Different inverter sizes within the same system

[Esi]

Hey guys,

I am new to the PV world and I was wondering: how do you decide whether to use two 15kW inverters or a 10kW and a 20kW inverter for a 30kW system, if the latter is allowed?

Thanks a lot

[sensij]

It might depend on how the strings going to each inverter are being designed.

[Esi]

Thanks for your reply sensij.
This is my case: I have three areas where I want to install modules because the space is limited.
Most of the modules are facing east, 15kWp and both MPPTs are hooked together.
Than there are modules facing directly south 4.08kWp hooked to one MPPT of a 10kWp inverter. Than, I have 4.76kWp of modules facing west, connected to the other MPPT of the same 10kWp invreter.
Is this an acceptable design? What would you recommend?

Thanks a lot

[sensij]

Depending on where you are located and the actual array orientation (tilt and azimuth), I would suggest looking at something like a 12 KW inverter for the 15 kW of panels (15 / 12 = 1.25 DC-AC ratio), and a 7 kW inverter for the (4.76 + 4.08 = 8.84) kW of panels. 8.84 / 7 = 1.26, but having the arrays facing different directions will lower the actual kWp. With that ratio, there will probably be a small amount of clipping during part of the year, but the value of the energy lost is likely to be substantially less than the cost to increase the size of the inverter.

Also keep in mind that the size of the inverter directly affects the size of the wiring and over current protection (circuit breaker), and potentially the size of the service panel to which the system is connected. This is a massive system, well outside my knowledge, and interconnect requirements may be different than for the typical <10 kW residential systems discussed most often here.

Edit: On second thought, an even better configuration might be putting half of the east array (7.5 kW) with the 4.76 kW west array, and the other half of the east array on a second inverter with the south array. That would be 12.26 kW and 11.58 kW, which could be handled by a 10 kW and a 9 kW inverter, or maybe even a pair of 9 kW inverters. SMA (TL series) and Fronius (Primo series) are both represented in this range. Having a different face on each MPPT input will lower the kWp required of the inverter by broadening the daily production curve.

[Esi]

I'm in the Mediterranean, Albania to be exact.
Both the tilt at the azimuth aren't ideal, except for the 4.08kW array, but I'm using CIGS modules, which should should work better than Si, under my circumstances. Yes, this is a commercial system.

Thank you for you suggestion. I don't have the actual practical aspect of sizing, noone does around here, hence my participation in this forum.
I'm using a software to simulate the results. As per your recommendations, I simulated a 12.5 and 8.2kW inverter instead of the 15 and 10 and got better yield, at least in the virtual world

[Esi]

Regarding your second though, thanks sensij. It does work better to 'split' the east modules into upper east hooked to the west modules and the bottom half of east modules hooked to south modules.
There is no 9kW inverters of Fronius for the 50Hz world, so I had to use two 10kW inverters.
But still, you saved me 5kW worth of inverters
Thanks