I had an interesting experience with a major supplier of solar systems in Johannesburg. 

When I tried to accept their Black Friday special offer on three extra panels for my existing system, their technical staff informed me that the panels will not work because the wattage differs from the existing. 

This flabbergasted me.

Existing is two strings of 3x365 W in series connected in parallel to provide potentially 2 190 W @ 144 Vdc and 15,2 amps.

The proposal is to add another string of 3x605 W panels in series connected to the existing string at the PV combiner box to provide potentially 4 105 W at 144 V and 28,5 amps.

All panels are 48 V and the system capacity is 6 000 W at 150 V and 100 amps with a 5 kW Growatt inverter and 2 x 3,3 kW Growatt lithium batteries. 

Therefore 3 sets of 3 panels in parallel, two of which are all 365 W and one set of which all are 605 W. 

I am well aware of the pitfalls in trying to combine different voltage panels but fail to understand how different wattages can create a problem. 

 

 

Different sized panels can be connected, but you would need to calculate different configurations to get the most optimal output.
But for the most part mixing different sizes almost always results in losses.

Power loss: Mixed voltage yes, mixed wattage no. 

1 hour ago, Langjan said:

All panels are 48 V

if all the existing panels are 48V, thus 3 X 48V = 144V per string and the new higher power panels are also 48V each and thus will provide loads more current, but also at 144V for the 3 panel string, then all should be ok, as long as your Growatt can handle the current, this should be fine without too much trouble, you presumably would have to wire in the new panels with new cabling, and not parallel them up on the roof or wherever the panels are located, since presumably the existing cabling isn't sized to carry the 4-odd kW's worth of current, but if it is, then even this won't need worrying about...

Agreed thanks and yes, I initially installed an extra set of PV cables for exactly this purpose. 

Just be sure to check your open-circuit values from the panel datasheets to make sure totals do not exceed the mppt max ratings of the inverter.
Replace below with your exact open circuit values to get the max values, which should ideally be below the inverter mppt max values.

 

6 minutes ago, WannabeSolarSparky said:

Just be sure to check your open-circuit values from the panel datasheets to make sure totals do not exceed the mppt max ratings of the inverter.
Replace below with your exact open circuit values to get the max values, which should ideally be below the inverter mppt max values.

 

 Thanks. As mentioned in my original post, the proposal is 4 105 W at 144 V and 28,5 amps.

The inverter capacity is 6 000 W at 150 V and 100 amps