PV System Limits

mantissa · Feb 16, 2011

Hi,

I am not an electrician, but am about to take the plunge on installing a PV system (or rather having one installed, I should say. By the way, any installers on the Hampshire/West Sussex borders are welcome to get in touch!)

My investigations have yielded some interesting (almost) conflicting information. I am keen to get the most out of the FIT in order to get past the payback point as soon as possible. For this reason, I would like to fit the maximum 4kWp of panels. However, there seems to be problem with fitting an inverter which chucks out >16A. I have an email from SSE (Scottish and Southern), my DNO and they said that until then end of March, they'll be waving through 4kW inverter output installs on G.83/1, but after that G.59/1 will apply.

I was wondering just how onerous a G.59 installation is for something that is greater than 3.68kW but less than 4kW. Does anyone have any experience of such a system. Of course, I'm talking domestic here...I have read in these forums the requirements for 99kWh, but I can't see that 3.88kW (4kW and a 97% efficient inverter) will cause eyebrows to be raised!

The alternative I'm considering is using a 4kWp array feeding an inverter which *can* be fitted under G.83, but from my looking at what's around, I don't see a 3.68kW inverter, so are they programmable to limit output current if required - as would be in this case?

Finally, my roof space is a little tight to get the 4kWp panels on anyway (so all the above may be moot!) as I only have 26.52m^2 (10.2m X 2.6m). How close to the roof edge is considered appropriate - are there any regs that apply here or is it just a rule of thumb? With Sanyo 235s I can keep it to 9.48m X 2.3m which is still tight, but I wondered if it was too tight?

Thanks in advance.

John

mmccx · Feb 16, 2011

you wont get 4 kWp on your roof, the slope height isn't enough to get 2 rows which you would need, also you dont have a long enough roof to install enough in 1 row of any module i can think of. You would more likely be able to get around 2.0kWp in portrait on your roof. I would be very wary of any company who says you could get 4kWp of modules on your roof. Please remember you need a 300mm border around array leaving you 9.6m x 2m of usable sapce

in regards to g59, It would not be economical to go g59, the cost to connect to the grid would far outweigh the benefits of having an 4000kW inverter, if you could could get 4kWp on your roof i would suggest a 3.6kW inverter as we generally undersize the inverters in this country.

edit: i just had a think you could get maybe a little more if you went landscape but i still dont think 4kWp would be possible on you roof dimensions. But if you go landscape it will cost extra as a sub frame would be required unless your rafters are exactly the same spacing center to center all the way along. The reason for the sub frame is because for the majority of modules we are not allowed to clamp on the short side of the modules so we need to orientate the mounting system vertically to all allow us to clamp on the long side

JBF · Feb 16, 2011

As the previous reply covers there are two problems here the first over the output. You can put 4kwp on your roof (if it fits) under g83/1 because it is the output of the inverter that is important not the panels (although they need to be matched) So as long as it is a g83/1 compliant inverter you are fine. Going towards g59 is not cost effective unless going much bigger due to greater costs and lower FIT's.

Second problem though is size of roof can't see you getting close to 4KWP with an on roof system may get a bit closer with an in roof system but again costs are going to go up so not ideal.

Hope that helps.

I am in south hampshire if you need any more info.

John

mantissa · Feb 16, 2011

Actually, I made a mistake with my dimensions: 10.2 is absolute roof length, 3.2 is roof depth and I had already 'derated' that to 2.6 to take into account the fact that I can't use the whole roof depth....So it's really 10.2 X 3.2!

Sounds like I have totally overcooked roof usage - although I have seen images of a roof with panels almost to the edge - not a pukka installation then perhaps?

mmccx · Feb 16, 2011

you still pushing it, you narrowly only be able to fit 2 rows of 9 Sharp 185 which is 3.33, you could put this on a SB3000 i believe or if not a SB3000hf, but still 4kWp is pushing it to say the least

Markc · Feb 16, 2011

you could consider waiting till the Sanyo 250wt panel has its MCS certificate in March 2011 (Hopefully). I believe you might get 4Kw with the space allowing. Output to grid will still need to be 3.68kW. I would consider this the maximum you are going to achieve.

mmccx · Feb 16, 2011

Markc said:
you could consider waiting till the Sanyo 250wt panel has its MCS certificate in March 2011 (Hopefully). I believe you might get 4Kw with the space allowing. Output to grid will still need to be 3.68kW. I would consider this the maximum you are going to achieve.

That module will be a minimum of 1.4m high tho surely? and even tho its a slimmer module you would still only manage 10 maybe 11 in a row and no longer be able to have 2 rows due to its height. or am i missing something?

mantissa · Feb 16, 2011

I think this is where my naivety is showing through

. I spent some time today looking at various panels and had 'decided' that with a row of 11 panels in portrait, followed by a second row of 6 in landscape, I could fit in 17 Sanyo 235 panels (1.58 X .798 m) in a total area of 9.48m X 2.378m. Of course, I have made absolutely no allowance for any panel gap due in part, I imagine. to mounting hardware and had assumed that it was possible to mix and match portrait and landscape panels in an installation. I'm good at the numbers and the electrics, but not the mechanics! Also, with an odd number of panels, it's right that I would need an inverter with two MPPT regulators to deal with different string sizes, isn't it?

mantissa · Feb 16, 2011

Is it possible to program an inverter for a maximum output, since I have yet to see an inverter with its output pegged at 3.68kW...?

mantissa · Feb 16, 2011

mmccx said:
you wont get 4 kWp on your roof, the slope height isn't enough to get 2 rows which you would need, also you dont have a long enough roof to install enough in 1 row of any module i can think of. You would more likely be able to get around 2.0kWp in portrait on your roof. I would be very wary of any company who says you could get 4kWp of modules on your roof. Please remember you need a 300mm border around array leaving you 9.6m x 2m of usable sapce

Where does that figure of 300mm come from? Is it just a common sense figure to allow access to the array once installed without walking on the panels or is it a regulatory requirement?

Mark.W · Feb 16, 2011

As MarkC said you could possibly get 4kW on your roof. If you wait until the 250w panel gets its MCS Cert you could get 10/11 panels in portrait and a further 6 in Landscape above the portrait panels. This is all subject to a survey!

Hope this helps

Sorry you beat me to it!

Mark.W · Feb 16, 2011

It's not 300mm as stated it is actually 200mm! Its so that wind loading etc can be taken into account.

Mark

mmccx · Feb 16, 2011

that 300mm figure is generally accepted, its to do with disturbing the ridge eve and edge tiles, i.e the ridge and edge tiles are concreted in, and the eve tiles are doubled up! isnt incredibly difficult and dodgy to move these tiles, they form an integral part of the weatherproofing and you dont want to disturb them.

In regards to your portrait landscape system, it would look very out of place, IMO crap and be a pain in the neck for an installer, if i were you i would keep it all portrait or all landscape, Portrait will be over all cheaper. If you do go with portrait landcsape remember you will have to live with how it looks....