What are you currently generating ?

[Eco Deal]

I like to point out STC (standard test conditions ) are as follows

25 oC
1000 wM2 (watts meter squared)
1.5 Air mass

but some sales guys will tell you a right load of cods wholup

''our panel only need 3 hours sunlight''
''our panels are the best ever ''
''our panels work off moon light''

 

[FB.]

Thank you, yes that makes sense.
Is there any truth in the fact(!) that the panels are under rated when new so that as they degrade over time they will still be theoretially putting out 90% of their maximum when they get to 20 years old? Or is that just wishful thinking?

I don't think panels are under-rated. They are correctly rated at the standard test conditions, and are often sorted into 0~+5W power bands in the factory (e.g. panels "flash-tested" at 240-244.9W go into the 240W pallets).

As for long-term power output: I think the panels will hold up fairly well.
I have been told by someone who studies such things that even older solar panels haven't lost as much performance as would be expected - even 20yr old early-1990's-manufactured solar panels still perform within about 10% of their original power. A few fail completely or almost completely, while many get externally tatty, but most hold up surprisingly well in terms of output despite often looking in poor condition.
They reckoned that the worst loss in power output is early in the panel life, as dodgy cells fail within a year or two, and as a balance of dirt builds on the glass. I think they said about a 2-3% power reduction each year for the first year or two is common, but after that the power reduction is virtually nil each year (less than 0.25% drop per year on average from years 3-20), except when the panels get really old and the seals get weathered, then the whole panel can malfunction due to large amounts of damp getting in.
The guy reckoned that about half of all solar panels ought to still be functioning at more than 80% of nominal output even when they are 35 years old.

 

[MicraShed]

except when the panels get really old and the seals get weathered, then the whole panel can malfunction due to large amounts of damp getting in.

Reassuring, though I popped out this morning and noticed dirt (bird muck I think ) on a couple of panels. So that left me praying for rain!

What happens when a panel fails though in say 5 or 10 years? Can it be replaced or would it be impossible if that panel was no longer available as it would not be possible to "mix and match".
Would I be looking at replacing the whole lot if a couple went duff.

Im being pessimistic here assuming that my installer wont be around in 10 years time and my guarantee wont be worth the paper its printed on (just like double glazing guarantees I have for the porch, it may be guaranteed for 15 years, but as the company went bust not long ago its now worthless).

One of the reasons I paid on the credit card, so that I can try and invoke section 75 if need be. It probably wont work, but I can hope!

 

[FB.]

Reassuring, though I popped out this morning and noticed dirt (bird muck I think ) on a couple of panels. So that left me praying for rain!

What happens when a panel fails though in say 5 or 10 years? Can it be replaced or would it be impossible if that panel was no longer available as it would not be possible to "mix and match".
Would I be looking at replacing the whole lot if a couple went duff.

Im being pessimistic here assuming that my installer wont be around in 10 years time and my guarantee wont be worth the paper its printed on (just like double glazing guarantees I have for the porch, it may be guaranteed for 15 years, but as the company went bust not long ago its now worthless).

One of the reasons I paid on the credit card, so that I can try and invoke section 75 if need be. It probably wont work, but I can hope!

I reckon in several years time there will be a big market in spares for the recently-installed solar arrays, as the systems age and faults develop. Only a tiny number of the panels/inverters installed in the last year need to fail in order to create quite a business opportunity.

If one panel failed, I suspect that you'd either have to:

1.
If the string voltage is high enough to disconnect that dud panel and still meet minimum inverter startup voltage, then carry on with one less panel but leave the dud one on the roof for cosmetic purposes.

2.
If you have two strings, cannibalise one string to make the other work.

3.
Replace all your panels, and sell them on the secondhand market which will probably be booming by that time as other people with the same dilemma desperately seek a certain make/model of panel are are prepared to pay £££ to keep their array looking neat.

 

[SolarCity]

To be honest, I don't think there will be much of a market for 2nd hand panels. They'll be as cheap as a tramps sock and they'll very rarely fail.

 

[Gavin A]

fwiw, IMO the main failure point in the panels electrically will be the bypass diodes, which I believe should usually fail closed, so that effectively it just removed the bypass functionality, though I suppose some will fail in such a way that they bypass that circuit.

either way, replacing a bypass diode ought to be pretty simple (once the panel's off), so I can see this being a worthwhile sideline / business in 10 years time once the warranties are up.

Incidentally, I reckon it's likely that this is an area the cheapest manufacturers will be scrimping on, but have no specific info on that, just my hunch. One day I might get bored enough to destruction test the bypass diodes from a high end and cheapo panel....

 

[FB.]

fwiw, IMO the main failure point in the panels electrically will be the bypass diodes,....

Yes, I think you're probably right.
I think bypass diodes will be/have been skimped on.

I would also expect that the more a bypass diode is used, the greater the likelihood of its premature failure. I therefore expect arrays with shading issues to be among the most likely to have problems.

Thinking about it a bit further, I wonder whether a panel with failed bypass would cause similar symptoms to the peak-power-drop-off which was extensively reported last year.
If a panel with failed bypass is in shade while the others in the string are in sun, it could cause a big drop in production as a result of the failed bypass diodes not allowing the power to flow past the shaded panel.

 

[SibertSolar]

Diode failure is extremely uncommon to be honest, unless the manufacturer used a very inferior product in the first place. In my experience with junction boxes, the diodes are typically very over-specified and, as a rule, are not particularly expensive anyway. New package design shottky diodes, typically with very low volt-drop, are hitting the market although tried and tested axial diodes from the likes of Panjit and ST are very mainstream. Remember also that the IEC tests require each individual diode to be rated for all potential failures associated with the whole module. Given that most crystalline modules have 3 or more sub-strings, each protected by a single bypass diode, you could deduce that the diodes are 3x over-specified.

Diodes can fail, especially where the thermal dynamics of the junction box don't allow enough heat dissipation and "thermal runaway" occurs but it is extremely rare. I have only ever come across this once under lab test conditions - and that was one diode of three that appeared to have an inherent issue that caused it to go short-circuit then open circuit. I've never yet come across a field-based diode failure but, of course, it could happen....