East and west facing roof

[stubo300]

we have a roof which is facing east and west, is it best to fit 10 panels on each side, if so what type of inverter would i need for this

 

[Markc]

I would suggest a twin MPP tracker inverter to minimise yield. Failing that possibly two inverters one for each side.

 

[Ramjam]

I guess you meant maximise?

Can you not fit everything on the west facing roof? West is better than east because it gets the best of the afternoon sun which is invariably better than the morning sun. If you can fit everything on the west and use one inverter then it's a better system.

 

[claypole]

I've run this through pvgis at several locations and east consistantly forecasts slightly higher yield than west. It is very slight, like about 0.5%but certainly not less than. I also think one potential reason for the difference is that on the whole the ambient temperature increases through the day and therefore on average, cells will be running slightly cooler (and therefore with slightly more power) for the morning period than the afternoon period. I suspect in most situations there will be some other criteria which makes more difference than this.

 

[Ramjam]

two inverters doesn't half bump up the cost! Twice the scaffold etc! all for 0.5%?

 

[claypole]

No argument there, it was just the "west is better than east" comment that I've struggled to prove correct, albeit my initial gut instinct was the same as yours. I'd have to keep (or find) proper records to be sure but when I thought about it some more it's also my perception that during the summer months when I'm working at say 6am there are more clear morning skies than there are in the evenings at home when the clouds have built up more - as I say, only a percieved recollection that would need verifying. I don't believe my previous post advocated install on both faces although clearly circumstances (available space mainly) might dictate that would indeed be the best option. And the extra cost for something like theSB 4000TL really isn't that great.

 

[Ramjam]

I'm with you completely. The problem with the likes of the 4000TL is that it wants big strings which we care rarely get at less than 4kWp. I'm doing an install next week with a 4000TL and 21 sharp 185s it wants one string of 14 and one of 7 modules. I'm still mystified as to how we are supposed to understand how many modules to put on a string to get the highest yield because is one string of 14 and one of 7 better than one of 11 and one of 10, surely any improvement by taking one string to 14 is offset by reducing the other to 7. Changing it to 11 and 10 doesn't seem to show any differences in sunny design.

I'd never done that comparison in PVGIS before but you're right, I got 0.27% more with east facing than west. Given that info I'd be inclined to recommended the most aesthetic side of the property in future?

 

[edexlab]

Its to do with having the maximum on one string to input A while keeping the other above the minimum allowable for input B
as you know you need equal strings on one input (A) less than 15 A same alignment ,tilt etc

 

[claypole]

Yep, also slightly baffled with the odd numbered configs given by Sunny Design. However each time I checked it the predicted output of say 10 and 6 was identical if I force changed it to 8 and 8, presumably for the exact same reason you state above with 1 offsetting the other. I concluded the software was set by default to maximise 1 string voltage (as far as is possible without 2nd string falling too short) and that you would be fine with any configuration in between the 2 extremes.

 

[Markc]

I guess you meant maximise?

Can you not fit everything on the west facing roof? West is better than east because it gets the best of the afternoon sun which is invariably better than the morning sun. If you can fit everything on the west and use one inverter then it's a better system.

Yes I did. Fingers working quicker than the brain.

Sap ratings are the same East and West so using these the design should be the same.

 

[Heather]

I was told the maximum allowed by our wonderful penny-pinching Gov. is now 18 per dwelling. Is this correct or have I been misled?
Was told by installers that farmers abused the system by installling 40 or 50 PV panels at a time to get rich quick! Can't blame them either. At least it's a legit. way to earn money and helps reduce need for nuclear + other polluting methods of power generation.

 

[Worcester]

@Heather someone (a salesman? ) is spinning you a line (or 3)

The only limit to what you can install is your budget (and space whether that's roof or ground mounted) and whether you want the challenge of planning permission for a non-roof mounted install.

If you install significantly more than you'll reasonably consume then the income may be subject to tax (see the finance act 2008) if you have a large roof and consume lots of eclectricity then you could install 50 panels yourself.

The 'get rich' issue is to do with the very large ground array systems (1-5 MW) = thousands of panels that the governemnt hadn't planned for and is mainly financed by city investors, and the farmers getting a rent and revenue share.

 

[JD6400]

@Heather someone (a salesman? ) is spinning you a line (or 3)

The only limit to what you can install is your budget (and space whether that's roof or ground mounted) and whether you want the challenge of planning permission for a non-roof mounted install.

If you install significantly more than you'll reasonably consume then the income may be subject to tax (see the finance act 2008) if you have a large roof and consume lots of eclectricity then you could install 50 panels yourself.

The 'get rich' issue is to do with the very large ground array systems (1-5 MW) = thousands of panels that the governemnt hadn't planned for and is mainly financed by city investors, and the farmers getting a rent and revenue share.

Hi Worcester
As i understand it you can only receive upto the 10 kw bracket tax free if installed on buildings within the cartilage of of the domestic property , if you are likely to use said output and it must be fed into the same mpan as the house is supplied from .
I would love to hear if you know different , as i know from following past threads on this forum that you have a very good working knowledge of our industry .
Many thanks
Dave
DP Electrical Ltd

 

[Worcester]

@dpelectrical

Our coprorate accountants did tonnes of research on this and after discussion with HMRC it is Section 20 of the Finance Act 2007 that applies:

HM Revenue & Customs:Individuals - Tax Credits, Child Benefit, Personal Tax, National Insurance Contributions

7.37 Households can also play a part in generating low-carbon energy by installing smallscale renewable electricity generation on site. Feed-in tariffs, worth on average £900 in 2010 for households, will provide a financial incentive to install renewable technologies, allowing homes to generate around half of their own electricity. The Pre-Budget Report confirms that households who use renewable technology to generate electricity mainly for their own use will not be subject to income tax on feed-in tariffs. This will save households paying the basic rate of tax £180 in 2010. If necessary, legislation to ensure this treatment will be introduced with effect from 1 April 2010.

(My Emphasis)

Extract from the Finance Act 2007:

20 Income tax exemption for domestic microgeneration
(1) In ITTOIA 2005, after section 782 insert—
“782A Domestic microgeneration
(1) No liability to income tax arises in respect of income arising to an individual from the sale of electricity generated by a microgeneration system if—
(a) the system is installed at or near domestic premises occupied by the individual, and
(b) the individual intends that the amount of electricity generated by it will not significantly exceed the amount of electricity consumed in those premises.
(2) In subsection (1)—
“domestic premises” means premises used wholly or mainly as a separate private dwelling, and
“microgeneration system” has the same meaning as in section 4 of
the Climate Change and Sustainable Energy Act 2006.”

As such theoretically even someone with a 3.68kW system may be liable to tax if they are a very small user of electricity.

However so long as you are a high consumer or aren't worried about paying tax on the profit, the most likely limitations are going to be roof space, along with the usual DNO permissions (if over 3.68kW), and if necessary the appropriate planning applications or exemptions, so the limitations are usually practical, not regulatory.

The interpretation of "at or near domestic premises" has some interesting results where there are joint business and domestic premises and the house roof isn't suitable or large enough.