Anyone here with a Residential PV Array (solar energy system)?

[moshe_levy]

All-

 

The "Hybrid Hatred" thread has evolved now into a discussion on PV Arrays. I am in process of getting one installed on my roof. I am curious as to whether any other board members have one, and if so if they would be so kind as to share their experiences.

 

-MKL

[CoarsegoldKid]

Yes. 2.3Kwh

[moshe_levy]

Joe-

 

Excellent. As mine is not up and running, I'd love to hear of your experiences. Cost, maintenance, do you sell SRECs, is the generation of said SRECs what your installer said it would be, are your expectations fulfilled, etc. etc. Please share.

 

-MKL

[rob1100r]

Do you know how long the panels are suppposed to last? Do they recycle the old ones, or charge to get rid of them....Got a nice empty spot in the yard for something like that. Thought about it in the past..

[moshe_levy]

The warranty on mine is 20 years. I imagine after that much time they're way obsolete in terms of efficiency and output anyway. Re disposal I'm not sure but I imagine most of it is recyclable.

 

-MKL

[CoarsegoldKid]

There are a lot of variables to examine. House built in 2004. AC is 13.5 seer if I recall. They are all important if you want the best performance for your personal electrical company.

Our system is actually at 2.6kwh system. As you know you will never see 2.6kwh due to losses that are out of your control. 2.3kwh was the amount stated for the tax credit and state/electrical company rebate. The best output I have ever seen is, and note that a person would have to watch the output constantly to see the max peak and I only look at it when I walk by, 2.25kwh is my observed peak. The expectation has been met. The array is mounted parallel to the roof with no added angle. In sunny winter days the optimum angle is not achieved. During the summer max angle is achieved. More hours of sun in summer provides more daily output than any other time of the year. However heat causes losses so the max output is during cool fall and spring days but less hours of sun so daily output is lower than in summer. If the array could be cooled during the summer daily output would be much higher. Cleaning them will also make a small difference. Did that make sense?

 

Only changing light bulbs to CFLs when incandescent blow and unplugging as many e-bricks as I can during none operation my average monthly PG&E electrical bill was 85 dollars per month pre solar array. Late spring, all summer, and early fall seasons the AC comes on to keep the home at around 76 degrees. We are retired so we are mostly home and the cats get AC when we aren't. Good to be a pet, eh.

 

The attic insulation was less than the R38 it was purported to be. Half the house was upgraded to R50 this past fall. All windows are double pane Milgard models of 2004. Walls are 2x4 so I think only R13 is there. Two skylights(soon to be removed) on the roof facing the western sun don't help. The long edge of the house faces east(higher percent of the glazing) and west which are both totally wrong as far as placement and glass is concerned with regard to passive solar designs. Our electrical usage routinely went into the 3rd tier of billing at 24 cents per kwh.

 

So with that in mind my post array PG&E bill averages 35 dollars per month. A reduction of 50 dollars. Our kwh cost now is 1st tier 11.5 cents all year round with the solar. We don't get into the 2nd or 3rd tier these days.

 

I consume all the kwh production from my array so there isn't any to really sell back after the true-up is calculated at the end of the 12 month period. California has changed the method of calculation since I installed the array. It was net zero. Now it is fairer. I think it is now whatever you don't use you sell. If a person was away at work all day not needing the AC on they would be selling back plenty of their production.

 

Maintenance= so far is just dusting them off. Reaching them means I must ladder up to the roof and use a long duster. I looked into window washer equipment but the cost to take our minerals out of the water and the pressure needed and the brushes and poles and extensions came to about 1200 dollars so I could not justify it just to clean them. I really wanted a Ducati instead. If I had a two story home and lots of high windows that would be a different calculus.

 

Cost of system install was about 18,000 not including the rebates and tax credit. All in all it was about 13.5K I think.

 

Determining when the system pays off in really difficult but if PG&Es historic 10% increase year over year continues it will take 14 years maybe, maybe less. If I can get more heat out of the attic during the summers without spending too much to do so it will pay off sooner because I can sell power. My gut tells me that as time goes on America's electrical usage will increase faster as the population grows outstripping the production and hence higher costs per kwh so who really knows how it will play out. Your mileage may vary.

[CoarsegoldKid]

Panels should last 30 years unless damaged by something. The electrical converters are currently warranteed for 5-10 years depending on company and should last about 15 from what I hear. Recycling of panels- well I installed 10 year old panels removed from a school in a trial program in Sacramento on to a school in the Sierras with a "course on installing solar systems". So I guess recycled panels do exist. The one's on my home are new and installed by a contractor.

[moshe_levy]

Joe-

 

Good info there. My house was new in 2002 with good quality Anderson windows and good insulation throughout. My array is rated for just under 7.4KW, using 32 panels of Sunpower's high efficiency 230W modules.

 

Unlike older designs which route the panels to a centralized inverter, on my system, each panel has its own inverter. As such, even if more than half the panels are shaded, the others will keep operating and producing. This enhances efficiency greatly since the system can't be dragged down by any number of panels being shaded.

 

Here in NJ my understand is that SRECs are generated according to production, irrespective of consumption. Meaning it doesn't matter if I use some or all of my production - my output is measured independently of my consumption. So I'm not selling what's "left over" after I consume, but rather what I produced in the first place.

 

Our utility costs are rising about 10% per year as well. But again, from what I've been able to discover, saving on the electric bill would take decades to pay this system off. It's the SRECs that are going to pay this off quickly.

 

How much is an SREC worth in California? Here it's a little over $600 currently.

 

-MKL

[CoarsegoldKid]

Not sure what a SREC is? The California rebate was about 1.70 per watt of real electricity. 1.70 x 2.3kwh = xxxx

The resale to the grid is whatever it costs you to buy from the grid not to exceed tier 1.

The tax credit was a percentage of cost minus rebate.

[moshe_levy]

Oh, wow, totally different than NJ.

 

OK, here, we have SRECs, or Solar Renewable Energy Credits. Coupons, basically. The official explanation:

 

"Each time a system generates 1,000 kWh of electricity an SREC is earned and placed in the customer's electronic account. SRECs can then be sold on the SREC tracking system, providing revenue for the first 15 years of the system's life.

 

Electricity suppliers, the primary purchasers of SRECs, are required to pay a Solar Alternative Compliance Payment (SACP) if they do not meet the requirements of New Jersey’s Solar Renewable Portfolio Standard (RPS). One way they can meet their RPS requirements is by purchasing SRECs. As SRECs are traded in a competitive market, the price may vary significantly. The actual price of an SREC during a trading period can and will fluctuate depending on supply and demand."

 

A 7.4KW system like mine is forecasted to produce between 7-9 SRECs per year. Right now an SREC fetches about $600 on the market.

 

Pricing for SRECs is monitored here http://www.njcleanenergy.com/renewable-energy/project-activity-reports/srec-pricing/srec-pricing

 

For those that want to reduce risk, there are state programs and even companies that specialize in loans or incentives to the PV owner in exchange for long term "fixed" SREC contracts. In effect you take fixed lower prices on your SRECs that are steady over time instead of being tied to the fluctuating market. Personally I'm going with the market. Riskier, along with higher chances of larger returns. The usual tradeoff.

 

-MKL

 

 

[ESokoloff]

........ I am curious as to whether any other board members have one, and if so if they would be so kind as to share their experiences.

 

Jamie/KMG_365, please pick up the white courtesy phone

[Mike]

We're looking at the full range of energy-saving possibilities for the cabin we hope to build (someday) on our lot in Wisconsin. This nearby company sells photovoltaic systems mounted on solar tracking platforms. It's a pretty cool concept, but I have yet to determine if it's worth the extra cost, as opposed to a more conventional photovoltaic array.

[moshe_levy]

Mike-

 

Everything I've read says that a tracking array will generation some 35-40% more per year than fixed, assuming fixed isn't adjusted seasonally. For us here in dense NJ, there is no space in the backyard for such a beast, and I'm not climbing on my roof every season either, so we're kind of stuck with fixed here.

 

Next step for you is to see what kind of assistance you're going to get from your utility company and your state in the form of rebates and other breaks on taxes, etc. Here there are plenty (no sales tax, arrays cannot be used in reassessment of property values, per watt credit given for special financing programs, utility rebates, in addition to 30% federal tax credit, etc.). It pays to find out what incentives are in place. To my knowledge only CA and NJ are serious about their solar incentives, however. The other states are lagging behind.

 

This is why I often say there is no national energy policy - every state does its own thing.

 

-MKL

[Mike]

Moshe--

 

I've done some very cursory research, and Wisconsin does have some incentives, including sales tax abatement and the exclusion of solar systems in property tax value computations. The local utility does buy back excess electricity, though I'm not sure of the specifics of that program; some of their customers have reported zero electricity bills over a period of years. If and when we get serious about building, I'll post my progress here.

 

-Mike

[moshe_levy]

Sounds good Mike. I'd be most curious. Forgot to mention, and perhaps the same is true in your state, that any income generated is 100% tax free.

 

-MKL

[Foot]

I have been off the grid and fully solar since 1993. It was too expensive to run power to my location, so I made my own!

 

Alan

[Deadboy]

A $50 a month savings for an investment of 14K (more or less) just doesn't make sense to me. If society deems this necessary we should be installing panels in the Nevada desert on a large scale (they are actually) not small residential systems....make no more sense to me than building a small nuke plant or hydroelectric system to power a single residence.

 

I know it makes people feel warm and fuzzy and that is fine I guess but financially I just don't see it....especially with the subsidized prices and tax incentives that are just making other people pay to help reduce their neighbor's cost.

 

[moshe_levy]

Deadboy-

 

The savings on your electric bill are not the financial incentive, it's just icing on the cake. The "selling back" of the energy you produce is where the money's at. The incentives are not just from government, but from local utilities in many cases. Ever consider that perhaps incentives are cheaper for them to offer than actually updating or expanding our woefully ancient power grid?

 

Alan, here, if you go off the grid, you bear 100% of the cost of the system - quite alot to swallow for the average homeowner! But you can then have a true "backup" system, meaning solar charging of a battery backup system. That would be a neat item to have! But alas, not possible here without a seriously fat wallet....

 

-MKL

[Deadboy]

The selling back incentive is interesting but still hardly worth the initial investment for a small system. Even if you get 100% "off the grid" and save $150 a month you are still talking about a long long time to see any real economic return. From what I have seen the sold back power is at the lowest rate the utility sells it at (tier 1).

 

Turn off a few lights, adjust the AC/heat, install a whole house fan and pocket the savings today, not in a decade (or more)....plus the systems degrade yearly....so your return is dimishing and pushing out the break even even further.

 

http://www.smud.org/en/community-environment/solar/Pages/solarFAQs.aspx

 

[Linz]

I know I'm in Australia but we have similar systems.

 

I have a 3.0Kwh system installed in the first week of October 2010. That gives me two quarterly power bills through this period. We feed our excess through to the grid and are paid for it.

 

So far, we've earned $19.00 and $5.00 and saved about $2500 in power bills.

 

Our system will pay for itself in around 3.5 years if electricity prices stay roughly the same.

 

Linz

[Deadboy]

Wow, your rates are considerably higher (we have a lot of hydro electric sources out west). I don't pay much more than $2500 a year.....

[moshe_levy]

Interesting link. You can consider us all living in separate countries, since the only thing linking us is the federal policy which is currently a 30% rebate on the cost of the system with the usual caveats. Here in NJ, the SRECs are the payoff. I linked above to the state page which shows their values, currently about $600 each. That's for every 1000KWh you produce. Local utilities seem to have different rates of rebates as well. I agree, a larger system will produce more faster - but here another caveat to rebates is you can't size a system larger than your average use.

 

I thought of a way around this: Just max out your use for a year (AC on ALL the time, all lights on ALL the time, etc.) and thenyou can get away with alot more. But in the end I just did it the honest way, and still wound up with a 7.36KW system.

 

Linz, I was just in OZ - didn't see much in the way of arrays down there and I understand incentives are quite limited, though taxes on carbon emissions seem to be in the air....

 

-MKL

[Nice n Easy Rider]

My brother-in-law, living in the Berkshire foothills in western Massachusetts, installed a 24 panel (192 total SF) system last year. Even with the Federal and State incentives, his break even point on the $28,000 system is estimated to be 7 years. Down here in North Carolina, where we have even more sun, the companies I talked to were estimating break even points of 10 years (I guess our power is cheaper here). Unless I knew that I was going to be in my house another 20 years (which I'm not) I'm not willing to invest in this technology yet.

[moshe_levy]

George-

 

I don't blame you. My friend in Charlotte pays something like $130 a month on dead summer with the AC on full blast all day and night, to cool his 3.3k sq/ft house. Here in NJ, I'd usually pay upwards of $450-500 per month. If state and / or utility rebates are low, and the only real savings of an array is the electrical bill, the payoff period would stretch into an uncomfortably long length.

 

As I said, we're all in different countries, as each state does its own thing, and the utilities in that state each do their own thing too. There is no unified energy policy to speak of, which this a very localized discussion since what makes sense here in NJ more than likely doesn't make much sense in most other states.

 

-MKL

[Mike]

The selling back incentive is interesting but still hardly worth the initial investment for a small system. Even if you get 100% "off the grid" and save $150 a month you are still talking about a long long time to see any real economic return. From what I have seen the sold back power is at the lowest rate the utility sells it at (tier 1).

 

Turn off a few lights, adjust the AC/heat, install a whole house fan and pocket the savings today, not in a decade (or more)....plus the systems degrade yearly....so your return is dimishing and pushing out the break even even further.

 

http://www.smud.org/en/community-environment/solar/Pages/solarFAQs.aspx

 

I think you have some good points, and anyone considering a PV system needs to sit down and try to crank out some numbers. How it works out is really is going to vary considerably from one home to the next, and you need to be careful to avoid the generalizations you read from proponents or opponents of what you're considering.

 

I've recently had some experience with a much simpler situation, when I replaced an 80-gallon electric hot water heater with a natural gas on-demand heater. Most of what I read on the subject suggested that the payback period would run at least 8 - 10 years (some suggested 15 was more realistic), but I knew our situation was quite a bit different than the assumptions built into most predictions. Based on what I've seen so far, my best guess is that we'll break even after 3 - 4 years.

 

It's much the same with solar electric generation. My guesstimates--admittedly pretty rough at this stage--are that it would take me about 15 years to recoup the cost of a system. My current take on it is that I'd be better off to invest some of what I might put into a PV system into a geothermal heat pump, then wait until I have the cash to install the PV system. As I see it, photovoltaics are going to continue to get cheaper and more efficient over the next few years, so there may be benefits to holding off. Having said that, the current incentives in many places are pretty substantial, with no guarantee as to whether they'll still be around in five or ten years.

 

It ends up being a little difficult to know exactly how to allocate limited resources. But, with knowledge you can make an educated guess.

[moshe_levy]

Mike-

 

Here geothermal doesn't get anywhere near the incentives of solar. By all estimates on a house of 3k sq/ft you're looking at something like $60k! I researched it after reading in US News and World Report that a system typically costs $5k - turns out that was a misprint. For $5k it's a no-brainer since heating or AC runs me upwards of $500 a month here in NJ. But $60k is another story.

 

IKEA is a large proponent of geothermal. They usually pony up for it at their various, enormous stores. I think for a homeowner it's a bit much to swallow, though.

 

ne thing we agree on is it's a case by case basis. The solar proponents themselves know this - there is a serious vetting process before you can even buy one. A computerized sun / shade analysis on your specific house must be presented and approved by various regulatory bodies before you can move forward. At least here.

 

-MKL

[Mike]

Mike-

 

Here geothermal doesn't get anywhere near the incentives of solar. By all estimates on a house of 3k sq/ft you're looking at something like $60k! I researched it after reading in US News and World Report that a system typically costs $5k - turns out that was a misprint. For $5k it's a no-brainer since heating or AC runs me upwards of $500 a month here in NJ. But $60k is another story.

 

-MKL

 

At that cost, it wouldn't even be a close call. We haven't settled on the final design of the home, so I can't yet determine the size of the unit we'd need. However, a lot of people are turning to geothermal in the area of our land and the cost around SW Wisconsin, exclusive of tax credits and other incentives, generally runs between $15,000 and $25,000. Again, preliminary findings only, but the closest guesstimate I've gotten is roughly an additional $7,500 for the heat pump and installation, and another $11,000 - $15,0000 for installing the loop. That's a lot of money, but if you're not replacing a conventional HVAC unit, you're a little ahead of the game.

 

Vertical loops are more expensive, but we have plenty of land, including a three-acre field that is clear of any obstructions, that would permit us to avoid a vertical loop and, instead, trench the loop. I've discussed building a pond with the guy who's going to do our site prep, but that doesn't appear to be a viable option in our location.

 

Anyway, whether it's $20,000 or $60,000, the cost is very significant. In our building location, natural gas is not available, leaving propane as the least expensive "conventional" fuel option (leaving wood pellets and the like aside). Propane heating costs can be outrageous, so, again, it becomes a little easier to justify a high initial equipment cost. As you note, YMMV.

[CoarsegoldKid]

Mike- in selecting a home design look for home designs from specialists in passive solar and contractors that are at least know the concept of passive solar. Passive solar homes are designed to be oriented to the sun and make use of solar angle, shading and insulation. It's an emerging concept in home design forgotten in years of abundant cheap energy.

 

Geothermal is a no brainer as far as I'm concerned when building a new home on acreage with plenty of space. The tractors are already there grading for the site and septic so digging a few trenches is just a few hours more. You have lots of options when building a new home.

 

Like a home that has an extra bedroom or a remodeled kitchen there is value in the solar system upon selling the home compared to others that don't have those things. According to an estimate a small system like mine would realize about 13K more in the sales price of comparable homes. I don't plan on testing the theory for at least another 10-15 years. It could all be BS. I sold a home 6 years ago with a 4 year old remodeled kitchen and the "long time in the business" realtor told me well all homes have kitchens so why would yours be worth more.

 

Houses that are more energy efficient are going to be worth more certainly in the not to distant future. Would you buy a R19 insulated home or a R50 insulated home. I'd take the R50 even if it did cost a few grand more. Price insulation and you can see why. Same with insulated windows.

 

I like solar and anything I can do to my home to reduce energy consumption because I like to stick it to PG&E.

[moshe_levy]

+1 on that. The sky's the limit when building from scratch. Simple things in planning can reap huge dividends, vs. the rest of us who are stuck with constraints in purchasing pre-existing structures which are not optimized for efficiency.

 

-MKL

[Shaman97]

Do the rebates apply if you DIY your own system? They're tedious to make, but if you want to save some $$$...

[moshe_levy]

I dunno. I wouldn't even consider it, personally. Fear of heights for installation, fear of red tape and endless applications and interactions with town and state inspectors before, during, and after, etc. Nahhhh, that's a service I'd gladly pay someone else to deal with and warranty.

 

-MKL

[Shaman97]

Moshe, I hear what you're saying. But it would cut down on the payback period significantly. Quick look at a few products would yield a 3K system (panels, inverter and monitor) for $6K - $8k.

 

Too, the local Community College offers courses (certification, actually) in Solar installation.

 

Not something I'm about to jump into, but I'd be looking at all the options.

 

On the other hand, the pace that I complete Home projects would make completion of the new WTC look fast.

[ghaverkamp]

On the other hand, the pace that I complete Home projects would make completion of the new WTC look fast.

 

In other words, you could have it paid off the day you turn it on.

[CoarsegoldKid]

Wow, your rates are considerably higher (we have a lot of hydro electric sources out west). I don't pay much more than $2500 a year.....

I paid less than 350 dollars for electricity last year.

[CoarsegoldKid]

Do the rebates apply if you DIY your own system? They're tedious to make, but if you want to save some $$$...

The rebates are on a sliding scale that is about to go to zero. The PG&E website should explain it all.

[Shaman97]

On the other hand, the pace that I complete Home projects would make completion of the new WTC look fast.

 

In other words, you could have it paid off the day you turn it on.

 

 

Dollars ahead, even.

[moshe_levy]

Moshe, I hear what you're saying. But it would cut down on the payback period significantly. Quick look at a few products would yield a 3K system (panels, inverter and monitor) for $6K - $8k.

 

Not sure- never researched it. I'm sure there's a way to DIY, but honestly I don't know enough. $6-8k won't buy you much. Think of this like a computer you're going to use for 20 years. Best to buy the state of the art, not something that is already obsolete tomorrow. That means high efficiency panels, each with its own imbedded inverter if you expect any shading (so partial shade doesn't drag the whole system down, since each panel stands on its own), etc. My system's pretty good - Sunpower 230W modules each running an individual Enphase inverter with well over 90% efficiency. It will be a few years at least before that becomes old news.

 

Here's an article from late 2009 which I think is still true today re NJ payback. Reading it I see why some of you guys that don't live here are skeptical - your paybacks are multiples longer than here.

 

http://blogs.reuters.com/environment/2009/09/16/new-jersey-has-best-paypack-on-residential-solar-in-us/

 

I think this is the first time I've EVER thought or said this, but for once, I'm happy to be an NJ resident. ONLY THIS ONCE!!!! Boy, did that feel strange to say!

 

-MKL

[Deadboy]

Interesting but I would be more curious about what you paid prior to installing your system. My bill is high for my area but I work from home, have 2 hot water heaters, multiple computers, electric heat and a 425 foot deep well.....and I was probably exaggerating a bit but I am sure it is around 2k per year. If you dropped from 2k to $350 per year that is a significant improvement.

[CoarsegoldKid]

I know folks that routinely pay upwards of 300 per month. A lot of wasted money in my opinion. I averaged 85 pre solar and last year it was 35. I don't think you can put one of these systems up and think it's going to pay for itself in 10-14 years. Too many variables. It will pay for itself. How soon depends on how much PG&E raises the cost. They will.

[Smoky]

Here in BC, I heat with gas.

 

My electric bill doesn't exceed $600 year.

 

My gas bill is about twice that. I have 5 gas appliances.

 

BC generates almost all its electric from hydro dams.

[ESokoloff]

A bit disappointed that Jamie has not thrown is 2 cents in on this.

 

After reading some of these post I'm wondering if panels/installations could be made to be more efficient.

What would it take to be able to adjust the angel of the panels 4 times per year to optimize exposure?

Also if cooler panels are significantly more efficient, then why aren't they water cooled?

[moshe_levy]

Eric-

 

Andusting angle depends entirely on your mounting system and ease of access. It is feasable to do for someone with an adjustable mounting set and the knowledge to adjust to proper angles without an issue.

 

Water cooled panels have been toyed with - see http://www.usc.edu/CSSF/History/2008/Projects/J0913.pdf and I'm sure there are more examples - but knowing what I know about electronics, I would venture to say that the inverter loses more than the panel in high heat conditions. For example, the inverters my company manufactures for the military like this http://www.novaelectric.com/dc_ac_inverters/nova_invert20-32.php typically are derated about 2.5-3% for every +1C over +50C.

 

FYI, a water cooled variant of any of our air-cooled inverters costs on the order of 10-15x more than air cooled as well.

 

For this reason I think it ultimately comes down to cost / benefit. Sure we can water cool inverters and the panels, for exponentially higher price (and look at how people are complaining NOW about the price - imagine then). Of course there is the second issue of people maybe not feeling comfortable with an elaborate irrigation system on their roof. Plus, to pump water you need an irrigation system and a pump which consume energy, and just for example with my system, would have 32 entries and 32 exits for the inverters, on top of 32 entries and 32 exits for the panels. That's ALOT of potential for failure, leaking, etc.

 

One of the nice things about a lower tech system is total lack of maintenance. When my system runs, it is silent and maintenance free. Everything is solid state. The rain cleans off the panels once in awhile. There's nothing to do, at all, no maintenance schedule. The only time you do anything is if a component fails outright. In that case the panels are covered by a 25 year warranty and the inverters by a 15 year warranty. So it's sort of set-it-and-forget-it. Water cooling adds a whole layer of complexity to that which likely comes nowhere near to actually paying off. Just my opinion there - no facts as I haven't seen hard numbers. If you see numbers on water cooling please share.

 

-MKL

[Shaman97]

There are companies that make Fresnel lenses for solar panels. It boosts the output of the panel, but there are some issues with overheating the panel. There are a few videos.

 

Here's one to demonstrate the power of a Fresnel lens:

 

Fresnel lens

 

Now, here's one where the voltage difference is measured:

 

 

Admittedly one of the videos isn't as informative as it could be, but the end result speaks for itself.

 

 

[CoarsegoldKid]

In 2005 I asked the question of liquid cooling to the instructors at SEI. They looked at me like I was from another planet. Then again.

 

I think taking one panel into an experiment of liquid cooling would be a good exercise. As said the complexity of multiple panels being liquid cooled as we now know it soon becomes enormous.

 

Inverters have plenty of heatsink cooling so I don't think there would be much to gain.

 

Keeping them clean is an advantage. Where I live the pollen this time of year covers the panels. If I'm lucky a late spring rain will help cleanse them. Then the dust that is normally in the air comes into play for the next 6 months of the hottest/driest part of the year. Roof mounted arrays present problems in cleaning. Some roofs are too steep. Some are too slippery especially with pollen and dust on them. Some are too high to reach from the ground. Some are too wide and long to access all points with common telescoping poles. Window washing equipment could be used but make sure your water is demineralized. All this costs money and maybe it's worth it. Maybe it's not. Most people would not think of cleaning them or keeping them cool. That's why these systems are derated.

[moshe_levy]

Joe-

 

Derating is typically estimated @ 1% or less per year, obviously depending on a great many factors (mostly environmental). Here in NJ it's not too dusty and it rains frequently, so the installers rarely quote any maintenance or related plans. I would guess around here derating is less than 1% per annum, which is to say your panel's efficiency is becoming obsolete due to technological advance at a higher rate than from derating due to normal wear and tear. I don't see it as a major issue.

 

-MKL