Panasonic vs Sunpower

[jflorey2]

What is the consensus for minimum angulation to be "self cleaning"?

I would not try to go flatter than 10 degrees. Flatter than that and you start to get pooling. It won't clean well at 10 degrees, but at least you won't get precipitated dirt from puddles on the panels.

[SunEagle]

What is the consensus for minimum angulation to be "self cleaning"?
I recognize that the ideal angulation for generation would approach 42 degrees which is our latitude here in MA.
My roof is not easily accessible and I was told that anything beyond a 10 degree angle will require anchoring in the flat roof which I don't want to do because of the potential for leaks/roof issues.

If you are not anchoring the panels to your flat roof how are you keeping them from flying off in a big wind event?

[J.P.M.]

What is the consensus for minimum angulation to be "self cleaning"?
I recognize that the ideal angulation for generation would approach 42 degrees which is our latitude here in MA.
My roof is not easily accessible and I was told that anything beyond a 10 degree angle will require anchoring in the flat roof which I don't want to do because of the potential for leaks/roof issues.

There is no self cleaning panel at this time, any more than there is a self cleaning window. Maybe a "more tilt, more self cleaner panel" might be a better description.

As a general concept, and as Jflorey2 notes, lower angles with respect to horizontal orientation will collect and retain more dust/atmos. pollutants and turn into mud pans. The closer to horizontal, the larger the sump area and the more effect over shorter time periods. Simple to understand concept that gets involved once wind/rain/pollution are considered, and probably very local in terms of pattern/effect. By observation and some measurement, my array fouls somewhat differently than others in my neighborhood even with generally similar tilts/orientations.

As Suneagle notes, or Socratically asks, arrays must be anchored. As a SWAG, figure a couple of roof penetrations in the form of lag screws, etc. per panel but know and make no mistake that there are a bunch of holes in every roof that you see with an array on it. Disabuse yourself of the idea of ballasting anything on a horizontal roof.

Spend 20 bucks. buy and read "Solar Power Your Home for Dummies". At bookstores or Amazon. You need an education in practical solar energy and how it's done in residential applications.

[ay79]

I would not try to go flatter than 10 degrees. Flatter than that and you start to get pooling. It won't clean well at 10 degrees, but at least you won't get precipitated dirt from puddles on the panels.

Noted. We get rain relatively frequently here in the northeast so I'd expect 10 degrees to run off and rinse dust and debris regularly.

As Suneagle notes, or Socratically asks, arrays must be anchored. As a SWAG, figure a couple of roof penetrations in the form of lag screws, etc. per panel but know and make no mistake that there are a bunch of holes in every roof that you see with an array on it. Disabuse yourself of the idea of ballasting anything on a horizontal roof.

THAT is news to me. I have been told that a ballast system would be adequate for a horizontal array. We had some very strong winds today so I'm fairly sure anything that wasn't horizontal on the roof would have taken flight. It seems the lack of any tree cover is only part of the story here. The biggest factor will be angulation (freshman physics coming back to me).

Solar installations in Boston that are always titled at the latitude of Boston (Average Tilt at Latitude or ATaL) average 4.57 kWh/m2/day, or about 19% greater than the average monthly GHI of 3.84 kWh/m2/day and approximately 21% greater than the average monthly DNI of 3.79 kWh/m2/day.
-https://solarenergylocal.com/states/massachusetts/boston/

So by installing horizontally I can expect to make 19% less power. Clearly this is outweighs a panel that is 2-3% more efficient. The problem that then arises is that angled panels require more spacing so fewer would fit on the roof which suddenly becomes constrained. I suspect it will come down to my confidence in an installer/roofer to anchor the racks and not leave us with leaks in a down pour. What seemed like a great idea is becoming a lot more involved!

Spend 20 bucks. buy and read "Solar Power Your Home for Dummies". At bookstores or Amazon. You need an education in practical solar energy and how it's done in residential applications.

Not a bad idea.

Thank you all for your advice and the benefit of your experience. I gather most of the contributors are in warmer drier states than I am.
Did weather and roof health play a large role in your selection of installers and of systems?
Are you aware of friends or people that you may have advised that opted to not go ahead after considering factors such as potential for roof damage?

[Mike90250]

Did weather and roof health play a large role in your selection of installers and of systems?
Are you aware of friends or people that you may have advised that opted to not go ahead after considering factors such as potential for roof damage?

I went with pole mount arrays, since we have 6 months wet and 6 months dry (fire season & flood season in Nor Calif) so the arrays are angled right, and the roof stays sound.

[J.P.M.]

Noted. We get rain relatively frequently here in the northeast so I'd expect 10 degrees to run off and rinse dust and debris regularly.




THAT is news to me. I have been told that a ballast system would be adequate for a horizontal array. We had some very strong winds today so I'm fairly sure anything that wasn't horizontal on the roof would have taken flight. It seems the lack of any tree cover is only part of the story here. The biggest factor will be angulation (freshman physics coming back to me).

Solar installations in Boston that are always titled at the latitude of Boston (Average Tilt at Latitude or ATaL) average 4.57 kWh/m2/day, or about 19% greater than the average monthly GHI of 3.84 kWh/m2/day and approximately 21% greater than the average monthly DNI of 3.79 kWh/m2/day.
-https://solarenergylocal.com/states/massachusetts/boston/

So by installing horizontally I can expect to make 19% less power. Clearly this is outweighs a panel that is 2-3% more efficient. The problem that then arises is that angled panels require more spacing so fewer would fit on the roof which suddenly becomes constrained. I suspect it will come down to my confidence in an installer/roofer to anchor the racks and not leave us with leaks in a down pour. What seemed like a great idea is becoming a lot more involved!



Not a bad idea.

Thank you all for your advice and the benefit of your experience. I gather most of the contributors are in warmer drier states than I am.
Did weather and roof health play a large role in your selection of installers and of systems?
Are you aware of friends or people that you may have advised that opted to not go ahead after considering factors such as potential for roof damage?

1.) With frequent rain, 10 deg. will be better than horizontal, which will always be a sump, rain or shine, but that's only one issue. For any reasonable hope of cost effectiveness/ STC W you will most likely need panels mounted at an angle to the horizontal probably around your local latitude, maybe a bit less (but not "always"- just usually and mostly).

2.) The reality of mounting panels on a horizontal roof is that for best (most) annual production, for most locations, panels need to be tilted to the horizontal. That usually means panels mounted in rows (hopefully) aligned generally E-W in sawtooth fashion with space between rows to help avoid (major) shading of the rows farther from the equator by the rows closer to the equator. Usually, the farther north the array's geographic location, the greater the row spacing that's required. Here's a trick: To a first approximation (only), when mounting panels in sawtooth fashion on a horizontal roof, and because of that shading, the most annual production possible - regardless of the panel tilt - is about the same as that of a horizontal array on the roof, or less.

So, one way to roughly estimate output on an otherwise unshaded horizontal roof is to use a model's (like PVWatts for example) horizontal array output, and first see if it it's close to whatever you want for an annual output. If it's way below what you want, you might want to rethink whether or not PV is worth it. Then, using the model, design for the best annual output by changing the panels' tilt and the row spacing and study up on solar geometry, including something called the "profile angle" to avoid adjacent row spacing, say +/- 3 to 3 1/2 hrs. on either side of solar noon. Then, see if the cost makes any economic sense by your criteria. You or your designer will probably need to run several scenarios of various tilts and row spaces, and with the panels in both landscape and portrait orientation to maximize annual output. You'll probably wind up close, +/- some, to that horizontal array output.

3.) Be prepared for some sticker and code compliance shock when you find out the cost and hoops to jump through to put a sawtooth array on a horizontal roof.

To your idea or notion of a ballasted system: Some of the problem is unseen and usually not considered. That being that the existing flat roof was probably not designed to take what may be a rather large ballast load, in addition to the other dead loads and live loads. It may be OK, but the point is, it must be checked, usually by a P.E. (read $$ and time). Making things more complicated is the idea that it may not be possible to do that for any amount of $$ if drawings are not available to see what the design actually is. as an extreme , an alternate is to take the roof apart and verify what's there and then check it for the various loading conditions expected to be encountered by adding an array. Then, either beef it up and reroof, or simply button it up.

All this is often necessary on a horizontal roof because the ballast load can be substantial. Not only that, but as a practical matter and sometimes a code issue, the ballast itself must be attached to the roof by - you guessed it - mounting it to the roof so it doesn't "walk" over time, or blocking it with stops that are fixed by anchoring to the roof. There are ways to seal penetrations that have been around as long as roofs have been prevented from leaking. That some solar installers do a poor job of sealing and flashing penetrations or use recently conjured up methods that may not stand the test of time doesn't mean sealing is an impossible task - just that it takes more attention to detail than more than a few installer take or may even be aware of.

If a sawtooth (or any orientation) array is attached to any roof, regardless of that roof's tilt, if it's in an area where building codes exist, it must usually be shown to be affixed according to code requirements. Because arrays not parallel to their mounting surface may and usually do present additional forces and moments on the mounting surface/structure that arrays parallel to the mounting surface and structure do not present in the same magnitude or fashion, an additional engineering charge to estimate and design for those (additional) loads may be necessary.

I'd suggest you do your own investigation by calling your local AHJ (Authority Having Jurisdiction - the building code dept./building inspector/etc.) and asking them what's required.

As a practical matter, ballasted systems are not as reliable in a wind storm, or in an overall sense, as practical as a well designed fixation method. The key words there being well designed.

A suggestion: If you proceed and do nothing about it, your solar ignorance will cost you money. that's a correctable situation, Start by getting, reading and studying the book before you do anything else. Then run PVWatts after reading the help/info screens a couple of times. Then, fill in any knowledge blanks you create by your knowledge quest you can't answer on your own by asking informed questions here.

Take what you want of the above. Scrap the rest.

Add: BTW: As for being in a warm climate, I grew up and lived in central and western NY state for close to a half a century. That's also where I first learned about alternate energy and cut my solar teeth.

The criteria for good design is variable and somewhat climate dependent. The need and requirement for good design is independent of location.

[ay79]

1.) With frequent rain, 10 deg. will be better than horizontal, which will always be a sump, rain or shine, but that's only one issue. For any reasonable hope of cost effectiveness/ STC W you will most likely need panels mounted at an angle to the horizontal probably around your local latitude, maybe a bit less (but not "always"- just usually and mostly).

PVWatts is very helpful. Hard to estimate the snow factor since it snows a lot here in 2-3 big storms that then generally melt and it's hard to estimate ice buildup that may occur and how much it would impact production.
Interesting to note the law of diminishing return on the angle of the array. Holding all else equal, when going from 0^o to 10^o I gain 725 kWh. Going from 10^o to 40^o I pick up only an extra 600kWh. Engineering and ballast/retention should be far less significant holding at 10 degrees even with the axis of my roof being oriented 45^o towards southwest (225^o). I'll take a crack at designing the array using some high school trigonometry. A good exercise either way.

As a practical matter, ballasted systems are not as reliable in a wind storm, or in an overall sense, as practical as a well designed fixation method. The key words there being well designed.

A suggestion: If you proceed and do nothing about it, your solar ignorance will cost you money. that's a correctable situation, Start by getting, reading and studying the book before you do anything else. Then run PVWatts after reading the help/info screens a couple of times. Then, fill in any knowledge blanks you create by your knowledge quest you can't answer on your own by asking informed questions here.

The criteria for good design is variable and somewhat climate dependent. The need and requirement for good design is independent of location.

I'm off to pick up the book from a nearby library.

[ButchDeal]

Interesting to note the law of diminishing return on the angle of the array. Holding all else equal, when going from 0^o to 10^o I gain 725 kWh. Going from 10^o to 40^o I pick up only an extra 600kWh. Engineering and ballast/retention should be far less significant holding at 10 degrees even with the axis of my roof being oriented 45^o towards southwest (225^o). I'll take a crack at designing the array using some high school trigonometry. A good exercise either way.

Have you looked at putting the PV modules flat to the roof instead of trying to tilt them? What is the pitch of the roof?

Have you looked at the code requirements for ballasted systems on a residential system?

Also have you looked at the spacing differences from installing at 10^o vs 46^o vs flat to the roof facing southwest?

[ay79]

Have you looked at putting the PV modules flat to the roof instead of trying to tilt them? What is the pitch of the roof?

Have you looked at the code requirements for ballasted systems on a residential system?

Also have you looked at the spacing differences from installing at 10^o vs 46^o vs flat to the roof facing southwest?

The initial proposal was 0^o but the opinions on the forum have been to avoid a fully horizontal installation. The roof is flat/horizontal.

I've not looked at the codes but the installer can look into that - I'm not making this a DIY project.

I haven't done the math to establish the spacing - the installer's designer is running numbers now to figure that out.

[ButchDeal]

The initial proposal was 0^o but the opinions on the forum have been to avoid a fully horizontal installation. The roof is flat/horizontal.

I've not looked at the codes but the installer can look into that - I'm not making this a DIY project.

I haven't done the math to establish the spacing - the installer's designer is running numbers now to figure that out.

Residential flat roofs are usualaly pitched about 5 degrees. Proposals often use 0 degrees as short hand.
What is the actual pitch of the roof?

[ay79]

Residential flat roofs are usualaly pitched about 5 degrees. Proposals often use 0 degrees as short hand.
What is the actual pitch of the roof?

Pretty sure it's flat - 0^o. I'm looking at quotes that use a 10^o inclining racking system.

[ay79]

Can anyone offer thoughts on "overclocking" a system. I have a proposal for 9900W worth of panels (3 x 330 Panasonic) but the installer is recommending a 7.6kW Solaredge inverter. He says that they're more efficient than if I used a 10kW inverter.
Am I not just leaving 2300W on the table here? Granted that's only at peak but still. What am I missing?

[ButchDeal]

Pretty sure it's flat - 0^o. I'm looking at quotes that use a 10^o inclining racking system.

It is common to put a pitch on a FLAT roof even though the FLAT roof is pitched. Sometimes the pitch is in the wrong direction or just not enough.

If the roof is 0^o then there would be pooling of water on it.
Most of the residential ones we worked on were about 5^o some just 2.5^o

https://longroofing.com/blog/a-comprehensive-guide-to-flat-roof-systems/

[ButchDeal]

Can anyone offer thoughts on "overclocking" a system. I have a proposal for 9900W worth of panels (3 x 330 Panasonic) but the installer is recommending a 7.6kW Solaredge inverter. He says that they're more efficient than if I used a 10kW inverter.
Am I not just leaving 2300W on the table here? Granted that's only at peak but still. What am I missing?

it is not over clocking but over paneling.
It can be done without clipping if there are shadows or a lot of different angels.

The SolarEdge 7.6kW is just as efficient as the 10kW though both are HDWave models of the inverter and there would be no efficiency difference.
One difference would be that 7.6kw is often the largest you can interconnect in the breaker panel MSP, where a 10kw inverter generally requires derating the MSP or a line side tap. Both would cost a little more in labor. You would not be leaving 2.3kw on the table as the PV modules will rarely perform at STC (standard Test Condition) rating. Depending on your local conditions, tilt, azimuth, and shadows you might be leaving something much smaller on the table or nothing. You are most likely to hit over 7.6kw production on cool clear days which in your case are also lowest production with a low tilt and winter sun. It would have to be modeled to see though.

Did you mean 30 x 330?

[ay79]

it is not over clocking but over paneling.
It can be done without clipping if there are shadows or a lot of different angels.

The SolarEdge 7.6kW is just as efficient as the 10kW though both are HDWave models of the inverter and there would be no efficiency difference.
One difference would be that 7.6kw is often the largest you can interconnect in the breaker panel MSP, where a 10kw inverter generally requires derating the MSP or a line side tap. Both would cost a little more in labor. You would not be leaving 2.3kw on the table as the PV modules will rarely perform at STC (standard Test Condition) rating. Depending on your local conditions, tilt, azimuth, and shadows you might be leaving something much smaller on the table or nothing. You are most likely to hit over 7.6kw production on cool clear days which in your case are also lowest production with a low tilt and winter sun. It would have to be modeled to see though.

Did you mean 30 x 330?

I did mean 30 x 330.
no shade since I'm the tallest thing around (3rd story) and no trees nearby.
Azimuth is 228^o
Northeast so it's cool and we're not talking 100^o Arizona days
If both are efficient then why would I not spend the extra $175 quoted and get the 10kW inverter to minimize clipping?