Solar Panel and GTI Questions

[007 RED]

I’m seeking some information/advice from our ‘resident’ experts please.

 

Background:  Have been monitoring this forum since @Crossy opened it with his “How about a solar carport on a budget” some 2 years ago.  It has been really interesting and educational particularly during these ‘strange times’ were currently living in.  Thanks, @Crossy and all the other contributors.

 

I have been toying for quite some time with the idea of installing a small grid tied system on my carport.  We (my good lady and I… plus 3 cats) don’t use a great deal of electricity.  Looking at our electricity bill printouts for the past 12 months, we use on average approximately 9 units/day.  OK, I know electricity is cheap, so why bother?  That’s a whole different argument for another day.

 

I am considering DIY project, something in the order of a 1KW system to produce about 3 units/day (a reduction of approx.. 1/3rd current usage) so as not to arouse suspicions in the local PEA office.

 

I’m looking at a similar system to that constructed by @BritManToo namely: a Suoer 1000W GTI plus 2 or 3 solar panels.

 

Admission:  Electrical expertise = NIL, but more than capable DIY and following good practice/rules etc. So please treat gently????

 

My questions:

 

Q # 1.  What is the difference (in lay person terms please) between Poly Full, Mono Full and Mono Half solar panels (apart from the cost), and which is the best?

 

Q # 2.  If I use a GTI rated at 1000W and use 3 x 340W normal poly panels (potential of 1020W), I’m aware that the panels could (under perfect conditions) produce 20W more than the GTI is capable of handling and ‘clipping’ will occur.  What affect will this have on the GTI?

 

Q # 3.  If I use a GTI rated at 1000W and use 2 x 460W mono half cell panels (potential of 920W), what affect will this reduced input have on the GTI?

 

[BritManToo]

16 minutes ago, 007 RED said:

Q # 2.  If I use a GTI rated at 1000W and use 3 x 340W normal poly panels (potential of 1020W), I’m aware that the panels could (under perfect conditions) produce 20W more than the GTI is capable of handling and ‘clipping’ will occur.  What affect will this have on the GTI?

It'll never happen.

My 3x 340w have never produced more than 650W even in the strongest sun.

Today in bright mid-day(ish) sun I'm getting 500W, thinking of adding another 1 or 2 panels to the string.

 

Safe to assume (under perfect conditions) your panels will produce AT LEAST 30% less than the advertised Watts.

Edited August 23, 2021 by BritManToo

[Crossy]

What type of meter do you have, whizzy disc or electronic?

 

If it's the disc type it will happily go backwards if your production exceeds current useage, BUT it is imperative that your meter reader doesn't see it going backwards. PEA do seem to be clamping down more and more on "unofficial" net-metering.

 

Electronic meters don't go backwards, they may even bill you for the energy you export!

 

So it might be an idea to invest in GTIs that can do "no export" and then arrange to enable this facility on "meter reader" days.

 

Something like this https://www.aliexpress.com/item/4001054628033.html?spm=a2g0s.8937460.0.0.5c562e0e9gsTMg

 

[Crossy]

For your panels, mono are more efficient (and more expensive), but poly apparently perform better in diffuse light (cloudy days) than mono do.

 

Half-cell panels are effectively two smaller panels in parallel. They are supposed to be more reliable and perform better than full-cell when one half of the panel is shaded.

[007 RED]

53 minutes ago, Crossy said:

What type of meter do you have, whizzy disc or electronic?

 

If it's the disc type it will happily go backwards if your production exceeds current useage, BUT it is imperative that your meter reader doesn't see it going backwards. PEA do seem to be clamping down more and more on "unofficial" net-metering.

 

Electronic meters don't go backwards, they may even bill you for the energy you export!

 

So it might be an idea to invest in GTIs that can do "no export" and then arrange to enable this facility on "meter reader" days.

 

Something like this https://www.aliexpress.com/item/4001054628033.html?spm=a2g0s.8937460.0.0.5c562e0e9gsTMg

 

Thanks for your comments and suggested GTI. 

 

We have a 'whizzy' disc meter and I am mindful from your previous postings of the potential problems that can arise if the meter reader spots it going backwards.

 

I note from our past bills that the meter reader normally comes between 9 and 11am, on the 15th or 16th of every month, so like you, I could make sure that Mrs MoneyBaht has the washing machine and the oven at full blast when he comes (maybe triggered by facial recognition on our external CCTV looking down the road). 

 

Joking apart, the problem I would have is connecting the inhibitor supplied with the GTI that you suggested to the feed wires (from the pole in the road outside the house) to the house.  The GTI will be mounted under the roof of the carport and the distance from there to the feed wires is approximately 20 meters as the crow flies.  I think that the simplest solution initially will be to put a alarm reminder in our smart phone calendars and disconnect the AC output until he has gone.

 

Edited August 23, 2021 by 007 RED

[Crossy]

Although I've sorted an automagic system to stop the export our meter reader invariably comes at 6AM on the 19th, until the days he doesn't!!

 

So I'll happily throw away some energy by not exporting all day on the earliest through to the latest days he's ever been.

 

20m between the CT and the controller is nothing, our CTs are 30m from the inverter, limit according to the inverter manual is 200m. Also note that that little inverter talks to the limiter using RS-485 which should be good for 500m or so ???? 

 

 

 

[007 RED]

1 hour ago, BritManToo said:

It'll never happen.

My 3x 340w have never produced more than 650W even in the strongest sun.

Today in bright mid-day(ish) sun I'm getting 500W, thinking of adding another 1 or 2 panels to the string.

 

Safe to assume (under perfect conditions) your panels will produce AT LEAST 30% less than the advertised Watts.

Many thanks for your assurance. 

 

I recall that I read in one of the 'Idiot's Guides to Solar Panel Systems' that power output of the solar panel given by the manufacturer is for the ideal situation and not the real world with clouds, dust, heat etc affecting the efficiency of the panel.  As you said, the real world factors will reduce the power output considerably.

 

So with your 3 x 340W panels your GTI copes OK and doesn't get overheated?

[Crossy]

Over-panelling a bit isn't usually a problem, the inverter will simply not use any excess power should you actualy make any. We've got 6.5kW of panels on the 6kW inverter.

 

The ONE thing you must never do, on pain of the exit of the Magic Smoke, is exceed the maximum open-circuit voltage of the solar input. The panels will go to their O/C voltage very quickly as soon as it gets light even if no useful power is being generated.

[BritManToo]

1 minute ago, 007 RED said:

Many thanks for your assurance. 

 

I recall that I read in one of the 'Idiot's Guides to Solar Panel Systems' that power output of the solar panel given by the manufacturer is for the ideal situation and not the real world with clouds, dust, heat etc affecting the efficiency of the panel.  As you said, the real world factors will reduce the power output considerably.

 

So with your 3 x 340W panels your GTI copes OK and doesn't get overheated?

Fan comes on maybe 5 minutes in every hour 9am to 3pm.

[007 RED]

10 minutes ago, Crossy said:

Although I've sorted an automagic system to stop the export our meter reader invariably comes at 6AM on the 19th, until the days he doesn't!!

 

So I'll happily throw away some energy by not exporting all day on the earliest through to the latest days he's ever been.

 

20m between the CT and the controller is nothing, our CTs are 30m from the inverter, limit according to the inverter manual is 200m. Also note that that little inverter talks to the limiter using RS-485 which should be good for 500m or so ???? 

 

 

 

Again, many thanks for that information.  You absolutely correct about the meter reader potentially coming on a different day than expected.  What's it called?  Murphy' Law!

 

You now have me really 'fired up' with the information regarding the distance between the GTI and the feed wires.  Definitely worth considering.

[BritManToo]

Just now, Crossy said:

The ONE thing you must never do, on pain of the exit of the Magic Smoke, is exceed the maximum open-circuit voltage of the solar input. The panels will go to their O/C voltage very quickly as soon as it gets light even if no useful power is being generated.

Yep, idiot builder connected 2 panels in series on his Suoer GTI ......... instant BANG.

PS. My meter reader either doesn't look, or doesn't care.

Edited August 23, 2021 by BritManToo

[Crossy]

Just for interest, our 6,500W of panels is generating 5,184W at the inverter output right now ???? 

 

EDIT Just peaked at 5,545W ???? 

[007 RED]

1 hour ago, Crossy said:

Just for interest, our 6,500W of panels is generating 5,184W at the inverter output right now ???? 

 

EDIT Just peaked at 5,545W ???? 

OK, the initial project started as a Solar Carport on a budget, but now its become a Solar carport on a massive overdraft ????

[Muhendis]

A word or two about that sticky label on the back of solar panels. The information on that label tells us what the panel can do in terms of Voc (open circuit voltage), Vmp (voltage at maximum power) and Imp (current at maximum power). These characteristics have very little to do with the real world. They are the measurements made under standard test conditions or STC. All panels are tested when they are produced. Testing is done with everything at 25ºC. The light source is a brief flash of sunlight at 1kw/m² and the results used to select the panels into different power groups. So you might for example, have some panels which measure 300watts and some which measurer 320 watts from the same production line. 

Following on from this comes real world and what a difference that can make. Solar panels have a linear voltage relationship with temperature. As the temperature goes up, the voltage goes down. This is at a rate between 0.3 to 0.5% per ºC and is referenced to 25ºC also this is not the ambient conditions but is the actual junction temperature of the silicone. In Thailand this could be as high as 60ºC. So the temperature difference could be 35ºC which means the panel Vmp would be between 10 and 17.5% lower. Where does this heat come from? Well the sun is the major source of course but there is also a bit of self heating going on. Each cell of the panel produces somewhere between 0.5 and 0.6 volts (nominally 0.58v) at 25°C so if you are taking 5amps from the panel there is a small heating effect of nominally 2.9watts per cell. Not much is it, but there are 72 cells in some panels so the self heating power for the whole panel in this case will be 208Watts. 

How does this information help us? The specification for the charge controller might restrict the maximum power to 5kw but the solar panel labels tell us that the maximum panel array output is 5.3kw. Corrected for the real world temperature this output is really less than 4.8 kw so no problem.

Incidentally there is an American standard for panel characterisation called PTC which measures the panel output at 45ºC.

[007 RED]

Many thanks for all of your advice/information regarding solar panels and GTI’s.  Certainly, gave me ‘food for thought’ last night, plus the additional contribution this morning from @Muhendis.

 

Although I intend to stick with my original spec, that is to say generate about only enough power to reduce our monthly bill by 1/3rd (about 3 units/day), @Crossy’ suggested alternative GTI with a limiter made me consider the possibility of a more sophisticated GTI e.g. Sofar.

 

Looking at their product lists, the smallest Sofar GTI available is 1.6W (yes, I know its 3 times more expensive than the Suoer 1000W) but it does come with lots of 'bells and whistles like limiter and wifi monitoring etc.

 

The question is, if I install say 4 x 340W panels to the Sofar 1.6W, and let’s say a 25% efficiency reduction, that combination might just produce 1000W which will still meet my original spec.

 

So what affect will this have on the Sofar GTI if the input power is considerably less than the potential output of the unit.  Will this be detrimental to the GTI, if so how?

 

My thought being that such a GTI could allow me to increase the number of panels and hence increase the power output at some later date if I decide it worthwhile/prudent to do so.

 

I appreciate that the increased cost of a 1.6W Sofar GTI over the cheaper makes will mean that the ‘payback’ time will increase considerably, but funds are not a problem. My main objective is a long term saving.

 

Once again, thanks for treating me gently ????

[Crossy]

So long as your string voltage at max power is somewhere within the MPPT range it will all work ok. The Sofar units do seem to be pretty tolerant. With 4 x 340W panels you are well within the happy zone of a 1.6kW inverter.

 

My main reason for going with Sofar was that they are fanless and IP65, it's located in a potentially damp area with plentiful wildlife.

 

[sezze]

Not to forget that Sofar units are good priced vs the cheaper but less efficient Souer units ( about 2500baht, 1000watt ) . I just checked 1.6kW Sofar for 9700 baht , JFY also for less then 10k available (1500 watt) , INVT is just above 10000 baht .