solar panel output/production questions

emjayjee · 6 June 2012 at 7:53PM

Hello

We had 3.67KW installation at the end of Feb this year. We are based in Birmingham, and have generated 768 KWh on our meter (all of March, April and May). Does that sound right? We are facing south with no shade. All panels are installed facing same direction.

We had 15 x 245 LDK panels installed with a Ever solar TL 4000 (http://www.ever-solar.com/product_details.php?id=4).

On the day, one panel was broken, so the electrician had to come twice. He explained that the array is linked as 5 panels and then 10 panels. There is 4 cables running into the inverter. One from either end of the 10 panels (2 rows of 5) and then one from either end of the 5 panels.
I am unsure whether this means there are 2 strings? and whether my inverter can handle this?

The predicted output over march/april/may come to approx 900 KWh

Can anyone answer the questions, or advise how I can investigate. I want to be sure before I call the company in case they fob me off with technical talk.

Thanks in advance

WestonDave · 6 June 2012 at 8:17PM

Does seem a bit light - PVGIS suggests 997 for Birmingham for a system your size, and whilst April and May were poor, I think people were still getting near expected levels. Might be worth having a look somewhere like PVoutput (website) to see what people with similar sized systems in your postcode area have recorded in the same period - if theirs is significantly more it might be worth asking why yours aren't performing.

Your inverter only has one tracker but whether the system is set up as two strings is really for the installers to say.

Martyn1981 · 6 June 2012 at 8:22PM

emjayjee, rather than answer you, can I throw out some extra questions to others on here, to see if my thinking is correct.

Checking that inverter from the link provided I see that it can take 2 strings, but only has 1 MPPT, in which case, I'd have thought that both strings have to match each other exactly (type and size of panels, orientation, pitch and crucially number of panels) otherwise the inverter will ignore / miss the lower voltage from the smaller string?

Of course there is no way to run an odd number of panels across 2 strings, and 1 MPPT?

Checking location against PVGIS, only a guesstimate, but for Mch to May) I got 945 (classic) and 1063 (climate). 768 is approx 2/3 of 1063, does that suggest the smaller string is being ignored or am I adding 2 + 2 and getting 22?

Something doesn't sound right here!

Mart.

Martyn1981 · 6 June 2012 at 8:35PM

Just trying to think round this, and was wondering if it could be set up as 3 strings of 5, since the inverter specs state a low min voltage of 125V, and those panels are 30.1V each, so 5 would be 150V. BUT .... can't see how 3 strings could be wired, and the ampage would be 24A exceeding the inverter max of 20A.

Edit: Other option of one string of 15 panels is also out (I think) since the inverter voltage limit will be exceeded if more than 13 panels fitted on a single string. But will depend on the exact panel model. There are 3 EU 245 panels, and they all have differing voltages - do you know the specific model number?

So back to first thoughts, that there are 2 unbalanced strings of 5 and 10 panels going into a single MPPT, so presumably the inverter is 'failing to see' most of the 5 panel string output.

I don't really know much about this stuff, but if my suspicions are right, then the installers must have known that this wouldn't work. In fact, simply proposing an odd number of panels was probably a no no before work even started.

Mart.

emjayjee · 6 June 2012 at 9:20PM

I have just had a call from the electrician who installed (freelance sent by the company) and he says it definitely was a 2 string installation and that he thinks the inverter should take it.

What you suggest Mart does sound true to me. It is approx 1/3 less (? the 5 panels on a separate string). I did ask him very specifically can the 2 strings be of different number of panels, and he said that running 10 panels on one string and 5 on another is not a problem.

I am very confused :S

Martyn1981 · 6 June 2012 at 9:32PM

Hello again.

Got to be careful, because reading the tech sheets and playing with calculators and common sense, is of course no substitute for actual experience.

So there's a very good chance I've got all of this wrong, but I just don't see how there set up can work, unless I'm missing something!

I'd suggest sending an e-mail to the 'contact us' page on the inverter link you posted, and asking about this specific situation. They should be able to advise immediately whether or not a 5 / 10 split is possible.

It may be that you'll need an inverter that can cope with a single 15 string, or more likely one with dual MPPT which can be wired with two strings of different numbers, probably 7 & 8 panels.

I think you've GOT to look further into this. Sorry.

Edit: Other option is to contact the solar company directly after all if they subcontracted the work, it's their problem not yours, since your contract was with them. Might be best to pre-arm yourself with some info from the inverter peeps first. A quick fix would be for the company to rewire the system as two strings of 7, if only to add 4 more panels back in. Oh, and one more question, what were you getting as daily highs in late May, 25+ or 17's?

Mart.

WestonDave · 6 June 2012 at 9:34PM

I'd agree with Martyn - it looks like the 5 string isn't feeding - only other way to tell is to see if you can get peak values from the inverter. At some stage over the last 3 months it should have put out nearly 3.6kw as a peak generation figure - if its never gone much above 2 then you would appear to be missing a third of your panels.

emjayjee · 6 June 2012 at 9:47PM

Martyn - bang on. Max we got in May was 17! I was surprised how low this acutally was given the amount of sunshine.

WestonDave - I need to get the data from the meter thing to confirm, but I am thinking I have only been running at 66%.

I guess at least I know now rather than a few years down the line.

However does it mean, I have to have an even number of panels? I mean, I paid for a 3.67 KW system, but have an odd number of panels. I dont have room for anymore than 15 on my main roof, unless another one is put around the corner which may complicate things. The other option would be to downgrade to 14, which would be silly too (except could be seen as 4 gained - but I have paid for it all!)

I actually consider myself clued on, but it does worry me that had I not checked and accepted this (like many people probably have) then this anomaly would have been missed for a long time

Martyn1981 · 6 June 2012 at 9:57PM

17 was a guess, but several people had been reporting 28's a week ago from their south facing 4kWp systems, so .....

(28 / 4) * 3.67 * 2/3 = 17

Losing a panel was just a suggested quick fix. The real solution will be to replace your inverter with one that has dual MPPT. There are loads eg SB3600TL-20.

By having dual MPPT the only requirement will be to have enough panels on one string to meet the minimum voltage (probably 5 or 6 panels). If you haven't got any shading issues, then logically you'd probably want a 7+8 configuration.

I'd strongly suggest not calling the spark, but calling the company instead. Don't forget to tell them that you expect to be compensated for the missing 1/3.

Mart.

emjayjee · 6 June 2012 at 10:42PM

I have emailed the inverter company so I have that official information, and will then call the company. I ensure I partly paid the company by credit card, so hopefully that will offer protection on the goods/services I bought should it become messy with the company.

Will keep you informed....

Martyn1981 · 7 June 2012 at 7:22AM

CC protection should help you, and I can't see anyway they can escape paying you for the lost 400(ish) units. (Plus the consultancy fees you've incurred :beer: )

I'm sure that they will swop out the inverter for a dual, it'll be cheaper than my silly quick fix, since they'd have to pay for scaffolding, rewire into 2 strings of 7, refund 1/15 of the cost (£500+?), and presumably leave the 15th panel in place for aesthetic reasons.

Post on here what they suggest as a solution, after you've confronted them. Might need to check the inverter they suggest, as a string of 5 panels may not have enough voltage for some inverters, so they'll probably have to pop back onto the roof regardless, to change the string sizes.

Best of luck, and happy hunting.

Mart.

Edit: To get a pretty accurate estimate of the generation you should have got, try this site:-

http://solargis.info/

select PVspot Beta, then follow the instructions on the LHS to register. This will then give you daily and monthly generation numbers for your system, which you should be able to use when 'negotiating', especially if you check a few and see if they come out at 67% (or to be more precise, the site will be 150%). M.

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solar panel output/production questions

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