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Axpert MKS II 5K sweet spot for upgrade


Giel
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I designed my solar system and had the following installed: 2 x Axpert MKS II 5K in parallel, 14 x 330W Canadian solar panels in 2 strings of 7 and 3 x Narada 48NPFC100. This system allows me to switch the grid off as long as the sun shines and a bit longer. Initially the installer installed the 2 strings of panels to one inverter, but I split it to have 7 panels per inverter. The system was installed early in December 2019. Everything still works fine. We are in Pretoria and generally we have more power than what we can use.

On cloudy days we do survive, but when the clouds are very low, the yield is low. I kind of figured my batteries charge to around 70% to 80% or sometimes a bit less on heavy cloudy days. I do understand this, but I am considering to add a few panels for such days.

 

The inverter spec is max 4500W, max VoC is 450V, and the MPPT range is 120V to 430V. The panel spec is 330W at Vmp of 37.2V with a VoC of 45.6V.

There are a number of options to increase solar capacity as follows:

1.    Add up to a maximum of 2 more panels for each string to increase it to 8 or 9. The power for 9 panels will be 2970W per inverter, 410.4 Voc and Vmp of 334.8.

2.    Splitting the system into 3 strings of 6 panels with one inverter having two strings and the other 1 string. The power to one inverter will be 3960W and to the other 1980W with the Vmp = 223.2V and the Voc = 273.6V.

3.    Another option is to have 4 strings of 5 panels, 2 per inverter. So each inverter will have 3300W in total and the per string Vmp = 186V and Voc = 228V.

Each option has pros and cons.

Option 1 is easy, but will need to split it anyway if I need to upgrade beyond 5940W solar. Also, to use rule of thumb safety, the Voc x 1.1 is close to the inverter Voc max for those cold winter days. The advantage of option 1 is that the MPPT minimum voltage will be met earlier in the morning and last later in the afternoon, getting perhaps a slightly longer number of hours out of it.

Option 2 is somewhere in the middle, is unbalanced, but allows for easy future expansion. A disadvantage is that one inverter will carry a higher load in terms of battery charging and might reduce its longevity.

Option 3 is a balanced system, but the Vmp is to the lower end of the MPPT range. So it will start charging later in the morning and stop charging earlier in the afternoon.

The middle of the MPPT range is at 275V. My current installation is 260.4V and sometimes shows 270V or more. With Option 1 the calculated MPPT voltage is 334.8V, with option 2 it is 223.2V and with option 3 it is 186V as mentioned above.

A)   Does anyone have experience on the “sweet spot” for this inverter in terms of MPPT range and does it matter where in the MPPT range the inverter solar input is?

B)   Is it a problem to have an unbalanced system where one inverter has to charge the battery more than the other? In other words, is option 2 ok?

C)   Any expansion just for the sake of cloudy days is not really worth it in terms of return on investment, but at least I will be more independent of any potential grid power failures on such days? When the system was installed in December, we had lots of rain and cloudy days and also load shedding. So those days I will be better off. Any comments?

D)   Which of the above or other options will you recommend?

E)   Any other views and suggestions are welcome.

 

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17 hours ago, Giel said:

1.    Add up to a maximum of 2 more panels for each string to increase it to 8 or 9. The power for 9 panels will be 2970W per inverter, 410.4 Voc and Vmp of 334.8.

That's at 25°C. My rule of thumb is to add 7% for 0°C winter mornings; then Voc is 410.4 × 1.07 = 439 V. That's still OK, not even derating at that voltage.

17 hours ago, Giel said:

Is it a problem to have an unbalanced system where one inverter has to charge the battery more than the other? In other words, is option 2 ok?

That's fine. A fair bit more wiring, though. But it leaves you open to adding yet another string of 6 panels 😈

17 hours ago, Giel said:

When the system was installed in December, we had lots of rain and cloudy days and also load shedding. So those days I will be better off.

You'll also be better off in winter, when the days are shorter (less solar input) and the nights are longer (run the lights for longer).

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