Help understanding inverters and which one is the right one for my roof?

[mattiker]

Hi,

I've been getting various quotes for a solar PV system - my roof is not a big flat south facing job, it has a few different sections. To fit solar to my roof you can get perhaps 6 panels on the main roof section - pretty much facing south with no shading issues, and you could get maybe 4 panels on another section of roof facing the same southerly direction but potentially facing some shading from the main roof section as it is set back a little.

My question is this...if a company quotes me for putting 6 panels on the main section and 4 panels on the other section of roof - what are the technical requirements for the inverter? From my limited knowledge I believe it would have to have 2 MPP's as the two strings of panels would be generating different voltages. What if the company quoted for equal sized strings but on different roof sections?

Please help me understand....I dont what to pay £1000s for an inefficient system, I dont think some of the installers I've dealt with so far really care about this point, either that or they are more used to simple flat roofs.

Cheers,

Matt

 

[Noshocks]

Four panels on a single string is asking for trouble, the combined voltage is too low, the only solution is two separate inverters, or modules with higher voltage like Sanyo etc, even then five is usually a minimum from experience. Just my opinion.

 

[mattiker]

Four panels on a single string is asking for trouble, the combined voltage is too low, the only solution is two separate inverters, or modules with higher voltage like Sanyo etc, even then five is usually a minimum from experience. Just my opinion.

Just for info, the panels quoted were Risen SYP250M - so would be a 1KW string.

 

[FB.]

Due to different orientation and light conditions, your two panel arrays would be best as separate strings (a string of six and a string of four) and it would be best to have an inverter which can handle more than one input, or an inverter for each string (which can get expensive!).

However, if minimum functional voltage for the inverter can't be achieved for the second string (*), or if the second string has troublesome shading, it may be best to just go with one group of six panels.

Also consider the (somewhat expensive) panels such as Sanyo which are about 10% smaller for the same output, so can be useful to squeeze an extra panel or two onto a roof with limited space.

(*)
My home system has a Power One Aurora PVI-3600 inverter with two independent strings of panels feeding it: 8x250W and 7x250W = 3750W (3.75kWp).
An extract from the manual of my Power One ("Uno" = one in Italian!):
"......One of the key benefits of the Uno family of single-phase inverters is the dual input section to process two strings with independent MPPT especially useful for rooftop installations with two different orientations (ie East and West)....."

 

[sharpener]

Have a look at at the Stecagrid range STECA StecaGrid 2000+. They claim to "specialise in difficult roofs", there are two independent trackers and the MPP voltage can go as low as 80V. My installer fitted their 4kW Master/Slave combo when unable to supply an SMA 400TL and I have to say I am pleasantly surprised, it seems to be well designed and well made and the internet interface is a useful add-on.

Another option would be to fit Enecsys micro-inverters to each panel then you can pretty much design the layout how you want.

 

[solarsavings]

Use an Aurora 3.0 Power-One String Tool

 

[dansk]

For multi aspect roofs and shading issues just use Solar edge, this are single MPP per panel systems making each panel individual of each other, so if ones in shade the others still work, attached is a photo of a job were the chimney is a problem and attached is a screen shot of the monitoring showing some panels affected by shade on the same job, next doors roof also catch's this array.