Solar PV: SE/SW split or all on SW-facing roof?

MEVW62

I'm planning a 4kWp solar PV system and trying to decide whether to put everything on the main SW-facing roof or to put some panels on the smaller and sometimes shaded SE-facing roof in order to get some generation for more hours in the day.

There are some shading issues with the SW roof so I'm looking at SolarEdge anyway, which I understand gives the flexibility to arrange the panels pretty much however I like. In spring and autumn the SE roof is shaded during the early part of the morning by the roof of the house next door; in winter the sun doesn't even rise until it's further round than that and in summer the sun is high enough to shine on my SE roof from around 5.30 - 6 am.

[I'd post a screengrab but MSE won't let me.]

The SE roof is slightly lower than the main SW one. As far as I can work out, the winter sun would still come over the top of the SW roof ridge onto the SE face, but I'd think it a good idea to put the panels as high up on there as possible. One installer (who hasn't visited the site) has suggested putting four there and 12 on the SW face, but I wonder if 2 and 14 would be better, and the 2 in 'landscape' orientation.

My reasons to consider splitting the array are:

1. I work from home so there's a fairly constant but not huge day time base load from the computer plus various tank-dwelling creatures that have heaters, lights, filters, etc., as well as the usual fridge and freezer, so getting more/ earlier morning generation seems sensible

2. The SE face is actually at about 35 degrees from S, the SW is at 55 so the orientation is marginally better

Does it makes sense to put only 2 panels on the SE roof? It's a trade off between all-day generation and maximum afternoon output (mostly in winter, in summer the SE-facing panels would be in sun till well into the afternoon). It seems to me that even a couple of hundred watts during the morning would be useful.

Or is it better just to keep things simple, and put it all on the SW roof?

Martyn1981

MEVW62 wrote: »

It seems to me that even a couple of hundred watts during the morning would be useful.

Or is it better just to keep things simple, and put it all on the SW roof?

I think you've explained it quite well, and it does, partly come down to the above statements. For FiT income SE & SW should be the same, so maxing out the better roof makes sense. But for leccy savings, having a lower (less export) but wider generation from a mix of rooves works best.

Firstly, if there were no other issues, then SE and SW would make an ideal setup. So, let's not worry too much about SE generation in the winter, it's less, and poorer weather, so shading will have less effect. But the amount of spring and autumn shading is important. If it's only early morning then my gut says that's not too bad at all as long as you are going down the route of a SolarEdge system anyway.

SolarEdge will mean you don't have to worry too much about keeping all panels out of the shade at the same time. That might mean it's better to place as many (as is sensible) on the SE roof, up to 50% ..... after all, no point putting more on the SE than the SW since it's harder to avoid evening demand, such as cooking tea.

Remember you aren't limited to 4kWp, you could put 5kWp, say 14 panels SW plus 6 panels SE and run them through a 3.68kW capped inverter. Your roof pitches will work in your favour, the SE having a summer boost, and the SW a winter boost, that'll help spread generation and limit any capping.

There's also the option of using slightly more efficient panels such as 285Wp (rather than expensive high efficiency panels). Each 7 285's will replace 8 250's, and cost roughly the same per Wp.

I think it all boils down to just how bad the spring/autumn shading is, and how much of the SE roof is covered at any one time. Could you put the screengrab link on, but add some spaces / make some changes, explain what you did, then we can still see it.

Mart.

MEVW62

Thanks Martyn

I hadn't thought of 'over-speccing' the generation capacity to allow for optimum summer/winter generation. Could possibly get 6 panels on the SE roof, definitely 14 on the SW.

The screengrab is at [usual hypertext prefix] bit dot ly forward slash 1Qmu7Jo

It's from SunCalc, showing midwinter sunrise (yellow) and -set (red).

Martyn1981

MEVW62 wrote: »

Thanks Martyn

I hadn't thought of 'over-speccing' the generation capacity to allow for optimum summer/winter generation. Could possibly get 6 panels on the SE roof, definitely 14 on the SW.

The screengrab is at [usual hypertext prefix] bit dot ly forward slash 1Qmu7Jo

It's from SunCalc, showing midwinter sunrise (yellow) and -set (red).

Here's your pic.

Forgot to say, if you install more than 4kWp then you get a 10% lower FiT rate. But it might work out, see recent post:

Martyn1981 wrote: »

T and I both did later extensions. That way the earlier system locks in its FiT rate, and only the extension gets the lower rate. But the 4-10kWp rate difference is now very small, only 10% (1.3p). I'd suspect the cost of going bigger in one go would balance out, you just need to get a price that is proportionately 10% less, eg £5k for 4kWp, or £5.6k for 5kWp.

There's a section on extensions, and info on going bigger in section 10 of the FAQs.

I take it the SE roof is the smaller roof to the top right of the yellow line? I can see why you mention shading. You'll know best what it's like in Spring/Autumn as we are nearly into Summer it's a bit too late to go and look now, but if the shading isn't too much it makes sense to split generation.

However, I fully appreciate that the SW roof is big and easy, so nobody would blame you for going with a simple single roof install ........ but I do like big systems.

Get a range of quotes and see how the numbers stack up.

Mart.

MEVW62

Went for 14 x 250Wp on SW and 2 x 250 Wp on the SE roof for 4kWp total, Phono Solar with SolarEdge plus iBoost. Installed last Thursday by TP Solar who were attentive and helpful during the planning phase and did a good tidy job on-site. I would recommend them.
Haven't had any full-on sunny days since (typically, the day of installation was the hottest/sunniest in the last couple of weeks) but managed nearly 8kWh on Saturday (damp, overcast nearly all day), a bit over 13kWh yesterday (very heavy overcast and rain all morning, brightened mid/late afternoon) and best so far, 19.3 kWh on Friday (dry, white clouds and frequent sunny spells, the system went on generating until 9.20pm). Currently at 9.8kWh for today (now 16:15, been solidly overcast nearly all day but brightened in the last hour).
Very pleased so far and no shortage of hot water yet, just bracing myself for the fact that it won't be like this in the winter...