MSE guide: plug-in solar panels

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82122186#Comment_82122186

Firstly, and dare I say obviously, I was explaining (in line with all of my previouis comments, and others) that a larger PV system will generate more power at any given time than a smaller one. Whilst equally obviously understanding that in the case of these regs, there would be a limit of 800W AC side.

Secondly you stated - "If the panels produce more, where does the extra go?". If an even larger system was generating too much for the inverter and being clipped, then the panels, would not be producing more, they would be clipped. So based on your wording, if the panels can and are producing more power (edit … then it has not reached 800W and is not beinmg clipped and ….), then it would go to the house circuit as I described.

Thirdly, originally I did include a caveat to make it clear, but felt that was unnecessary, as we are all now well aware of the 800W limit. So in a good faith discussion it was not necessary to further clarify the issue, so removed it, just to see if you would respond, as you did.

TBH, I was hoping you wouldn't go down that route. 😔

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82122174#Comment_82122174

My understanding, and it may be incorrect, is that the inverter will deliberately move away from the MPP (maximum power point) in order to reduce generation, even down to ~zero if necessary.

QrizB

Picking up on this:

However, if the inverter takes a 200% overload (ie. double), where does the energy go - does the inverter just disspipate it as heat?

The power never gets to the inverter. It stays in the panel, which isn't cooled (by power leaving the panel and entering the electrical system) as much as it might otherwise be.

A panel just lying in the sun, not connected to anything, would get hot. When you hook it up to an inverter and draw power from it, removing that power means the panel doesn't get as hot as it otherwise would have been. It never becomes cooler than ambient (that would violate the second law of thermodynamics) but it's not as hot as an identical, unconnected panel would be.

A "standard sun" (as used when rating panels) is 1000 watts per square metre. A typical modern panel might be 23% efficient, so a square metre of panel under a standard sun would generate 230 watts leaving 770 watts to heat up the panel (or get reflected, or pass straight through it, or whatever).

If you overpanel your inverter, the electronics in the inverter will manage the impedance the panel sees to prevent the panel output from exceeding the inverter capacity. The excess power never leaves the panel and just goes to make the panel slightly hotter than it would other be (bu still cooler than if you weren't drawing any power at all).

My understanding, and it may be incorrect, is that the inverter will deliberately move away from the MPP (maximum power point) in order to reduce generation, even down to ~zero if necessary.

I believe you're correct.

I had quite a long and unproductive discussion with another forumite a few years ago who was arguing that using renewable energy to heat homes and dry clothes was increasing the rate of global warming, because the heat eventually ended up in the environment. I don't think I ever managed to convince them that the heat in your home was heat that had been extracted from hot solar panels or warm moving air elsewhere, and so the overall effect was zero.

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82122331#Comment_82122331

Not just you, I had the same discussion early this year, on another forum, where I explained that RE powered heat pumps only move heat, they don't add any. The OP pointed me to the additional energy used to power the HP, but before I could reply, others had pointed to the caveat (I included on that occassion) that RE removes energy from the environment, which is then released when the leccy is used.

I also followed a thread a few years ago, experimenting with a couple of solar panels and a thermal monitor. I know it has to be true, but was still fascinated to see a panel plugged in, being cooler than one not plugged in.

It's a bit like the theory (that Mythbusters confirmed), that if you fire a bullet horizontal to the ground, it will touch the ground at the same time as a bullet simply dropped from the same height at the same time. Fun, but near magic to my ickle brain.

prowla

https://forums.moneysavingexpert.com/discussion/comment/82121647#Comment_82121647

(I missed your post earlier.)

It's not the advertising it's the actual offering of the item for sale.

Regarding the "not seem to accept the perfectly valid point" regarding overpanelling there; I believe I made that precise valid point earlier in this thread:

https://forums.moneysavingexpert.com/discussion/comment/82118638#Comment_82118638

Can you point out where I've said it couldn't?

But the fact is that a plug-in system cannot produce more than 800W at any point and of course that means it can never produce any extra over 800kWh.

The posts "just trying to simplify what non-technical readers struggle to understand" are actually creating confusion, for example when I asked "If the panels produce more, where does the extra go?", one asserted that it went "Into the home circuit, reducing import, or exported to the local grid", which is incorrect.

As you say "It is the inverter load that does not allow that potential higher power to be produced. No power is lost.", there is no extra power.

And that is precisely why a UK-compliant plugin system is never more than an 800W system, regardless of the spec of the panels.

prowla

https://forums.moneysavingexpert.com/discussion/comment/82122276#Comment_82122276

You are still persisting in the assertion that ("obviously") "a larger PV system will generate more power at any given time than a smaller one".

As I and others have explained, that is simply incorrect.

If the system is running at its maximum output of 800W then the larger system will not produce more than that 800W, regardless of how much bigger the panels are.

From a reductio-ad-absurdum perspective, if it had 2kW panels on the best sunshine day of the year, then they would be producing an excess of over 1kW.

I asked where that excess would go and you said into the house or onto the grid, but that is impossible, as the output of the system is limited to 800W.

Therefore that supposed extra 1kW must be lost somewhere.

If it's lost at the inverter then it would have to be dissipated as heat, but that would be equivalent to a 1kW electric fire and make the poor little inverter glow red-hot and fry its components.

Therefore, and as explained by others, the supposed "extra" power is not output by the panels.

And thus the panels spec is not the rating of the plugin system, ie. the plugin system in question is an 800Wp one not 890.

(As has been mentioned, overpanelling a plugin system can help it capture more sunlight over a larger area and so help keep the system yield up in less than ideal conditions, but it will still be an 800W system and never more.)

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82122567#Comment_82122567

You are still persisting in the assertion that ("obviously") "a larger PV system will generate more power at any given time than a smaller one".

Yes the larger system will produce more power at any given time than a smaller one, up to the 800W AC limit. At this point, you simply appear to be going in circles and refusing to accept this fact.

Regarding the extra power, as already explained, if it is producing extra power, then it clearly is not at max, and being clipped, so that extra power will go to the house, as explained.

I really do not understand what your issue with the 890Wp system is.

From a reductio-ad-absurdum perspective, if it had 2kW panels on the best sunshine day of the year, then they would be producing an excess of over 1kW.

No they would not be producing an excess of over 1kW. I think you mean that they would be capable of producing more power if there was a larger, unrestricted inverter, but there isn't. So they would be capped at 800W AC of power.

Therefore, and as explained by others, the supposed "extra" power is not output by the panels.

And thus the panels spec is not the rating of the plugin system, ie. the plugin system in question is an 800Wp one not 890.

No, the system is an 890Wp PV system as I and everyone else keeps explaining. PV systems are described by the panel capacity, not the inverter. And as I have stated over and over, in the case of an 890Wp system connected to an 800W capped inverter, there will be no/negligible clipping, and the annual generation will be roughly 890:800 higher. Do you disagree regarding annual generation, I don't believe you have responded to this point made by myself (and others)?

(As has been mentioned, overpanelling a plugin system can help it capture more sunlight over a larger area and so help keep the system yield up in less than ideal conditions, but it will still be an 800W system and never more.)

You have said similar repeatedly. I believe you are wrong, but at this point it is clear that your mind is made up. However, I feel that it was worth explaining the difference so that at least others with no PV system experience, looking at PnP solar will understand. Hopefully they will get, where possible, a larger PnP system and benefit from the larger system size. Just to repeat, I would suggest something around 1,200Wp would be a good match.

Sorry I could not help you.

Netexporter

Yes the larger system will produce more power at any given time

Point of pedantry. A larger array will generally produce more energy (kWh) with an inverter of a given power (kW) rating.

Martyn1981

https://forums.moneysavingexpert.com/discussion/comment/82122881#Comment_82122881

Yes, and the larger system will produce more power at any given time than a smaller one, up to the 800W AC limit.

Probably should caveat that by saying, when it's generating, not in the middle of the night. ;-)

prowla

https://forums.moneysavingexpert.com/discussion/comment/82122908#Comment_82122908

There y'go!

And thus a plug-in system cannot be rated at more than 800W.