Am I losing income on the sunny days?

[SnaxMuppet]

My 2.4kWp system has been up and running now for the best part of 3 months. The problem is that I am having concerns over the amount of generated power that I am losing because the inverter is maxxed out.

The modules are Phono Solar PS240M-20/U x 10 in a single string. The inverter is a Fronius IG20.

I did post here a few concerns I had before they did the install and there were some who felt that this inverter was too small for these modules. However, the installer was completely insistent that this was the right inverter for the job and so I pretty much had to let him install the IG20. He did say that if I had further concerns once it has been in for a while then to get back to him.

Well, the system seems to be working quite well. It generates some power even on the cloudiest, dullest of days but on the sunniest of days it often hits its 2kW output limit. The problem is this: at STC these modules produce 2.4kW but the inverter can only output 2kW.

I have got back to the installer explaining that I am losing income on these sunny days and he has assured me many times now that OK, I am losing a bit on income on the very sunniest of days but that will be made up by the extra gains this inverter will give on the less bright days. Overall, he says that this is the best inverter for this module arrangement (10 x 240kWp). He also says that yes, the modules can potentially generate 2.4kW in the right circumstances but once they get warm in summer, and what with system losses (his estimate 15%) then the maximum the array is likely going to produce ever is about 2kW and so even if it is over it certainly isn't by much.

Now I want to believe him and just forget about the solar PV system and get on with my life but when I see the inverter sitting maxxed out at 2kW I can't help but wonder how much income and power is being wasted.

I know that there are a lot of knowledgeable and experienced people on this forum and I am putting out a plea to you... for the sake of my sanity please help! I want the installer to show me somehow that I am really not losing that much. I imagine that I could be getting 200-400W extra with a different inverter and even with the British weather that would be significant.

Am I worrying over absolutely nothing?
Is this inverter losing me significant amounts of power and is it enough to consider changing the inverter to something else?
Is there a way to prove to me, or for me to prove to myself, that OK, it is maxxing out but the losses are not significant?

I am pestering my installer now and he is getting a bit fed up telling me that we have the right inverter. Actually, I don't blame him because I just don't believe him even though he has asked Fronius to size the system and they say IG20. He has put it through PVSOL and that says IG20. So what more can he to do to convince me? It still feels wrong that I should be losing potential income and potential power. It is getting to the point now that the installer and I are going to fall out in a big way if I continue to push him for more confirmation that this is the right inverter.

Perhaps I should see a shrink? Or perhaps... he is wrong and this inverter is losing me a significant amount of income...

Can anyone please help?

PS If anyone is willing to do a comparative design, showing what the best inverter for this system would be when compared with a Fronius IG20 then I would be hugely grateful.

[billi]

Hi

I cannot say  much about what you loose , but my 2.4 PV    is generating  quite often over or around 2000 watt

To me it sounds  to install (Fronius IG20) an inverter that generates nominal 1800 watt (2000 watt peak) 

Not only that you loose units  (we all suffer from that  , cause you  feeding the grid  ) , but is the inverter not  under more stress and can fail earlier   

The  next bigger (Fronius IG 30)  is just costing 50 GBP more   

Billi

[SnaxMuppet]

The installer has said that by using the IG30 we will lose out at the bottom end. So we gain at the top and lose at the bottom. Their argument is that we have much fewer sunny days than dull days so the IG20 is better. I look at the specs for the IG20 and IG30 and I see very little difference at the bottom end... same MPP range etc. To me it seems that the IG30 is likely to be very similar to the IG20 on dull days but it won't clip at the top on the sunny days.

Also, Fronius say that the IG20 is the better inverter... that is what I don't understand. Everything to me suggests that if we have to have a Fronius inverter then the IG30 would be the better option but as I am no expert in any of this it is difficult for me to argue against my installer and Fronius and that is the problem I have.

It seems that I am smack bang in between the IG20 and IG30 which makes me think that a different manufacturer might have something more suitable... Sunnyboy 2500HF perhaps? But I look at the specs and I see the the SB2500HF has a minimum MPP voltage of 175V... the IG20 is 150V so it will generate power with less light than the SB won't it? So it is a balance.

It needs a proper comparison I think to get to the bottom of it. My installer says he has not done such a comparison as they like to us Fronius whenever possible. Being so in the middle of the Fronius models I feel that perhaps we should look at other makes but they are saying it is not necessary.

[A.L.]

hello SnaxMuppet,

go here www.eci.ox.ac.uk/research/energy/downloads/pv-inthe-uk.pdf

On page 16 the report shows that most of our solar energy arrives at less than 800W/m², taking a simple view your system is only likely to 'max out' at about 830W/m² and you will only lose the excess over  830W/m² and given that panels will be warm at this irradiation level the loss will be even smaller. So given the increased part load efficiency of the inverter at 100-300W/m² I think you will still come out ahead.

There may be a small graphical glitch on page 16, there is when I view it.

[claypole]

Yep, another good article from sma here
http://download.sma.de/smaprosa/dateien/7418/TempDerating-UEN103910.pdf
When I was finalising my system design I was convinced my inverter must be at least same max output as nominal array peak but the software was always undersizing for max kwh/kwp. This article helped me to understand and visualise the way more frequent low irradiation gains far outweigh the few lost peaks. But...like you I can't find the low end differences between the IG 20 and 30. With sma I could clearly see from the efficiency curves how the smaller inverters would gain. I can't find those curves on the Fronius. If they are indeed identical then it does seem daft to lose the peaks for minimal extra cost of IG30 but I wouldn't think the loss would be very great for the reasons given by AL.

[profp]

hello SnaxMuppet,

go here www.eci.ox.ac.uk/research/energy/downloads/pv-inthe-uk.pdf

On page 16 the report shows that most of our solar energy arrives at less than 800W/m², taking a simple view your system is only likely to 'max out' at about 830W/m² and you will only lose the excess over  830W/m² and given that panels will be warm at this irradiation level the loss will be even smaller. So given the increased part load efficiency of the inverter at 100-300W/m² I think you will still come out ahead.

There may be a small graphical glitch on page 16, there is when I view it.

SnaxMuppet is in the South West. Last year I measured insolation for a period, and regularly recorded values over 830W/m2 (the insolation meter maxes out at 1.2kW/m2, and was off the chart around midday on occasion in June/July). I no longer have access to the meter, and our local micro-climate is a bit unusual (although we are not that far from Plymouth) -- but to cut a long story short, it might be worth the OP taking insolation measurements himself.

[Jober]

Don't forget there is the overheating issue if a non-transformerless inverter works at max for a long period in summer, particularly when it is in a loft. I know our loft gets unbearably hot in summer.

[Alan]

Quote “ Don't forget there is the overheating issue, if a non-transformerless inverter works at max for a long period in summer “.

Not correct.
An inverter with a transformer will have standing heat radiation losses that increase with the power generated.

A transformer less inverter will match the power generated ( If commissioned correctly ) at all times with less losses.

Regards

Alan

[Alan]

Following on from last post.

There will not be an over heating issue with a correctly designed inverter with a transformer. It just costs you money to run it.
Hopfully the solar panels mounted on the roof will reduce the actual heat in the loft space that would have radiated from the roof tiles exposed to the sun.
Air ducted from out side on a north facing elevation would still be a good for inverter cooling.

With an inverter with a transformer at maximum generated power typically you will be loosing about 3% of the power you are generating in localised heat inside the inverter. Due to copper resistance losses in the transformer.
At lower export levels. (  Cloudy / overcast day ) the losses relate to eddy current and hysteresis losses  which is the majority of the no load loss.

My inverter with out a transformer has an import / export meter that is fitted.

The inverter has exported about 4800 k.W.H. ( units of electricity )
The inverter  has imported 0.1 k.W.H during the same period of time.

Inverters with out a transformer are definitely the way to go.

Which is what Mr Jober was saying. The term ( non-transformerless inverter ) took a bit of decoding overnight to understand.

Regards

Alan

[Cozworth806]

Sorry to reply to an old thread but have come across this on a couple of quotes I received.

In one the 2.88kw Sanyo panels were matched with an Eversolar TL2500 which has a max input of 2.7kw, and the everplan software I found suggests that it means it is 10% over threshold at peak input and flagged up a warning about this, but the TL2500 result came out just ahead of the TL3200.

In another the installer looked at the figures and downsized the inverters quoted due to the effieciency/output.

I live in West Yorkshire so I guess that peak input for me will be probably less than someone in the South, PVGIS suggests max KWp/m2 is 813kw and therefore it may be a case of I will see those gains at the bottom end rather than at the higher scale.

Quick question is what does an inverter do when it gets a overloaded input? Will it cope with it or just drop it (thereby losing production?)

[Laurence]

Your standard 13 Amp socket is able to supply up to 3kW. When you plug in a 100W light bulb, how much does it draw? 100W. More power may be "available", but the load will only draw the current it can, based on its resistance and Ohms Law. The power is not wasted - it is simply not drawn from the supply.

As long as your PV array cannot put out a higher voltage than the inverter is designed for, you cannot damage the inverter with arrays that might be capable of delivering more power (amps). The inverter simply wont draw beyond its design load (unless its a cr@p inverter, bent on self destruction). Again, in strict energy terms, the power is not wasted - it was simply never called from the PV array. What is wasted, though, was at least some opportunity to generated the max possible power.

As to the original post - I think when you look at the typical power curves over several months, the odd bit of maxing out of the inverter is not a significant loss of kWhr. Unless, of course, your inverter is regularly spending several hours a day on the limit!