Solar Panels on a Flat Roof

[Bad Dolphin]

That sounds great for a low-profile, yet low maintenance install. The install looks great and the price is great, too! I have an almost flat room myself (self-installed) as posted in the other thread with the panel mounted flat to the roof, which ends up being 3-4 degrees and it's not enough angle to fully self clean the panels. 15 degrees should be enough. But I need to add more solar and am trying to decide between doing it myself, getting a local contractor and using Tesla. It's also about time to reroof, too. How old is your roof, it looks basically new?


Yeah, a rough rule of thumb for optimal year round production is to mount the panels at the same angle as your latitude. It's especially important to mount them steeper if you want to maximize winter time production, though depending on typical cloud cover, you may get more annual production by using a flatter angle. Lots of tradeoffs involved.

Our understanding is the same—latitude for panel angle. Here in Hawaii, we’re at 22 degrees.

Not clear on what you’re suggesting for flatter angle for cloudy days? Our rainy season is winter, when the sun’s angle is lower in the sky. Since power is generated by light—whether it’s direct sun or through clouds—I would think that the steeper angle is better so the one Abel is closest to pointing directly at the sun? Unless you’re suggesting that the clouds scatter the light, making it less directional and coming more uniformly from the whole sky so flat panels might be better?

 

[Zeronet]

I have a home with a flat roof and when I posted some questions about the install a few months ago it seems like there was some doubt as to whether Tesla did panels on flat roofs.

I was able to fit 18 panels on my 25' x 23' flat-roofed garage by adding a small patio roof on the south side (left).

This allowed space for an 18 panel, 5 kW array, that not only covers the home electric including all of the heating and cooling HVAC (electric heat pump), but all Tesla charging with the garage home charger (from Tesla). This is possible because the house has foot-thick ICP walls and an ~ R100 spray foam ceiling as well as some passive solar features. You results could differ.

Solar panels are a no-briainer for anyone in a sunny southern state that can get a "net-metering" agreement with their electric company. Net-metering means that they give you a kWh credit for every kWh you generate. At night and in the winter, you draw down your kWh credits to pay for electricity, making it essentially free after you pay for the array and electric inverter. In Arizona I have to pay an $18 monthly fee for the priviledge of being connected. And they have cut off all new net-metering agreements because the home solar was competing with their giant coal-burning power plants which are after all, their primary business. But if you can get the net-metering agreement, you could be grandfathered in for 10-20 years.

My system cost about $30,000 (including the new roof) and was installed by a local electrician and renewable energy contractor. It was eligible for about a 40% income tax credit reducing my costs to $18,000. You should check to insure that the federal tax credit and possible state tax credits are still available.

Electric energy is FREE from the air. As the solar panel cost keep going down, and their efficiency keeps improving, the electric industry is in for big changes.

 

[Dave EV]

Not clear on what you’re suggesting for flatter angle for cloudy days? Our rainy season is winter, when the sun’s angle is lower in the sky. Since power is generated by light—whether it’s direct sun or through clouds—I would think that the steeper angle is better so the one Abel is closest to pointing directly at the sun? Unless you’re suggesting that the clouds scatter the light, making it less directional and coming more uniformly from the whole sky so flat panels might be better?

Yeah, there's two things to consider when it's cloudy:
1. As you suggest, solar production is generally better with flatter angles than steeper angles due to the sun's rays being diffused. I don't know how much, though, I'm sure someone has the details.
2. If your goal is to maximize annual production and it's very cloudy in winter, you may be better off with less tilt angle and getting more energy in the summer.

 

[nickw252]

in a sunny southern state that can get a "net-metering" agreement with their electric company. Net-metering means that they give you a kWh credit for every kWh you generate. At night and in the winter, you draw down your kWh credits to pay for electricity, making it essentially free after you pay for the array and electric inverter. In Arizona I have to pay an $18 monthly fee for the priviledge of being connected. And they have cut off all new n

Your setup looks amazing. I also have a cabin in Pine with a shingle roof that faces due west. I'm considering getting solar there, however, my electric bills average about $100 per month (I have gas heat and water heater).

Do you have APS in Flagstaff? Pine has APS and I haven't looked at their solar plans and connection fees.

 

[brur]

APS drops you in a special energy agreement when you install grid-tie solar. Peak energy costs climb while solar producing times are reduced almost to a tenth. So what you return to sender earns you 3 cents, while energy during peak usage is 24 cents. Peak is from 3 to 8 wintertime iirc. You can still make the economics work but it is frustrating to see what the provider is doing.
If you were grandfathered in you have a much more innocent agreement, but those days are gone.

I am astonished to read the 5kw rooftop exceeds his tesla consumption. Perhaps he doesn't drive much?

 

[tezzzla]

How much less efficient is the flat installation compared to an optimum angle (approx. equal to latitude)? Or are panels better now and it's not so important. Owner of a flat roof at 34-degrees latitude.

 

[Dave EV]

How much less efficient is the flat installation compared to an optimum angle (approx. equal to latitude)? Or are panels better now and it's not so important. Owner of a flat roof at 34-degrees latitude.

Depends - easiest way to estimate for yourself is to use the PVwatts website.

 

[tezzzla]

Depends - easiest way to estimate for yourself is to use the PVwatts website.

Thanks. Between 0 and 33 degrees, an approx 9 percent difference. But only 1 percent between 23 and 33 degrees

 

[tezzzla]

I have rolled asphalt, which has been covered by elastomeric coating.

How are the mounts attached? Look like they are bonded somehow which would be nice. We have the same roof, elastomeric coating this year. No hurricanes in Southern California.

And how did they pick the angle?

 

[nickw252]

Depends - easiest way to estimate for yourself is to use the PVwatts website.

That website is amazing. I compared my setup (15* tilt) to what it would be with a normal roof (30* to 45* tilt) and it looks like my low tilt is better for the summer but not as good in the winter. This works for me though because I have a gas furnace and gas water heaters.

 

[nickw252]

How are the mounts attached? Look like they are bonded somehow which would be nice. We have the same roof, elastomeric coating this year. No hurricanes in Southern California.

And how did they pick the angle?

The mounts are attached with lag bolts bolted into the roof rafters. This was made easy because I have the blueprints to my house showing exactly where the rafters are and what size they are. Tesla then sealed up every mount location with a caulk sealant called M1. It's sealed in three areas (under the mount casing, inside the mount casing, and outside the mount casing).

I've had roof issues before and made lots of repairs myself so I paid close attention to their work. It looks like they did an outstanding job on the install and I don't have leak concerns.

Also with respect to your "no hurricanes" comment - CA probably has even better weather than AZ because we have the summer monsoons with microbursts that can result in 60-80+ MPH winds. I don't think CA has that type of weather.

 

[nickw252]

First full day of generation. It was a perfect sunny December day in central Arizona. I'm happy with that amount of generation from a 10.54 KW system (31 panels).

 

[RideTheWood]

Nice! This was the email I got from Tesla about installing on my flat roof in Texas (they won't), but it looks like you're in a state where they have more options. This is what (and where) they'll install on:

• Composite Shingle
• Concrete Tile
• Standing Seam Metal
• Rolled Asphalt (Only available in AZ, CA, HI, NV, NM)
• Foam (Only available in AZ and NM)
• Corrugated Metal (Only available in HI)

In the list from Tesla, they include standing seam metal roofs - was that list specific to your flat roof, or did they indicate they'll install on all standing seam roof (within reasonable pitch angles)? I am putting on a new standing seam roof (7:12), which is the best option for the high elevation where I live (7800 feet) and heavy snow load we can receive (typ. 250" to 500" per year). Curious if anyone has had Tesla install on standing seam metal?

 

[Rebeccastanke]

It's a 10.54 KW system. It takes up about 700 - 800 square foot of roof space. The cost was just a shade over $21,000

I would love to know who you worked with at Tesla. I’m having issues with their designers trying to put flat panels on my flat roof. I need a designer who knows how to design for flat roofs. Can you call or text me the person contacts info at Tesla. Rebecca 480-227-0246

 

[Gulbuth]

In conversation with my Tesla contact, I asked about a 20 or 30 degree tilt angle for some of my panels. He said no, 10 degrees is the only tilt that Tesla will install. Has anyone been able to specify a particular tilt for their installation? Thanks!

 

[Dave EV]

In conversation with my Tesla contact, I asked about a 20 or 30 degree tilt angle for some of my panels. He said no, 10 degrees is the only tilt that Tesla will install. Has anyone been able to specify a particular tilt for their installation? Thanks!

Flatter panels aren't necessarily bad if you can install more panels since row spacing will be less with the lower tilt angle. You'll get more energy in the summer, and about the same in the winter and more energy annually.

All numbers below are approximate and generated using PVWatts in Tuscon as an example.

In Tucson, 1 kW of 10* tilted panels will generate 1715 kWh / year with a min of 102 kWh in Dec and 189 kWh in May.
1 kW of 25* tilted panels will generate 1803 kWh / year with a min of 127 kWh in Dec and 179 kWh in June.

But you have to space the panels about 23% farther apart to minimize shading in the winter with the 25* tilted panels, so if you went with the lower tilt, you can fit about 23% more panels which would generate 125 kWh in Dec, 233 kWh in May and 2110 kWh for the year.

 

[Gulbuth]

Flatter panels aren't necessarily bad if you can install more panels since row spacing will be less with the lower tilt angle. You'll get more energy in the summer, and about the same in the winter and more energy annually.

All numbers below are approximate and generated using PVWatts in Tuscon as an example.

In Tucson, 1 kW of 10* tilted panels will generate 1715 kWh / year with a min of 102 kWh in Dec and 189 kWh in May.
1 kW of 25* tilted panels will generate 1803 kWh / year with a min of 127 kWh in Dec and 179 kWh in June.

But you have to space the panels about 23% farther apart to minimize shading in the winter with the 25* tilted panels, so if you went with the lower tilt, you can fit about 23% more panels which would generate 125 kWh in Dec, 233 kWh in May and 2110 kWh for the year.

 

[Gulbuth]

Thank you Dave. That‘s helpful

 

[Vines]

Thank you Dave. That‘s helpful

Also, a tilt like 25 degrees is nearly the height of an average kitchen counter. This goes from an object low to the roof without much lever arm on the roof attachments, to a larger sail which creates more force from wind resistance, and also that overturning force is multiplied through a longer lever arm to the roof attachment.

When modules were very expensive ($4-5 per watt), then custom racking solutions to catch every watt mattered more since the module cost itself was such a huge portion of the cost. Now that modules are 20% of that or less, very often designers these days will rightfully give up some specific production (kWh per year per nameplate watt DC installed) for an overall cheaper design (kWh per Dollar) and a larger overall system with higher yearly production.

 

[Sascher18]

Any issues with the brackets screwed into foam roof?