PV split over two roof planes

[Triassic]

I’ve got a 4kw system on order. My original plan was to split them equally over two roofs, however, having tracked the sun over the last few weeks, and considering that the south facing roof is larger than the west facing roof, if was wondering if there would be any problem in putting more panels on the south facing and less on the west. 

[ProDave]

The panels are wired in strings, usually 2 strings for a 4KW system.  You really want to be putting all the panels in one string on the same orientation.  So that means a 50:50 split between the two roofs is best.

 

Unless you have micro inverters, one per panel.

[Triassic]

28 minutes ago, ProDave said:

Unless you have micro inverters, one per panel.

I’ll have a look at using micr inviters, the sun is on the south facing roof far longer than the west, so there should be some advantage to an uneven split of panels. 

 

Is there any particular make of micro inverters that are bst? 

[A_L]

44 minutes ago, Triassic said:

if was wondering if there would be any problem in putting more panels on the south facing and less on the west. 

 

Assuming your inverter has 2 MPPT trackers (usually is these days) and the open circuit voltage of both strings are within the MPPT voltage range of the inverter then no. There might be a slightly lower part load efficiency of the tracker with the smaller string but it should be greatly outweighed by the increased production.

[Ferdinand]

My system is:

 

27 panels facing East.

8 panels facing West. These 8 are split across 2 separate gables.

 

They are split across 2 inverters, each of which has 2 inputs (thought dual inputs were normal) with the strings defined so that I can move some of the East Panels (iirc up to 14 or 16) onto a putative South facing veranda roof.

Inverters are Solaredge, and I have a Solaredge thingamajig optimisation device on each panel.

 

So I think that it should be possible to do what you need, probably with a single inverter if chosen to be a dual input, and if you talk carefully to your supplier about your requirements, or gird your loins for a brain teaser if self-installing.

 

As people say above, there may be a few small compromises to live with.

 

F

Edited January 19, 2019 by Ferdinand

[Triassic]

1 hour ago, Ferdinand said:

gird your loins for a brain teaser if self-installing.

Luckily I have a tame electrician who can sort out the installation of the electrical stuff.

[Nickfromwales]

SolarEdge gets my vote. Good inverters with dry capacitors, and long warranty. SE inverters also feature 'export limitation' so anyone unsure of getting a yay from the DNO may wish to use them as a means to get a bigger system installed.

[Sue B]

I am thinking of flat roof (you have probably seen on other posts) with solar panels facing east, south and west.  I know there is software out these somewhere to help determine the configuration that suits your requirements best but not looked at it yet.

 

I know so little about solar panels so excuse my ignorance.  This is what I have gleaned so far - I think!

 

Monocrystalline panels are dearer than polycrystalline but are more effiecient

You are allowed 4 kw without permission

You need an inverter that can take 2 strings. 

Panels are connected by strings (not litereal strings - even I know that  )

Panels generally need to have the same orientation to go onto a string or the MPPT struggles a bit to do it's techie stuff

Therefore you could have an east and west split with an inverter with dual inputs

To get a three way split you would need a second inverter

 

So

Thinking about furture proofing and what we can afford inially and what we can add onto once battery storage becomes affordable:

 

We could do the east / west split now and have one inverter but get the cabling ready for a second inverter should we decide to do that later.

Get the roof ready for the south installation so that we won't be damaging the roof adding supports etc later on

Add a second inverter that is "off grid" and chuck the south facing panels into the battery  with no export to the grid

 

Is that a sensible option or is it a crazy idea and if so why?

 

Answers on a postcard please?

Sue

xx (because like Miss Piggy - I love you all) 

[Stones]

We had SE and SW orientated PV arrays at our last house, the SE array added about 12 months after the SW array (which took us from G83 to G59).  I had anticipate a much better spread of useable generation, but the reality was only really marginal in terms of extending the generation window.  Winter generation was still pitiful, and in summer, there was far too much to use (especially peak of the day).

 

 

[ProDave]

I have split mine E/W to try and make a longer (but lower) generation period throughout the day.

 

The fly in the ointment about adding a third string later to charge batteries, is to be completely "legal" that would have to be entirely off grid, i,e the loads the batteries supply are different circuits to the house.  As soon as you connect a battery storage system to the grid, that too has to be notified to the DNO. I have not read the rules about that yet.

[Stones]

I think that's what the G98 protocol covers (supersedes G83 as of April)

[Sue B]

Are the G38 and G98 regulations?  Not seen them in any reading so far.

 

but judging from your answers, the third string comnected to an off grid system sounds feasible - whether it is sensible or not is an entirely different matter.

[Jeremy Harris]

If connecting to the grid then the old regulations used to be G83/2-1 (for up to 16 A per phase) and G59/3-4 (for over 16 A per phase).  G83/2-1 has been superseded by G98/1 for up to 16 A per phase.

 

http://www.energynetworks.org/electricity/engineering/distributed-generation/engineering-recommendation-g83.html

 

and http://www.energynetworks.org/electricity/engineering/distributed-generation/engineering-recommendation-g59.html

 

These are "engineering recommendations", but in practice they have the power of being regulations because the DNO won't grant consent to attach any equipment to the grid, or allow equipment to be attached to the grid that doesn't require formal consent (in the case of G98/1), unless the equipment is tested and approved to these requirements.

 

The situation regarding engineering recommendations being given the power of regulation is much the same as existed for electric vehicle charge points prior to BS7671 18th Ed coming out, in that they also were covered by an engineering recommendation from IET that had to be complied with if the charge point was being funded by an OLEV grant (although practically all manufacturers just ignored that and fitted non-compliant charge points to save money...).

 

 

[Ed Davies]

Going with E/W first with the option to add S later seems back-to-front to me. For 1 kW of panels on Hengistbury Head angled at 35° elevation PVGIS (CMSAF) gives the following numbers of kWh for December and annually:

       Dec   Annual

South  34    1170

East   16     917

West   15     918

 

Since almost all the benefit of PV these days comes from self consumption and, as @Stones says, you have too much in the summer it seems to me that the main aim should be to maximise winter production even at the cost of reducing the overall production. If doing two sides first it seems to me that doing south and one of the others would be a better option. Which of the others is an interesting question depending somewhat on lifestyle, whether you have batteries immediately and so on but not terribly important, I don't think.

[Stones]

3 hours ago, JSHarris said:

If connecting to the grid then the old regulations used to be G83/2-1 (for up to 16 A per phase) and G59/3-4 (for over 16 A per phase).  G83/2-1 has been superseded by G98/1 for up to 16 A per phase.

 

http://www.energynetworks.org/electricity/engineering/distributed-generation/engineering-recommendation-g83.html

 

and http://www.energynetworks.org/electricity/engineering/distributed-generation/engineering-recommendation-g59.html

 

These are "engineering recommendations", but in practice they have the power of being regulations because the DNO won't grant consent to attach any equipment to the grid, or allow equipment to be attached to the grid that doesn't require formal consent (in the case of G98/1), unless the equipment is tested and approved to these requirements.

 

The situation regarding engineering recommendations being given the power of regulation is much the same as existed for electric vehicle charge points prior to BS7671 18th Ed coming out, in that they also were covered by an engineering recommendation from IET that had to be complied with if the charge point was being funded by an OLEV grant (although practically all manufacturers just ignored that and fitted non-compliant charge points to save money...).

 

 

 

Reading through the actual blurb, G98 doesn't take effect until April:

 

'Requirements for the connection of Fully Type Tested Micro-generators (up to and including 16 A per phase) in parallel with public Low Voltage Distribution Networks on or after 27 April 2019' 

[Stones]

Notwithstanding the above, @ProDave notified SSE about his system on a G98 form rather than G83 form.  Not quite sure why SSE advised him to do so given the taking effect date.  As I have a system to notify I'll be asking the question.

[Sue B]

11 minutes ago, Ed Davies said:

If doing two sides first it seems to me that doing south and one of the others would be a better option. Which of the others is an interesting question depending somewhat on lifestyle, whether you have batteries immediately and so on but not terribly important, I don't think.

 

That is exactly the information I was looking for - thanks Ed.

 

Looking at those numbers it makes sense to have either all south or south and west if I split them to get some time in the evening.  However, reading one of the other posts (I think it’s was in this thread) the gain from multi-direction was marginal.