How much do systems degrade? A friend's system seems like 33% in ten years.

[Southpasfan]

I am figuring out what size system to get. One worrying fact is that a good friend has a ten year old 7kwh system. At the moment, unless we are reading his bills incorrectly, its only producing 20kwh per day, in Southern California, mind you, when it should be more like 30kwh per day minimum.

Is that bullet proof evidence that something needs to be checked on his system? Or, if you buy a 10kwh system today do you have to assume it might be only a "6.6kwh" system in ten years due to ...... well, I don't know what it would be due to, that's why I'm asking.

 

[bkp_duke]

I am figuring out what size system to get. One worrying fact is that a good friend has a ten year old 7kwh system. At the moment, unless we are reading his bills incorrectly, its only producing 20kwh per day, in Southern California, mind you, when it should be more like 30kwh per day minimum.

Is that bullet proof evidence that something needs to be checked on his system? Or, if you buy a 10kwh system today do you have to assume it might be only a "6.6kwh" system in ten years due to ...... well, I don't know what it would be due to, that's why I'm asking.

Depends upon the panel type (premium panels like Sunpower have much lower degradation rates), inverter type (central, micro, central+optimizers).

33%, however, seems excessive even for the cheapest panels and inverters. If it were me, I would have someone check out the system to see if a panel has failed, inverter is failing, etc.


Oh, and TREES. If it is a central inverter system (10yo, most likely it is), and one panel on the string is shaded, then ALL of them will drop down to that lower level of production. 10 years is plenty of time for trees to grow, and might be time to trim some of them.

 

[BrettS]

It should degrade much less than 33% in 10 years, but keep in mind that degradation is measured from what it was producing in year 1 compared to what it is producing in year 10, not what it ‘should’ be producing. Using a calculator to try to figure out what it should be producing is just an estimate and there are a lot of reasons that panels won’t produce what the calculators say they should... shading, panel orientation, inverter inefficiencies, etc. If he can look back to see how much the system produced in the first year then that is what you should be looking at to see how much the system has degraded. It’s quite possible that it was never a very high performing system.

However, that said, whether you can find those first year numbers or not it might be worth taking some time to check out the system and see if you can find any issues with it. Look at panel orientation and shading to see if those are concerns. If this is a system with a string inverter with no power optimizers shading on one panel can impact the whole system’s performance.

 

[wjgjr]

I think for both solar panels and the solar roof (which is what we have) Tesla provides a guarantee - ours is 95% at 5 years, and no more than 0.5% degradation per year after (through year 25.) Now, this doesn't mean production will always be as high - @bkp_duke mentioned shading as an example, dirt can potentially be an issue, and weather patterns can affect annual production. But, Tesla does warrant the overall performance - the "Rated Peak Power" - won't degrade too quickly over time, so you should have a remedy with Tesla if something goes wrong with the panels/tiles that causes degraded performance

The 33% number over 10 years does seem like something is wrong with that system.

 

[Southpasfan]

That fact that shading of one panel can affect the whole system, if true, is an astonishing technical fact which simultaneously (1) does not surprise me that it would be true and (2) is the type of thing an owner might overlook.

Some owners might not overlook it, I grant you, but if, like my pal, putting solar in was more "the right thing to do" rather than "I need to make money on this" it is easy to see how he just accepted that the system was not that profitable.

 

[electracity]

Oversizing a system can really increase the payback period. Perhaps consider sizing the system as recommended by professionals, and planning on how you would add panels in the future.

Panels will probably make 30 years, but inverters probably won't. Most systems installed today will have a significant rehab at mid life.

 

[BrettS]

That fact that shading of one panel can affect the whole system, if true, is an astonishing technical fact which simultaneously (1) does not surprise me that it would be true and (2) is the type of thing an owner might overlook.

It is definitely much less of an issue now. By using micro inverters or string inverters with power optimizers any shading on one panel will only affect that one panel. Pretty much any system that you get today will have one or the other, unless shading is just not a problem in your location. However, I believe that 10 years ago it was much more common to get a string inverter without power optimizers, so given the age of your friend’s system this could be a factor there.

 

[DanCar]

How often are the panels cleaned?

 

[bkp_duke]

Dirty panels should not cause a 33% drop in output. 5, maybe 10% tops.

Is Solar Panel Cleaning A Waste Of Time And Money?

 

[Dave EV]

I track my solar system's energy production pretty closely (Enphase micro-inverter system, highly recommended) and I would estimate that my 10 year old system has degraded about 5%. Most of the variation I see month to month is due to weather. For example, the last half of June was particularly gloomy, so I only got about 89% of the historical average production. My worst deviation for a month was June 2019 with only 70% of my average. Part of my overall lower output, though, is that I don't clean off my panels as much as I used to, my panels are mounted on an almost flat roof so a decent amount of dirt accumulates on them. Your typical install won't have this problem. Even then, I typically only see about 10% difference between clean and dirty panels.

To really see what's going on, you need to track production of the entire system over an extended period of time and compare to the weather. If your system is down by more than 10% after 10 years, you almost certainly have a failed component in the system and should have it looked at. It's unlikely to be dirt unless the panels are visibly dusty/dirty when inspecting from the ground.

Does the system have any monitoring built-in? This is critical for diagnosing any issues. With my system I can easily see if any single panel is under-producing which makes it easy to see what's going on without any physical inspection.

 

[Southpasfan]

I track my solar system's energy production pretty closely (Enphase micro-inverter system, highly recommended) and I would estimate that my 10 year old system has degraded about 5%. Most of the variation I see month to month is due to weather. For example, the last half of June was particularly gloomy, so I only got about 89% of the historical average production. My worst deviation for a month was June 2019 with only 70% of my average. Part of my overall lower output, though, is that I don't clean off my panels as much as I used to, my panels are mounted on an almost flat roof so a decent amount of dirt accumulates on them. Your typical install won't have this problem. Even then, I typically only see about 10% difference between clean and dirty panels.

To really see what's going on, you need to track production of the entire system over an extended period of time and compare to the weather. If your system is down by more than 10% after 10 years, you almost certainly have a failed component in the system and should have it looked at. It's unlikely to be dirt unless the panels are visibly dusty/dirty when inspecting from the ground.

Does the system have any monitoring built-in? This is critical for diagnosing any issues. With my system I can easily see if any single panel is under-producing which makes it easy to see what's going on without any physical inspection.

Thanks for the response. How is your system comparing with (a) expected production v. (b) actual production?

 

[Southpasfan]

Ok all. I feel somewhat embarrassed, but I figured it out.

My friend was not the type of guy to get as far into the tech as some, and with a ten year old system his memory for comparing the before and after solar was not good. So, he didn't realize that his utility does not, on his bill, actually say how much his panels produce, they only show how much they supplied to him, and another number (presumably per hour that it occurred) his panels for lack of a better word "over produced."

His system is only covering about 1/3 of what is, and would be without solar, a rather large, likely about 3,000 to 4,000 per month kwh usage.

He has a system that produces probably 900 kwh average, but not much in terms of net back to the power company. Or, to put it another way, during the day, other than in summer, he's always using, for the most part, at least what his system produces.

 

[charlesj]

Dirty panels should not cause a 33% drop in output. 5, maybe 10% tops.

Is Solar Panel Cleaning A Waste Of Time And Money?

Yes, unless it was never cleaned?

 

[bkp_duke]

Yes, unless it was never cleaned?

Solar panels are designed to be very efficient, even if "never cleaned". By design, they are supposed to clean with just a moderate raining.

EDIT - you obviously didn't read the article I posted.

 

[charlesj]

I track my solar system's energy production pretty closely (Enphase micro-inverter system, highly recommended) and I would estimate that my 10 year old system has degraded about 5%. Most of the variation I see month to month is due to weather. For example, the last half of June was particularly gloomy, so I only got about 89% of the historical average production. My worst deviation for a month was June 2019 with only 70% of my average. Part of my overall lower output, though, is that I don't clean off my panels as much as I used to, my panels are mounted on an almost flat roof so a decent amount of dirt accumulates on them. Your typical install won't have this problem. Even then, I typically only see about 10% difference between clean and dirty panels.

To really see what's going on, you need to track production of the entire system over an extended period of time and compare to the weather. If your system is down by more than 10% after 10 years, you almost certainly have a failed component in the system and should have it looked at. It's unlikely to be dirt unless the panels are visibly dusty/dirty when inspecting from the ground.

Does the system have any monitoring built-in? This is critical for diagnosing any issues. With my system I can easily see if any single panel is under-producing which makes it easy to see what's going on without any physical inspection.

I have Enphase as well. Yes, they are great. Each panel production is independent of the others.
Also, as you know, weather patterns are changing from year to year, clearly documentable.
My best month was May for each of the first 3 Mays. then it shifted to Aug for year 4, then to June for year 5, Juy in year 6, then back to June for years 7&8.

 

[charlesj]

Ok all. I feel somewhat embarrassed, but I figured it out.

My friend was not the type of guy to get as far into the tech as some, and with a ten year old system his memory for comparing the before and after solar was not good. So, he didn't realize that his utility does not, on his bill, actually say how much his panels produce, they only show how much they supplied to him, and another number (presumably per hour that it occurred) his panels for lack of a better word "over produced."

His system is only covering about 1/3 of what is, and would be without solar, a rather large, likely about 3,000 to 4,000 per month kwh usage.

He has a system that produces probably 900 kwh average, but not much in terms of net back to the power company. Or, to put it another way, during the day, other than in summer, he's always using, for the most part, at least what his system produces.

Exactly. Utility bill is not where you get accurate solar production data as it also includes your consumption variations.

My first year was partial year, Yearly production as follows in sequence:
6.68mWh; 6.24; 6.33; 6.08; 6.17; 6.257; 6.05
You be the judge how much that is due to panel and micro inverter degradation and weather pattern changes. Impossible to tell due to variables and I bet weather changes may be the largest factor.

 

[charlesj]

Solar panels are designed to be very efficient, even if "never cleaned". By design, they are supposed to clean with just a moderate raining.

EDIT - you obviously didn't read the article I posted.

Actually I have seen such reports and also actual instrumented demonstration of panel output variations due to dirt. Unfortunately it didn't include a panel that was never cleaned for years.
Oh, link to that latter was posted not that long ago on this forum.

 

[bkp_duke]

Actually I have seen such reports and also actual instrumented demonstration of panel output variations due to dirt. Unfortunately it didn't include a panel that was never cleaned for years.
Oh, link to that latter was posted not that long ago on this forum.

Reference please.

Do you live in a place the NEVER gets rain?

 

[bkp_duke]

Power loss due to soiling on solar panel: A review - ScienceDirect

1 to 4.7 % loss depending upon location.

And that is a review article (i.e. an article that aggregates the bulk of available, published, information into one article).


I once paid a guy, who was on the roof for other reasons, $40 to hose off all our panels. This was after 6 months of no rain, and all the neighbors doing landscaping (lots of dust on the panels). I got a 5% or so improvement in performance. Not enough to justify the cost of cleaning a 12 kw system.

 

[charlesj]

Reference please.

Do you live in a place the NEVER gets rain?

Here is one biased reference:
Heliotex Solar Panel Cleaning Residential Home Services

The link posted on this forum is the best that was instrumented.
I guess I should have saved it but i am not clairvoyant.
I guess your kink is not biased but actual instrumented results?

If I find that link, I will post it here.

Well, Monterey doesn't get a lot of rain, gets lots of pine pollen.

To some a 5% is meaningful. Cost me nothing or a few cents for water.
You know, 5% here, 5% there and pretty soon it becomes a lot.