Help please - 2 parallel South South East facing roofs

[jdeep]

Semi ignorant member of the public here looking for advice from an expert not trying to sell to me. I am in NE Scotland.
Looking to purchase 16 X 250 W panels but too tight for space on largest SSE facing roof. I have 2 parallel SSE facing roofs about 8 feet apart. The smalIer roof will have shading issues from a chimney from late afternoon. I have had 5 companies out to quote and differing views of the best option. I would really appreciate some advice on the best option or any option that is a definite no no.

Option 1 - one continuous string over 2 roofs
Option 2 - 2 strings either 10 + 6 or 11 + 5 and one 2 string inverter
Option 3 - 2 strings and 2 inverters
Option 4 - an 11 + 5 split with the 5 going on a westerly facing roof to maximise my domestic usage of electricity in the evening.

Keen to purchase but need a confidence boost to know what I am buying and would be grateful for any help.

JIM

 

[dansk]

Hi - Two strings and Duel MPP inverter (opt 2) or micro inverter system like Solar Inverters, Power Optimizers and PV Monitoring | SolarEdge that would overcome any shading issues.

 

[SolarCity]

Can you offer any photos?

 

[SpitfireW'sale]

What a shame that the 5 companies that have visited could not convince you themselves!

My best guess with limited info would be that you will need 2 inverters. I may be corrected, but I don't know of any inverter that can effcicently handle 1 string of 5 and 1 string of 11 250W panels. So option 3 from your list.

 

[jdeep]

Hi Dansk and BiggsSolar - really grateful for your time, No pictures available and now probably too dark to see tonight.
The bigger roof is an extension roof at 90 degrees to back of the existing house, It joins just beyond a window about 5 feet from corner of house. from what I can work out space between last panel on main roof to first panel on smaller roof would be about 8 feet. Could cable run over roof or in loft space or not a good idea at all?

 

[dansk]

Well if i understand correctly they would be facing different orientations, so nope not a good idea, strings need to face the same way and pitch to be effective. Strings on different pitch and orientation need duel MPP or two inverters for example.

 

[jdeep]

Hi Dansk.
Sorry I must have given you the wrong impression with my description. The 2 roofs are parallel and one 40 degree and other 45 degree pitch. One roof is the side of the extention roof going in at 90 degrees to back of original property and the smaller roof is at the side of the original property.
Thanks

 

[dansk]

OK, well opt 2 still stands, use power one 3.6 inverter with 6 and 10 im sure that will be ok (ive not run this through pv sol, im just using an educated estimate)

 

[solarfred]

Not sure what panels but try a Steca 2000 + slave unit. Will give you x 4 tracker inputs and thus avoid the shading issue as much as possible for the best price. If you tell us the panel spec. can check the limits?

 

[jdeep]

Thanks solarfred and others
The panels suggested by the "best" sales person were Suntech STP250S.
Have looked at 2 "efficiency" tables found on internet and they seem to be ok at just over 15%

 

[SolarCity]

Don't get too confused by the efficiency. Efficiency simply means that a panel will produce more watts per square metre than another panel. Therefore one 250w panel @15% efficiency will simply be physically smaller than another 250w panel @14% efficiency. In theory, they should produce the same.

Of course, that's when you get in to temperature coefficients etc. and you really need to look at software to help you make a choice.

For what it's worth, Suntech panels are a superb option - both in terms of value for money and performance.

 

[jupestar]

I can't really picture it fully, but i think if i understand correctly you could use a SMA 4000TL. If you can get 6 panels on the smaller shaded roof, you can input this into one MPPT, and two strings of 5 panels on the other roof into the other MPPT. There is a new SMA 3600TL which might be worth a look (i don't know much about it- but sure it is very similar). Done a similar on a east/west roof, and it is producing very well.

Or you can try micro-inverters, i'm not a fan, but in this case they might be the right choice.

That would be where i start looking, would need to check voltages inverter spec's etc before confirming anything... shame your not comfortable with what people are quoting maybe worth getting some more people to look at it. Someone local. Put some photos up would be useful.

And as Biggs says - don;t read too much into efficiency's, its really a space issue. and some panel manufacturers slyly quote cell efficiency (which means nothing) instead of module efficiency.. It's all about build quality. how the cells are protected and how sterile the manufacturing process was. Suntechs have a good reputation.

 

[jdeep]

Hi Guys - really grateful for all this.
Sorry for the delay in replying but just had another wonderful experience - just been quoted over £22k for a 3.5 KWP system but with a guarantee if I took a "360 deal" they would give me my money back in 5 years. I asked him to leave and he eventually did. Not immediately though and not before a telephone call charade with his manager.

Think I am comfortable with the Suntechs and will restart conversation re inverter. They quoted for an Eltek Valere Theia He-t. It seems to have 3 string inputs. Any issues?

 

[solarfred]

Well it has got 3 inputs but only 1 MPPT tracker as far as I know and you need multiple trackers to get the best from your set up.

 

[jupestar]

I've heard this from a few people - did you let him stick around long enough to explain how this 'money back' deal is supposed to work?. If so would you mind explaining, as i'm curious.

Regarding the Eltek, I don't have first hand experience, i.e. I've never used on. But i do know the sales rep, and he is very knowledgeable about PV, and goes the extra mile to make sure deliveries and quality is 100%. So any problems it should be fairly easy to sort out.

 

[jdeep]

Dont believe any of this but as I understand his explanation - They use my job as a marketing opportunity for them and their much larger Parent Company. They spend nothing on general marketing but target around every successful job. This makes them more profit than 20% of my job over 5 years so they make a profit and I get my money back. However just in case it doesn't work in my case it is Insurnce backed guaranteed.
Been reading the other threads tonight and there are good guys and arses in every market. You guys have been excellent with me and I have no reason to mistrust most of the guys I have seen over the last few days. Good luck with this business long term

 

[screwdriver]

Go suntech 250 and power one pvi 3.6 inverter, dual tracker, can run unequal strings no probs and very good efficiency.

Oh and knock the chimney down!

 

[jdeep]

Cheers Screwdriver, Happy to take chimney down soon as its on its last legs

Don't know if this is the correct etiquette but

To Daytoa 600 - tried to reply to your personal message but your inbox is full

 

[jdeep]

Hi again - one last favour I promise. I have spoken to 2 suppliers and it looks like I now have 2 good options to choose from and any advice would be helpful.
1. a 4kWp 16 x 250 Suntech split over 2 roofs (11 + 5) with Aurora Power One 3.6 inverter as suggested above or
2. a 3.5kWp 14 x 250 Sanyo with the inverter of my choice to fit on the larger roof.

I believe the Sanyo system will be more expensive by around £900. Which would you choose please

 

[SolarCity]

That is a difficult question. The Suntech panels are pretty good but if you consider you will only being doing this once (hopefully), then it may be worth paying the extra and getting 14 panels on the one roof with the Sanyo system.

Both options appear pretty good though.

 

[jdeep]

Cheers Solar City. I am really tempted by the Sanyo. They seem to be the "best" panel and can all be placed on one roof at the back of the house.

Only one additional problem in my mind is that the company offering the Sanyo have only obtained their MCS certification in Nov last year. The started the company up in July 2011 and previously worked for other companies for years before. They are in REAL and MCS accredited and seem very genuine.
My head tells me to play safe with the Suntech unless the Sanyo is a much much better deal but am I worrying about nothing if they are MCS and in REAL.

 

[SolarCity]

The key is to talk to their past customers and go and take a look at their work.

I've seen work from MCS firms that have been registered for ages that is utterly shocking so it's not necessarily a good measure of quality.

 

[Earthstore]

On the main roof are they leaving a decent border around the edge, it should be 500mm, or are they squeezing as many panels on as possible?
Why not go for the Suntech and have two inverters, you may find you would get more out of your system that way.
Hope this helps.

 

[jdeep]

SloarCity - I now have 3 previous customers telephone numbers to chat with tomorrow
Earthstore - We estimate 300mm from Ridge to first and the same gap from bottom panel to guttering
Following earlier advice on here I looked at power one pvi 3.6 inverter, dual tracker to run 2 uneven strings (11 + 5) or (10 + 6) or to deal with shading issues.
I believe reducing the start up voltage on MPPT 2 helps.
Happy to consider 2 smaller inverters. Do their combined ratings need to be under 4kW. Will they still be guaranteed not to produce more than 16A

 

[SolarCity]

Something to consider when going for two inverters is that the efficiency of the smaller inverters will probably be lower than if you went for a single inverter. Transformerless inverters are a rare beast when you go below 2kWp.

I'm preparing to be proven wrong with this, but I don't consider a low start up voltage to be such a big issue. It certainly helps with designing a system but you really don't need much light to get a pretty high voltage so I don't see it increasing yield much, if at all.

 

[jdeep]

Hi SolarCity and others, First sunny weekend day for ages up here, Spent most of it on garage roof watching shadows and on SunCalc website. How sad am I.
I am almost certain to to go 4kW 16 X 250 Suntech (11 + 5 split). With power one pvi 3.6 inverter. I will have no shadow on 11 but some shadow from chimney on 5 from mid afternoon. Shadow should be reduced when sun higher in sky. Need to act quickly for March deadline but look to take down chimney during summer. Unless the installers tell me differently I will leave start up voltage at default. The company originally suggested 2982 kWh on a 3.5 kWp system. Hope for around 3200 for 4 kWp.
Thanks for all your help