String Design for Maximum Efficency

[Bdorman]

Hi all

I'm finalising a design for Solar Panels with Battery Storage on my home (I live in the South East of UK).

The plan is for:
21x 455W AIKO ABC N G2 Panels - 9.555 kW
FoxESS KH8 Inverter - 8kW
FoxESS ECS4300H-H4 Battery - 16.59kWh

All the panels will be installed on a single side of a south face roof organised in two portait rows of 8 and single landscape row of 5 (8/8/5).
The installer has suggested the panels be organised with the strings running vertical, this will allow the MPPT trackers to optimise as the sun rises and sets.
Image of proposed layout attached

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I have had conflicting advice that the panels should be split evenly (7/7/7), even if this causes a slightly odd layout of strings.
This would then split the load evenly across all three trackers, allowing for optimal output.

No one seems to have properly explained to me how the MPPTS work in detail and how the interact with inverter output.
Would anyone be able to kindly advise or provide infomration on the following?
- Is the total output based on the SUM across all three MPPTs OR some other function (MIN, MEAN, etc)?
- Does each MPPT have a maximum load?
- Does each MPPT have a minimum load before contributing to the total output?
- Balanced strings OR strings with panels in similar light conditions?

[Dave Foster]

I suspect that is because there are quite a few conflicting technical requirements and some inverters share strings on a single MPPT (but the KH has 3 independent MPPT’s), in an ideal world I would be trying to get the panels laid in such a way so that the MPPT for each string is not being affected by shadows or lower elevation of the sun.

On the panels laid evenly 7/7/7 - with the KH inverter independent MPPT’s each tracker would handle a seperate string and so balance is not an issue here, the inverter wouldn’t care whether there were 6 or 8 panels on a string. The MPPT’s would independently do what is necessary to maintain the best power output from each string and as you have more than 4 panels on each string there is sufficient start voltage available.

Where you have uneven strings the lower voltage on the smaller string would mean that string may start up slightly later than the other strings but it’s going to be a marginal effect at best.

But the next problem is maximum power, each string has a maximum power rating of 3,300 watts which would affect the panel distribution - 8 panels would exceed this at maximum power and so on perfect days where you have clear skies and cool panels the MPPT’s with 8 panels on would clip and you would lose approx ~300Wh per string - if you had 7/7/7 this would not happen and the installer is correct in advising you that this would be the optimum solution.

The position of panels on the roof and any low buildings or trees might in winter (because of low elevation) mean that the lower string may not be as effective as the upper strings and so they may slightly throttle the output of the panel on the upper string (but again only a marginal effect).

All in all that looks like a good design and a 7/7/7 would offer the better performance over 8/8/5.