Solis S6 output issues on Dual MPPT unit

[PW-EDI]

Hi Folks, nice to be among fellow DIY Solar Fans.

I have an issue with a Solis Dual MPPT S6 that I purchased (used but taken from working system) recently. There seems quite a difference in what is being input to what is being output, far more of a difference than my single MPPT S6.

My setup is quite simple, 5x 400w panels in a single string. My aim with the Dual MPPT to is to add a second string of panels, just waiting on install. The inverters are Grid Tie so we just feed the energy to the house, no feed in or battery attached.

My Single MPPT S6 produced this kind of conversion:

​	​	​	​	DC PV1 (W)​	Power Out (W)​	​
20​	25/12/2023 11:47:25 (UTC+00:00)​	Normal​	​	155​	120.000​	​
24​	25/12/2023 12:07:25 (UTC+00:00)​	Normal​	​	172​	150.000​	​
35​	25/12/2023 13:02:25 (UTC+00:00)​	Normal​	​	101​	80.000​	​

But on testing the Dual MPPT S6 today it only gets this conversion, bear in mind still only single string:

​	​	​	​	DC PvPV1(W)​	DC PvPV2(W)​	Total Power(W)​
20​	28/12/2023 10:39:29 (UTC+00:00)​	Normal​	​	117​	0​	40.000​
21​	28/12/2023 10:44:29 (UTC+00:00)​	Normal​	​	138​	0​	50.000​
53​	28/12/2023 13:31:50 (UTC+00:00)​	Normal​	​	0​	154​	80.000​
56​	28/12/2023 13:46:50 (UTC+00:00)​	Normal​	​	0​	172​	110.000​

So I seem to be getting far higher yield on PV2 than PV1 but the difference over the Dual to Single MPPT conversion is still looking like twice the size.

Is this one of the following:
1. A setting I can't see/find yet that will instruct single MPPT use in a Dual MPPT system
2. Is the unused MPPT actually consuming the extra energy?
3. I have a genuine fault on the inverter although no alarms or codes being shown

The Single MPPT is 1kW rated and the Dual MPPT is 3kW rated and the comparison was done on similar weather days (Not so sunny Scotland), panels and cabling identical between both systems. The S6 was factory reset prior to my install but I did notice that Max Power Output from previous user was only 34% which seemed very low.

My main goal here was to have 2 strings able to produce energy since the sun goes round the back of the house in the afternoon so my panels presently don't get a full day of sunlight, the second string would be in a different part of the garden to get more sun view.


Thanks folks, see if I can get to the bottom of this one,

cheers
Pete

 

[rpdom]

All I can think is that the dual MPPT model has a higher internal consumption - possibly because of the higher rating. I don't think the second MPPT would use any power if it has no input.

I have to admit I'm not familiar with those models, I've only had the hybrids with dual MPPT (only using one) and batteries.

 

[Solar Guppy]

An inactive mppt channel would save the switching losses for the boost converter as it would be inactive, maybe 10 watts as a guess. Also without external can high resolution power meters, one is depending on unit reported values and they tend to not be high precision

 

[PW-EDI]

Thanks guys, once I get my second string of panels online, will revisit this one, it may be the case I run with two smaller more efficient inverters rather than one larger less efficient unit.
All part of the learning curve I guess.

 

[PW-EDI]

Hi guys, to round off this original post, I now have the dual MPPT unit running and it does seem that it isn't happy running with only a single string.

Each MPPT will use about 30-40W of power during conversion, seems to be higher at lower input level so sub 100W from the panels they are less efficient at conversion, above 100W input they start to improve.

With both MPPT running the unit is much happier and its running quite well now. I will say that this inverter is slightly less efficient than my original 1kW unit so maybe a slight change in design there that causes this. This smaller inverter would only consume about 20W during conversion and it was pretty consistent.

 

[makii3]

the lower output during your earlier usage of the new dual mppt inverter maybe attributed to the "below than optimal" Vmp, since the new inverter is optimised for a higher output, thus higher input, compared to your old 1KW inverter. considering your 5 pv modules, your Vmp would likely be around only ~200Vdc. although by spec, your inverter would run without problems with this much voltage from your PV, it would be much "happier" if fed with voltage closer to 400Vdc.