Quick one: unused solar panel on the roof, better short or open circuit?

[AndyBeez]

To me this question is rather like asking, should I leave an unused alkaline battery in open or short circuit? The durability of a solar panel is not just a measure of the intrinsic loss of output over time but also the build standard of the panel. No point having high quality mono crystalline silicon when the panel edges and seal fails after five years. Cheap Chinese panels have a cheap Chinese lifespan. Buy German - it lasts.

Unless you are building a solar farm where ever joule counts towards the bottom line, arguing over a few amp-hours is a waste of time. Especially if you are designing for capacity redundancy. Let us not forget the inverters too which can burn the spreadsheet numbers when they fail. What is the MTBF of the inverters?
Tip: Google "solar farm used solar panels" - the solar farms often off load aging and under performing panels. Also, thanks to green energy grants, panels are financially depreciated to zero over 4 to 5 years. So to make the most of capital allowances, farms upgade to new panels just for the tax-free money. A five year old panel can be a real bargain - ball park, a panel still outputting 200W+ is us$100 re-fitted. So it may pay to use and replace 'preloved' panels for a short period, rather than try squeeze the claimed 25 year lifespan out of full price new ones.

[Siwastaja]

To me this question is rather like asking, should I leave an unused alkaline battery in open or short circuit?

But a battery is a "voltage source", whereas solar panel is a "current source". Especially for laymen, but also for semi-hardened professionals, voltage sources are pretty intuitive but current sources are confusing as heck. Therefore, people understand more easily how capacitors store (and deliver) energy than inductors; they understand how capacitor keeps stored energy by being open, but are confused as hell when they hear that inductors store the energy while being shorted. Because for laymen, "short circuit" means some kind of error condition, dangerous even, which involves heat, sparks, and whatnot. This is true when you short a voltage source. But for a current source, the opposite is true - opening it is dangerous as it generates an infinite voltage, while shorting it means simple that no work is being done.

Solar panel of course is not a simple current source. It's a clamped current source so that neither open nor short circuit condition is catastrophic at all.

[KE5FX]

If a metaphor is called for, an LED might make a good one.  An LED under external illumination could be crudely thought of as a current source with compliance limited primarily by the material's band gap, just like a PV panel.  Nobody worries about whether LEDs should be shorted when not in use, and it's not clear that any such concerns would necessarily become more pressing as the junction area grows.

[Zucca]

Thanks guys, I was not expecting 4 pages of discussion.
I am also very happy with all the technical/physics explanation. Remarkable.

Thanks again to everybody. And yes you changed my mind, I will leave them open, at least there is no heat going into the cables and risk of fire in my laughable wood made home in USA (K I S S).

I need a day with 48 hours to finish my solar system. It was not planned to be in a situation like this, I am sure you all don't have time or wanna read for my boring private life details...

I pray the eng who designed my solar panels did everything right and they will not degrade too much.
Attached the datasheet in case you are interested, I have the 400W variant.

[bdunham7]

Dou you mean IR radiation like a LED (not just the blackbody radiation due to surface temperature)?

Yes, sort of like that although less organized.  I can't remember where I saw that discussed.  If it pops into my head I'll post it.

[zilp]

I'm not convinced that when no current is drawn 100% of the light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than reflected.

Have you seen solar panels get brighter when you reduce the electric load?

I haven't.

[gnuarm]

I'm not convinced that when no current is drawn 100% of the light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than reflected.

Have you seen solar panels get brighter when you reduce the electric load?

I haven't.

I don't see IR.

[zilp]

I don't see IR.

Why do you think that solar cells generate electricity primarily from IR?

[gnuarm]

I don't see IR.

Why do you think that solar cells generate electricity primarily from IR?

We must be talking about two different things.  I was talking about current creating light.  Someone said they couldn't see it and I was explaining why.

[zilp]

We must be talking about two different things.  I was talking about current creating light.  Someone said they couldn't see it and I was explaining why.

No, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

[gnuarm]

We must be talking about two different things.  I was talking about current creating light.  Someone said they couldn't see it and I was explaining why.

No, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

No, I never said that. 

[zilp]

No, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

No, I never said that.

So ... what did you say, then? And no, I don't mean a quote of what you said.

[CatalinaWOW]

No, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

No, I never said that.

So ... what did you say, then? And no, I don't mean a quote of what you said.

I'll tell you how a person with an engineering or physics background interprets what he said.  All objects radiate energy, with a spectrum similar to a black body, but modified slightly by variations in spectral emissivity.  So a solar panel radiates energy with a spectrum roughly similar to a 300 to 325 degree Kelvin black body.  Such a black body has its strongest emissions in the LWIR band, and very little in the visible band.  If you take the derivative with respect to temperature the changes are also strongest in the LWIR band, so in addition to having very little signal in the visible band, the relative change in that signal for the few degree temperature changes is smaller.  So if you can't see in the infrared, you can't see how the waste heat in a solar panel is being radiated.  Now if you heat your panel up to a cherry red heat or hotter the visible band becomes relatively good for determining temperature.  But in that case shorting or opening the leads will make no difference.

There is no reason to get into a feud about something that is just the basic way the world works.

[gnuarm]

No, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

No, I never said that.

So ... what did you say, then? And no, I don't mean a quote of what you said.

I've written a lot here.  How about you reply to the message you are looking for a clarification to.

[zilp]

I'll tell you how a person with an engineering or physics background interprets what he said.  All objects radiate energy, with a spectrum similar to a black body, but modified slightly by variations in spectral emissivity.

That's funny, because in my world, no one with a physics background would ever interpret "reflected" to mean "thermally absorbed and re-emitted as black body radiation", because reflection, by definition, maintains wavelength.

Also, it obviously makes exactly zero sense that wraper supposedly intended to say that they weren't convinced that "light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than [absorbed by the panel and converted into heat]", rather than the obvious interpretation of "light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than [redirected to a different propagation angle at the same wavelength]".

There is no reason to get into a feud about something that is just the basic way the world works.

Exactly my thought.

[Dacian]

As bdunham7 already mentioned is a matter of energy conservation.

Solar PV cells are basically a very large diode.
Voltage drop across that diode (PV cell) is around 0.7V for a cold panel. Divide the open circuit voltage to number of PV cells.
Having the panel open or short circuit will make no difference so just leave it open but protect the connectors from water if they are exposed.

I actually use PV panels as heating panels to heat my house.
Here is a thermal image of a 60 cell PV panel used a heater
The 60 panels are split in to 3 groups of 20 cells and I shorted the 20 cell group in the middle of the panel to effectively have a 40 cell PV panel
The 40 cells * 0.7V around 28V depending on current it may get close to 0.75V so 30V and that is exactly the max power point of this this 60cell panels.
I have two 60 cell panels outside connected in parallel and then those supply the 40 cell PV panel used as heater witch is inside the house.
Using a PV panel as heater is a better option than a resistive heating element as it will always work at max power point.
In the thermal photos about 17A was flowing trough the heater panel with ambient temperature of +25C and the hottest part of the panel was around +50C so about 25C above ambient.