[RichardCullip]

On my 4kW system the highest daily total I've seen this month is 28.61 kWh. Two days later, a very cloudy day here in Poway, CA I saw a mere 7.35kWh total. If my math is right that's ~75% reduction from a sunny day to a cloudy day.

 

[jjrandorin]

On my 4kW system the highest daily total I've seen this month is 28.61 kWh. Two days later, a very cloudy day here in Poway, CA I saw a mere 7.35kWh total. If my math is right that's ~75% reduction from a sunny day to a cloudy day.

This post makes it sound like you think "a cloudy day" is something that can be standardized as far as production goes....

 

[wjgjr]

Yes, I was thinking the same thing for the normal days. My highest day of production has been 52kWh which I did not think I would get considering my horrible NE facing array.

Yeah, that all seems pretty normal, which is good news as there do seem to be a number of individuals with issues, and Tesla does tend to refuse to provide support until PTO, even if there is an obvious problem.

On my 4kW system the highest daily total I've seen this month is 28.61 kWh. Two days later, a very cloudy day here in Poway, CA I saw a mere 7.35kWh total. If my math is right that's ~75% reduction from a sunny day to a cloudy day.

That is certainly plausible. So much depends on what "cloudy" means. Heavy, dark rain clouds all day can easily cause a 90% drop in production, where a partly cloudy (say 50%) day with mostly light clouds can see a 25% or less reduction.

 

[Ampster]

I have a 4 panel cluster running on it's own string,

How have you cofirmed that those panels are the only one on that string? If you have access to the insides of the inverter as in the picture it would be easy enough to test your assumption by taking voltage readings. Usually inverter minimum voltage is based on voltage, not number of panels. The physical layout does not necessarily dictate string wiring.

 

[GJones]

Energy production and graphs - I am also in NorCal (Napa) - 10kW array (SunPower) and your production is almost exactly a mirror of mine in terms of total output and the general shape and pattern of the curve on those days. Enjoy your system - it is doing well.
Don't dwell on daily output except as entertainment. Based on over a decade of solar production:
Look at annualized data. You will find that length of day, cloud cover, temperature and general cleanliness will all have an impact on your daily production. With a 10 kW array, my highest daily production is 70 kWHrs and that only happens a couple days per year (usually in June).
Annualized - the production averages a bit over 44 kWHrs per day. My arrays face 80% @ 240 degrees (southwest) and 20% @ 150 degrees (southeast).