Ground mount vs. Roof mount

[894tom]

This is a topic that I have found pretty much the same information on whatever link I click on. They all look like cut and paste articles but with different headers.

Ok so why another one? I am looking for a specific discussion not addressed in these cut and paste articles. The article states that there is not alot of difference between production amounts when comparing a roof mount install compared to a ground mount install. They are refering to all things being equal....ie same amount of sun exposer per day, same length of wire used, same array angles. Ok, but they mention the heat factor. How can that not play a factor? Is there a specific off set (number of inches) on the roof mount where the heat issue becomes negligible when compared to a free standing ground array? In other words are they saying that once you raise the panels say...6" off the roof its basically the same as a free standing ground mount array?

[fafcosolar]

Ground mount vs. Roof mount

I am looking for a specific discussion not addressed in these cut and paste articles.

I think that you will find that the heat difference in negligible unless the panels are essentially against the roof, but anecdotally I can tell you that I have seen no discernible difference between ground and roof mounted arrays at similar tilt angles in close proximity. In fact, we installed many Zep Solar mounted arrays (rail-less mounting system), which place the modules extremely close to the roof surface. I looked at their power production compared to Unirac Solarmount mounted arrays, and I found no significant difference. I could swear that Zep produced a paper about air flow and temperatures at on point, but I can't find it right now. It might be worth contacting them, because I'm sure they have addressed this issue before.

In case you want to get into the technical details of temperature coefficients with solar modules, here is a Sandia National Labs report: http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf

Interesting question... but I think you are splitting hairs about this issue. With PV module prices plummeting, it's becoming less important to eek out every last watt of power from you modules. Just install more!

Jason Szumlanski
Operations Manager
Fafco Solar

[russ]

I have not seen a study showing the benefit of say 6'' vs 4'' between the panel backs and the roof. Probably not much as the heat transfer from the roof to the panels is by radiation and the air movement will be there in either case.

One benefit of the higher mount is that leaves etc should have less of a tendency to get caught.

In a ground mount the panel backs only have the ambient temperature effect - no reflected heat.

[Mike90250]

When installing a ground mount, aim is not as constrained as a roof would be.

[cebury]

Could a large part of the reason there isn't much difference be due to the solar panels providing shading which stops a significant amount of radiant heat gain onto the roof itself? At that point, it's radiant + conductive heat upward from the attic temps, due to heat-gain from other parts of the roof which are exposed.

I know nothing of racking & solar panels, but as far as radiant heat transfer for temps we're discuss (<175F), once you get past 1" air gaps I know it's reduced exponentially.

[nipsip]

Ballast type ground mount or pole? IMO, if your land is rock free, fairly level, put in pole mounts. While you are at it you can dig extra and put the sonotube in for future expansion.

[vinniethePVtech]

Ground mount has far more pros than cons vs roof mount.

Cons of roofmounted.

Composite and steal roofs collect heat, one draw back to amperes.
Fixed tilt that can't be adjusted.
Higher level of maintenance.


Ground mounted systems like shleter are easy to install with no need for concrete piers.
Adjustable tilt and angles for seasons can be achieved using other than shleter

[Naptown]

Ground mount has far more pros than cons vs roof mount.

Cons of roofmounted.

Composite and steal roofs collect heat, one draw back to amperes.
Fixed tilt that can't be adjusted.
Higher level of maintenance.


Ground mounted systems like shleter are easy to install with no need for concrete piers.
Adjustable tilt and angles for seasons can be achieved using other than shleter

only a 40K machine to install that most likely cannot be rented.
If room allows I have found that a roof mount is about 1/2 the cost of a ground mount as far as the racking and installation go provided the roof is walkable. Ground mounts work however for utility scale projects where there is never enough roof.

[vinniethePVtech]

only a 40K machine to install that most likely cannot be rented.
.

There are companies that rent the pile driver. $2000 per week, $700 insurance deposit, plus shipping charges out of southern California, plus a certified operator for the equipment. Probably looking at a total of $4000+? That's roughly 10% of the equipment cost if it can be done in a week. 500kw of framing can go up in about 3 days, crew of 4 people. Its pretty impressive. The real expense is a geo tech or engineering inspector with costs of $300 per day to approve the post depths.

You are correct though. I've seen some pretty cheap roof mount systems built out of uni strut, and more expensive ones built out of uni rack. I'm not impressed with uni rack weeb locks how ever it's a real pita on install. I would prefer uni strut, with sun power IFF clips and 1 ground lug/6awg per string.

The only roof mount system that installs faster is S5! clamps for steel roofs, using either sunpowers IFF clip or sun edisons clip.

[nipsip]

There are companies that rent the pile driver. $2000 per week, $700 insurance deposit, plus shipping charges out of southern California, plus a certified operator for the equipment. Probably looking at a total of $4000+? That's roughly 10% of the equipment cost if it can be done in a week. 500kw of framing can go up in about 3 days, crew of 4 people. Its pretty impressive. The real expense is a geo tech or engineering inspector with costs of $300 per day to approve the post depths.

You are correct though. I've seen some pretty cheap roof mount systems built out of uni strut, and more expensive ones built out of uni rack. I'm not impressed with uni rack weeb locks how ever it's a real pita on install. I would prefer uni strut, with sun power IFF clips and 1 ground lug/6awg per string.

The only roof mount system that installs faster is S5! clamps for steel roofs, using either sunpowers IFF clip or sun edisons clip.

I would use sonotubes and have $10 an hour people wheebarrow the concrete to the sonotubes if necessary. If it is rock free, you can drill with an auger, otherwise a mini excavator will do the trick.

[vinniethePVtech]

I would use sonotubes and have $10 an hour people wheebarrow the concrete to the sonotubes if necessary. If it is rock free, you can drill with an auger, otherwise a mini excavator will do the trick.

Just make sure you use stainless steel pipe, even galvanized has a te dance to rust out after 10 years unless you use sn extremely thick gauged pipe.

[AnotherOldGuy]

In a related vein: what about a free-standing 'carport' type structure rather than ground mount? I am considering this, as the additional shade would be useful for parking a truck and tractor under. A shed-roof at an appropriate angle (probably lower pitched to produce more power in summer...a/c and such) would work well. I have just signed up for the forum as I really can't seem to find much out there on this subject. There are commercial parking structures, but no homeowner projects.

Perhaps I ought to start a thread on this.

[Naptown]

In a related vein: what about a free-standing 'carport' type structure rather than ground mount? I am considering this, as the additional shade would be useful for parking a truck and tractor under. A shed-roof at an appropriate angle (probably lower pitched to produce more power in summer...a/c and such) would work well. I have just signed up for the forum as I really can't seem to find much out there on this subject. There are commercial parking structures, but no homeowner projects.

Perhaps I ought to start a thread on this.

There are commercially available residential carport structures specifically designed to mount PV on. They can be pricey thuogh. You could always build a pole structure and mount your panels on it. I have one in the works now that may go through.

[angelinwilliams]

Roof mount
->Depending on the pitch and orientation of your roof, it may be possible to install the solar panels out of view.
->Beyond standard equipment, rooftop solar energy systems typically don’t require a lot of additional racking and other mounting hardware that's needed for ground-mounted systems.

Ground mount
Solar panels perform best in cooler temperatures. More air tends to circulate behind the solar panels of a ground-mounted array than those of a rooftop system. Because ground-mounted panels manage to stay cooler, they often outperform their solar roof counterpart.

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