Quotes and install questions

pinnks · 4 August 2018 at 12:09PM

yep - does this get you closer to knowing where next? I would wait for comments to come in on the 3.68 v larger inverter

Zarch · 5 August 2018 at 6:51PM

Have been watching plenty of E and SW facing arrays today on PV Output (SAD!) :rotfl: Plus i've been watching my E facing roof and when the sun hits and shade comes in (sort of noon, 1pm onwards).

Which has helped me get a better view of how my array could look when installed and the subtleties of my potential 'best' generation graph.

Here's what i've come up with.

https://www.dropbox.com/s/y6a86dudiw7w7u2/Guestimate%20v3.png?dl=0

So I still can't justify the 320w panels over the 300w ones for £700 extra. All this graph shows that really all I will get from 320w panels is more when I'm unlikely to use it, ie when i'm already generation lots! So I think i'm still leaning towards 17 x 300w.

But what this has showed me is that even on the very best days, like today and other top days that i've found on PV Output for arrays pointing the same directions is that I could probably get away with a SolarEdge 3.68 inverter.

This could be £100 cheaper, I need to ask the installer. Yes, there would be short periods of the very best days (like today) where i'd be a few hundred Watts down on generation, but I don't think i'd ever get that £100 back.

Say worst case i'd miss out on 330w for 3 hours each day through the summer. That's 1kW a day for each day missed. So 7.5p (for each kW - edit, its 6.55p!!) i'd miss out on generation and export FIT per day. So i'd need 1,333 days or 44 months to recoup that £100. If my maths is right? Does that make sense? Its only the very tip of that graph above the flat line i'd be missing out on.

Plus, and this is question I don't know the answer to yet.

Is driving a 3.68kWh inverter close to or at maximum demand (ie 3.68) more efficient that driving a 5.00kWh inverter at much lower demands (ie 3.68)?

I've read this was the case with older inverters, but not sure with these new ones.

The question is the same through the winter when i'm maybe topping out at say 1000w generation. Will the 3.68kWh inverter handle that 27% usage better than the 5.00kWh one at 20%. Or does it matter? The SolarEdge blurb says >99% efficiency and makes no mention of any recommended minimums etc. Anyone know if this is an issue?

Final question regarding using a 3.68kWh inverter instead of a 5.00kWh one. Am I right in thinking that even though i'd be installing a 5.2kWh panel system, I could swerve the DNO application and any associated costs that come with hooking a larger inverter up because the 3.68kWh inverter limits the amount that goes back to the grid.

I think I was told or read these charges can now be £300? Did I hear that right? This charge could be what makes a 3.68kWh inverter a no brainer in my config.

Thanks as always. :beer:

pinnks · 5 August 2018 at 8:45PM

I think the maths is on the right lines but is 7.5p right (I thought it was about 3.9p for generation and 50% of 5p for export - but have not checked). That would be about 6.5p?

Anyway whatever the pence, the 44 months would be spread over the best days of the best months, so perhaps 3/4 of those days or 75% of 180 days a year. That is 135 days. So your 1333 days would take about 10 years to recoup the cost. If my maths is right.

On the DNO issue, I think you dodge the bullet as it were if the inverter is 3.6 as that is the max that could be exported notwithstanding the size of the panels. But others will confirm.

Personally, I think I would opt for the 3.6 inverter - save the £100, save the DNO costs which can be a couple of hundred or more and either save that money or put it towards a diverter or whatever. That is, unless someone could advise me why that would not be the better route...

Martyn1981 · 6 August 2018 at 6:44AM

Zarch wrote: »

I think I was told or read these charges can now be £300? Did I hear that right? This charge could be what makes a 3.68kWh inverter a no brainer in my config.

I don't know the exact cost, but that's correct, and your questions about efficiency are certainly true in the past, that a smaller inverter worked harder is more efficient than a large inverter run gently.

It's not a policy suitable for high sun areas, like Spain, but in the UK, sizing an inverter for 'messy' generation and poor weather is more sensible, especially when every Wh is important first thing in the morning, last thing in the evening, and all through the winter.

hawkwinddotcom · 6 August 2018 at 11:26AM

ofgem figures
FIT 3.93kWh
EXPORT 5.24 kWh

https://www.ofgem.gov.uk/environmental-programmes/fit/fit-tariff-rates

Zarch · 6 August 2018 at 11:45AM

hawkwinddotcom wrote: »

ofgem figures
FIT 3.93kWh
EXPORT 5.24 kWh

https://www.ofgem.gov.uk/environmental-programmes/fit/fit-tariff-rates

Yep, my mistake not 7.5p, but 6.555p combined per kWh generated and exported. :j

1 x 0.0393 + 0.5 x 0.0524 = 0.06555 = 6.5p (or 6.6p if we are rounding up)

pinnks · 6 August 2018 at 7:49PM

And we all wondered at school what we would do with all those maths lessons:rotfl::rotfl::rotfl:

Zarch · 8 August 2018 at 10:03AM

Can't believe i've not asked this before, but what generation can you expect from panels in full shade?

These East-ish facing 300w panels i'm hoping to put in will be in full shade from 2pm onwards. So it could still be a bright summers day everywhere else.

What performance do you get in full shade during the day? Good, bad of indifferent?

Dave_Fowler · 8 August 2018 at 11:11AM

Zarch wrote: »

Can't believe i've not asked this before, but what generation can you expect from panels in full shade?

These East-ish facing 300w panels i'm hoping to put in will be in full shade from 2pm onwards. So it could still be a bright summers day everywhere else.

What performance do you get in full shade during the day? Good, bad of indifferent?

It depends on the amount of cloud in the sky. If the sky is clear blue and the panels are shaded, there will be very low power production. However on a half and half cloudy day there will be a significant amount of light reflected from the clouds and that will produce a surprising amount of power.

Dave F

Coastalwatch · 8 August 2018 at 11:19AM

Zarch wrote: »

Can't believe i've not asked this before, but what generation can you expect from panels in full shade?

These East-ish facing 300w panels i'm hoping to put in will be in full shade from 2pm onwards. So it could still be a bright summers day everywhere else.

What performance do you get in full shade during the day? Good, bad of indifferent?

I've two panels vertically mounted on the WSW gable end so in full shade until around 1pm. The remainder are SSE at 23 degree pitch in full sun all day. Generally the two end panels generate between 30 & 50% in comparison depending upon the day. As I'm typing this at 11.20 am they are at 20%, but it was a cloudy start to the day!

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