East/west roofs -single or twin inverters?

JKenH

Hi

I am after a bit of technical advice, please. I have 12x 300w panels (3.6kwp) on the east side of my roof (105*east of south) and 14 x 300w panels (4.2kwp) on the west side (75*west of south). Each array is served by a Solis 3.6kw inverter -spec below:

Model Solis-1P3.6K-4G
Energy source PV
Input Side
Max. DC input power(kW) 4.2
Max.DC input voltage(V) 600
Start-up voltage(V) 120
MPPT voltage range(V) 100-500
Max.input current(A) 11+11
MPPT number/Max input strings number 2/2
Output side
Rated output power(KW) 3.6
Max. apparent output power(kVA) 4
Max. output power(kW) 4
Rated AC grid voltage(V) 230
Grid voltage range(V) 160-285
Operation phase Single
Rated grid output current(A) 15.7
Max. output current(A) 16
Power Factor (at rated output power) 0.8…1…0.8
THDi (at rated output power) <1.5%
DC injection cuttent(mA) <20
Grid frequency range(Hz) 47-52 or 57-62
Efficiency
Max.efficiency 97.8%
Euro efficiency 97.1%
MPPT efficiency >99.5%

At this time of year, particularly, both inverters are running well below their capacity. The maximum I see from the east roof is about 1.4kw and 2.2kw or so from the west roof. I didn’t start looking at my inverter outputs until late July but the best I saw then from the east roof was 2.1kw at 9am.The west was capped by the old inverter at 3.68kw so I don’t know what it might max out at but generally on sunny days it would be a bit over 3kw. Ignoring peaks on intermittently cloudy days the combined output would be about 5 kw max at about 2pm.

Because it is a 7.8kw installation with a combined inverter output of 7.2 kw it needed a G59 application but something went wrong and Northern Powergrid can’t trace the application or approval and to date my installer has failed to come up with it. I need to sort that out but it got me thinking what if we had to go to a 3.68kw limit?

Would that necessarily be a bad thing given that I am a late adopter and displacing grid consumption at 13.7p is more important than Fit/Export payments at a combined 6.6p? I have more solar than I need in summer but, like everyone else, not enough this time of year.

Is it possible to put the 12 panels on the east roof on one string and the 14 panels of the west roof on the other string of one of my 3.6kw inverters without damaging them? The max dc input voltage is 500v (26 panels would equate to 910v) so I suspect not. I know, however, there is a tendency to underspec inverters by 30% or more so maybe I am missing something here.

If I could use just one inverter would it improve the efficiency of the system at this time of year?

I realise that I would lose some peak solar in the summer but if the inverter would run at 3.6kw that would meet most of my needs but I would lose a few kw of FiT/export.

I would appreciate any technical advice anyone can give me. (I have mentioned the G59 aspect as context only so wasn’t really looking for comment on that unless anyone has experience of retrospective applications, if indeed that is what is necessary).

Thanks

Ken

pinnks

A very good question. Not sure I have an answer but assuming it is possible to have just one inverter and two strings, what is the real loss of production?

Peak production on the east roof will be at a time when production on the west roof is at best a couple of hundred watts as the panels generate very little without direct sunlight and that won't happen until the sun passes from the east side of south to the west of south. Equally, peak production on the west roof will be when there is very little happening on the east side.

Between about 11:00 and 13:00 the west roof will be ramping up production as the east roof will be ramping it down but this is when you are likely to get capped.

If you have graphical displays of your real-time production, choose a good summer's day (plenty of those this year - just choose the cleanest, cloud-free day) and combine the graphs. This will give an indication of what your real max output is at any time of the day. You might find that you virtually never actually exceed 3.68kW...

You could end up with a reserve inverter for when yours breaks, subject to the internal components aging through the passage of time even when the thing is disconnected from the system.

HTH

JKenH

pinnks wrote: »

If you have graphical displays of your real-time production, choose a good summer's day (plenty of those this year - just choose the cleanest, cloud-free day) and combine the graphs. This will give an indication of what your real max output is at any time of the day. You might find that you virtually never actually exceed 3.68kW...

HTH

Unfortunately I don’t have any graphical displays for individual inverters as the old inverters were difficult to interrogate. I only have graphs for the combined inverters from early September which was a poor month for me because of cloud but you are right there is very little sustained generation over 3.6 kW.

Zarch

JKenH wrote: »

Because it is a 7.8kw installation with a combined inverter output of 7.2 kw it needed a G59 application but something went wrong and Northern Powergrid can’t trace the application or approval and to date my installer has failed to come up with it. I need to sort that out but it got me thinking what if we had to go to a 3.68kw limit?

Just thinking having had my FIT contract documentation today for signing. That document had all the installation detail mentioned on it; declared capacity, install date, MCS certificate numbers etc. So maybe whoever deals with your FIT might be able to help with any missing detail that you could forward to Northern Powergrid?

Martyn1981

Some quick thoughts:

1. The specs you've posted say 2/2 for MPPT's and strings, so that means each MPPT can have two strings, so voltage should be fine as you are reducing it by up to 4, but you would also double the amperage so need to check that's ok.

2. You don't have to have a 3.68kW inverter, you have to have a 3.68kW capped inverter. Most inverters (famous last words) can have the max set by the installer, I think the SMA's can be set in increments of 1W. So you can install a larger inverter better matched to the KWp and particularly the peak kW's you get (around 5kW), but capped at 3,680W.

3. One inverter would be more efficient, but obviously the losses to capping would be significant, and the net impact will be a loss.

4. There are products around that can cap export to 3,680W (not generation), so household consumption can be taken into account. But you'll need to go Googling and see if they have reached a domestic level yet, and if any DNO's have approved them.

5. Bit like point 4 but the inverter is still capped at 3,680W but has a DC side battery, so the inverter 'diverts' excess up to the charge rate of the battery and till the battery is full, eg generating 5.5kW, with 1.5kW diverted to batts and the remaining 4kW to the inverter which caps at 3.68kW loosing 0.32kW. [Note numbers are pure speculation.]

6. Good luck with the DNO, I think approval is the best all round option.

michaels

I had contemplated doing this although I have solar edge invertor.

As you say issues are max voltage per string and for invertor as a whole.

There is a site that you can use to display your output that also shows curves for expected output which you could basically add your two proposed arrays and it would show you expected max for any date you choose - sorry can't remember what it is call - pvoutput or something?

theboylard

pvoutput.org - excellent site although took me a while and lots of reading early posts on here and there to figure out the settings - and I still can't figure out how to set export at 50% automatically, so I go in about twice a month and bang those figures in!!

To use the site is actually free, although you can enable extra functionality if you donate.

For JKenH - you'll need to find out if your inverters have an accessible API - this means your ouputs are loaded directly to pvoutput, so may only need a bit of manual tweaking afterwards?


There is even a MSE group, MSE Forumites but it's not that well populated, but it is a fascinating site and gives real world reports of actual generation across the country/world.

Zarch

theboylard wrote: »

pvoutput.org - excellent site although took me a while and lots of reading early posts on here and there to figure out the settings - and I still can't figure out how to set export at 50% automatically, so I go in about twice a month and bang those figures in!!

You're right, PVO doesn't allow you to put different export/generation/50% etc.

So the value to put in is (1 x your generation rate) + (0.5 x your export rate)

so for me: (1 x 0.393) + (0.5 x 0.0524) = 0.0655

So every KwH is worth 0.0655p, which is what I stick in the tariff section.

Hope this helps?

theboylard

Hi Zarch,


yes that's what I do. But I still add the 50% export figure (ie. the half of generation) as I don't measure export currently. That's the bit I'd like to automate!!

Zarch

theboylard wrote: »

Hi Zarch,

yes that's what I do. But I still add the 50% export figure (ie. the half of generation) as I don't measure export currently. That's the bit I'd like to automate!!

Right, get you, sorry.. You need something that can measures export and uploads to PVO.

I'm using an emonPi CT clamp based system.....
https://guide.openenergymonitor.org/applications/solar-pv/

I intend posting a summary thread soon about how it all hangs together.

theboylard

That would be useful, I've got one that is plugged in but has not been updated since i got it, right at the end of the oem kickstarter campaign for the emonpi!!!