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Author Topic: How would different inverter(s) alter my FIT rate?  (Read 2549 times)
john999boy
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« on: July 21, 2016, 10:07:59 PM »

This is something that I'm mulling over but would appreciate some feedback to ease the process:-

My virtually due south system of 36 panels was specified with each of the three Kaco 3002 inverters being fed by twelve Sharp 235w panels. This was done because I couldn't be sure how far (or if) the shadow from a neighbouring property's gable end was cast upon my roof. It turns out that it only partially affects one or two panels at once along the lower string so the top twenty four are always at full output and the lower twelve are running at a slightly lower output during the winter months only.

So the question is whether to have a new inverter that may be more efficient for the top twenty four panels (and that would release two 'spares' for the bottom twelve) or get just one new inverter that could cope with the shading issue of the twelve as well as the other twenty four 'unshaded' ones?

Ideally I'd like to stick with Kaco (I'm also open to other suggestions) as I've also got online monitoring but which inverters in those 2 scenarios would be a good replacement and what would be a typical cost?

Is there an option to add some battery storage with the new inverters which I could utilise after the sun goes down?

Most importantly, will any of this affect the rate of FIT that I'm currently on (with due care being taken after reading this http://www.navitron.org.uk/forum/index.php?topic=27195) as there won't be any extra panels - just a (hopefully) more efficient way of making use of everything that's 'captured'?
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8.46kWp solar PV - 169 - 40 slope - 53N. Immersun fuelled DHW.
Iain
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« Reply #1 on: July 21, 2016, 11:30:26 PM »

Hi
Just recently replaced my inverter with a newer model. Does not affect the FIT as it is all based on the panels. My old inverter was a SB 1700 and has been replaced with a SB 2.5. I informed the FIT provider for their records. They didn't ask for the serial number.
I suppose the only problem would occur if your existing inverter was set to cap the output and the new one didn't.

Iain
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20 x 65mm Thermal and 180ltr unvented
Powervault 4Kw - G200 Lithium-Ion (LiFePO4)
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M
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« Reply #2 on: July 22, 2016, 08:30:02 AM »

Hiya John, like Iain said the FiT is kWp based so no problems. But a couple of thoughts on inverters:

1. I tried to find a 3MPPT inverter years ago, but failed (there are master and slave systems that add units, but I was looking for a single unit). If you can't find one then you need to make sure the power, voltage and MPPT voltage range of the 2MPPT's can cope with the 24 v's 12 panel set up. I'm sure you'll find one. [Just went nosing and the blueplanet 7.5TL3 seems perfect(?) and it's 2.3% more efficient.]

2. Looking at the specs for your inverters, I see that they have a rated output of 2,500W. So a thought springs to mind (as I had to submit inverter specs to my DNO), are you on 3 phase, or did you get DNO permission for your setup? If permission, was it at 7.5kW? If so, then that's the sensible limit to aim for, to avoid having to ask their permission again. Mind you, for your system that sounds great anyway.

Lastly, you mentioned battery storage. What FiT rate are you on? A DC system linked to your inverter will be more efficient, but all losses will be reflected in your TGM readings, whereas a standalone AC system would have higher losses, but not affect your FiT income. So, for instance, let's say you use 1,000kWh of battery leccy in a year, then a DC system will probably have processed 1,100kWh, losing you 100kWh at the TGM. So factor that into your calculations and decision. For the later/lower FiT rates it gets far less important.

Mart.
« Last Edit: July 22, 2016, 08:33:14 AM by M » Logged

Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
john999boy
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« Reply #3 on: July 22, 2016, 05:35:01 PM »

Thanks both.

The system is single phase with DNO permission for the full 8.46kW capability so the only thing constraining the generation is the sun on the panels!  Wink At least I thought it was until Mart mentioned about the rated output of each inverter being 2.5kW when I thought they were 3kW.  Embarrassed That does explain why the system only peaks at 7.5kW on good generating days! This was something I'd previously noticed but didn't totally appreciate why it happened.



As I'm on the initial FIT rates, the best battery system in my case would be the AC one so that's what I'd prefer. I'll check out the inverter that was mentioned (does it allow battery connectivity I wonder?) and has anyone got a ball park figure for battery storage and how much (capacity) would be required for a 'normal' house? Talking of which, Tesla have literally just sent me this email after I'd enquired when they were first released:-
Quote
Thank you for your interest in reserving a Powerwall. We are pleased to announce that the Powerwall is now available in the United Kingdom.
We need a little more information to proceed with your reservation and ensure that your installation meets the specific needs of your home. We would therefore really appreciate it if you could fill in a short survey
but would I be correct in thinking that theirs is a DC setup?

I think that lots more research will be required.......
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brackwell
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« Reply #4 on: July 22, 2016, 06:12:12 PM »

John,

I am not quite sure what you are trying to achieve regarding replacing the inverters, but i would comment that at 53 N with 10% losses you are not going to exceed 7.5kw. Indeed the graph you show does not display any capping as this would appear as a horizontal line at 7.5kw if the inverters had been capped. If the inverters have not been capped then they would be able to take more than the rated 2.5kw.   It has always been well recognised that the inverter needs to be of the order of 10% below peak output because after all peak output is only for a few minutes a yr and for all the other times a smaller inverter is more efficient at partial load.

Ken
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M
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« Reply #5 on: July 22, 2016, 06:19:36 PM »

Hiya John, obviously I don't know anything (so to speak) so can't recommend any inverter, just looked at Kaco models as you said you fancied one. Good news about the DNO limit, but looking at the next model up (same tech page), it's rated at 8.6kW so you 'should' ask the DNO. However, my gut says the 7.5kW would be better. Having read comments on here, capping is less than expected, and you get a boost at low levels by having a smaller inverter.

This thread, and my July13 post is interesting, shocking even, showing a 3.2kWcap on a 4kWp system, with now obvious loss. Scaling that up to your system is 3.2x8.46/4 = 6.77kW, so even less than 7.5kW.

So possibly ideal.

No idea on prices, but I wonder how much you'd get on fleabay for the 3 inverters, they might cover a large chunk of the cost of the new toy?

[Edit: Ken beat me too it. Also his eyesight is better than mine, that is a lovely power curve, no real evidence of capping. At 89% your inverter(s) are well sized. M.]

Mart.
« Last Edit: July 22, 2016, 06:23:50 PM by M » Logged

Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
john999boy
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« Reply #6 on: July 22, 2016, 06:35:07 PM »

Just a quick reply ATM but my thoughts were that either 2 or 1 inverter would be 'less lossy' than having 3? Obviously if it could be coupled with additional 'night storage' then that too would be beneficial. Incidentally I've not yet completed the Tesla Survey but a quick google makes me wonder if I would need a house extension to fit one of their jobbies!

BTW, I'm not saying that the inverters are capped as I know they aren't - just maxing out at 1kW below the capacity of the panels. TBC....
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brackwell
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« Reply #7 on: July 22, 2016, 08:04:51 PM »

My understanding is that the shaded panels will bring down the whole if you have just one inverter. If the inverter was 2 tracks with 24 panels on one track and 12 on the other, i believe this would not be as ideal as what you have.

Ken
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M
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« Reply #8 on: July 23, 2016, 08:39:56 AM »

BTW, I'm not saying that the inverters are capped as I know they aren't - just maxing out at 1kW below the capacity of the panels. TBC....

Hiya John, I think we are all on the same track, I've just been a little lazy saying 'capped', when I suppose in this instance we should be saying 'maxed'. But what Ken spotted still stands true, it looks like your graph peaks with generation, not maxes out.

I'm not sure what the 'proper' terminology is, and whether capped and maxed are interchangeable? Perhaps maxed is better as it could mean hitting the inverters real or artificial limit, either way it's 'maxed'.

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
M
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« Reply #9 on: July 23, 2016, 08:48:52 AM »

My understanding is that the shaded panels will bring down the whole if you have just one inverter. If the inverter was 2 tracks with 24 panels on one track and 12 on the other, i believe this would not be as ideal as what you have.

Ken

Hiya Ken, I think that's a good question for the experts. I seem to remember someone (possibly Gavin) saying that a single MPPT inverter is slightly more efficient than a dual MPPT inverter, when there's no need to split the strings for shading.

But in this case, I'd have thought the higher efficiency of the larger inverter, plus the low generation benefit of having all the generation on one inverter (to start it up in the morning, and keep it going in the evening) would work well.

That said, if they don't need changing, I'd stick with em till one fails, then change them. That's exactly what I'm doing with my ESE system which has two inverters, but could be replaced by a single inverter with dual MPPT's. However, generation is in line with PVGIS, so I'll wait for a failure.

@John, looking at the generation figures, your system is 'interesting' sometimes near the top, occasionally near the bottom, and generally midfield. But on average looks to be doing fine in comparison.

Mart.
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Just call me Mart.     Cardiff: 5.58kWp PV - (3.58kWp SE3500 + 2kWp SE2200 WNW)
john999boy
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« Reply #10 on: August 02, 2016, 08:11:08 PM »

A little bit of an update as it seems that my inverter warranty has got a lot longer to go than I first thought.

That's good as one of them malfunctioned towards the end of June and I didn't immediately realise because the beginning of July was pants in the generation stakes. It turns out that there was an 'update' available for the other two and they were also done at the same time as the replacement was fitted - so now I've hopefully got a fully working system again. Full credit to the Kaco team for sorting out a problem that wasn't entirely down to them to solve.  genuflect

At least that means I've got a bit lot longer to evaluate the battery and updated inverter options that are out there. 
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8.46kWp solar PV - 169 - 40 slope - 53N. Immersun fuelled DHW.
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