Asking for performance advice

solarhours

Hello everyone,
After reading some of the posts here, I was hoping some of the bright minds that interact on this forum could help me with a system I am having installed at the end of Oct.

I need advice on what to ask for from the installers in order to get the most kWh production out of my system. I have a flat roof, no shading issues, modules will be placed no more than 60 ft. from my meter and, we get about 6.25 solar hours a day here.

The size of the system to be installed is 4.56 kWh. It consists of 20 M 190 Enphase microinverters, 20 228 Sun Power Serengeti modules and, they will be installed utilizing the Uni-Rac G10 Rapid Rac balast mounting system. I will also get 5 years of Enphase monitoring.

If anyone has any suggestions on how to get the most out of this system or, if anyone has any concerns with the system itself, I would love to hear from you. Thank you all for your time and thank you for educating the newbies like myself.

russ

Re: Asking for performance advice

What is your location? You design a system based on production in the winter normally - when there is far less than 6.5 hours of usable light.

Then the guys can most likely give decent suggestions/comments.

solarhours

Re: Asking for performance advice

I live in Rio Rancho New Mexico. Temps are hot in summer. I think the best production comes in fall and spring time. Thanks

solar_dave

Re: Asking for performance advice

From pvwatts web site for Albuquerque - about the same place as Rio Rancho:
http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/US/New_Mexico/Albuquerque.html
This assumes direct southern exposure with no shading and a 35 degree tilt on your panels.

"Station Identification"
"City:","Albuquerque"
"State:","New_Mexico"
"Lat (deg N):", 35.05
"Long (deg W):", 106.62
"Elev (m): ", 1619
"PV System Specifications"
"DC Rating:"," 4.6 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 35.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 5.33, 575, 50.02
2, 6.06, 577, 50.20
3, 6.44, 677, 58.90
4, 7.16, 697, 60.64
5, 7.40, 722, 62.81
6, 7.10, 653, 56.81
7, 7.13, 675, 58.73
8, 7.02, 671, 58.38
9, 6.71, 626, 54.46
10, 6.55, 659, 57.33
11, 5.73, 580, 50.46
12, 5.14, 556, 48.37
"Year", 6.48, 7667, 667.03

solarhours

Re: Asking for performance advice

Thanks for the info. That is actually a little over what my installer estimated but close. I am wondering if maybe those numbers are a little optimistic. My system will be installed with no penetrations using the Uni-Rack 60 system. I think I read where that means they will be installed at a 10 degree angle and not the optimum 35 or 36 degree angle you mentioned. Do you think that will make a big difference in production? Also, someone mentioned making sure the Solar Modules are installed 6 inches above the roof to improve ventalation and keep the panels cooler. Would that decrease the temp enough to help production and is it possible with a balast system like the one I am using to install it 6 inches above the roof? How about the Enphase microinverters? I know they have a longer warranty than most inverters but, I have no shading issues and was wondering if they impact output in any way. I do like the idea of being able to add on to my system later if needed but, I am hoping I don't have to pay a price for that. In other words, are the Enphase micro-inverters better to install or should I go with a string inverter? Finally, what about the cables? What size should be used to optimize output and minimize power loss? I am sorry. I know I have a lot of questions but, I have talked to so many installers, I just want to make sure I am getting the right info. Thanks to anyone who can help.

drees

Re: Asking for performance advice

Here are the PVwatts estimates for 10* mount:

"Station Identification"
"City:","Albuquerque"
"State:","New_Mexico"
"Lat (deg N):", 35.05
"Long (deg W):", 106.62
"Elev (m): ", 1619
"PV System Specifications"
"DC Rating:"," 4.6 kW"
"DC to AC Derate Factor:"," 0.770"
"AC Rating:"," 3.5 kW"
"Array Type: Fixed Tilt"
"Array Tilt:"," 10.0"
"Array Azimuth:","180.0"

"Energy Specifications"
"Cost of Electricity:"," 8.7 cents/kWh"

"Results"
"Month", "Solar Radiation (kWh/m^2/day)", "AC Energy (kWh)", "Energy Value ($)"
1, 3.93, 419, 36.45
2, 4.86, 463, 40.28
3, 5.81, 613, 53.33
4, 7.15, 701, 60.99
5, 7.99, 792, 68.90
6, 7.99, 744, 64.73
7, 7.80, 746, 64.90
8, 7.19, 691, 60.12
9, 6.24, 586, 50.98
10, 5.41, 547, 47.59
11, 4.31, 432, 37.58
12, 3.68, 387, 33.67
"Year", 6.04, 7120, 619.44

I'd expect that you'd see numbers about 10% higher than that.

With all your panels mounted the same angle/direction without shade, you aren't going to see a big difference in overall performance with micro inverters compared to a central inverter.

6" off the roof should be pretty good - a bit higher would be better with the temps you're going to be seeing, but won't be bad. Is your roof in good shape and highly reflective (white)? A white/reflective roof will help keep temps down, too.

If you could get more tilt that'd increase output, especially in the spring/winter/fall when temps are cooler.

With Enphase, you're using their cabling up on the roof typically - from there down to the panel you're probably looking at 10-12 GA depending on the length of the run. You'll probably have 11 panels/string so you won't be maxing out the current on the line. One trick to minimize wiring losses with Enphase is to tap the string in the middle instead of the end - I'm sure someone could calculate the difference that might make.

BB.

Re: Asking for performance advice

The other question was about ballasting vs bolting to the roof.

Assuming you are in Ohio--Do you have high winds (will the panels stay on your roof)? Do you have snow/freezing slush (weight of ballast on roof+other loads)? etc...

Normally, a licensed PE (Professional Engineer) would need to sign off for the building department approval of the structural loading of the roof and the array. Has anyone done that yet? A local PE would know more about the weather patterns in your area than most of us here. And his behind is on the line when he stamps the plans as approved (by the way, most building inspectors/departments in our area have no liability if they a approve a installation that does not meet code).

-Bill

solarhours

Re: Asking for performance advice

Thanks for the info. guys.

Drees,
10% higher than the number you came up with is about what the installer quoted me and that is fine with me. I think he said 7907kWh per year.

I thought about painting my roof white but in Albuquerque there was recently a controversy about how much more efficent a reflective white roof would make your house. The argument is that while it cools your house in the summer, it also cools it in the winter and increases your heating bills. Now that you mention roof and efficency, that reminds me of another question I would like to ask you. Most of my panels will go on the roof above my garage. While my house is well insulated, my garage has no insulation. Do you think that having no insulation in the roof between my garage ceiling and the shingles will cause my roof to be warmer or cooler? Usually insulation keeps the heat out which I assume would direct it back out to the attic and back into the shingles. But since I have no insulation and the heat comes into the garage itself, do you think my roof is lower in temperature than it would be if I had insulation? My house faces almost directly south so, I have a lot of exposure to the sun where my garage faces.

One last thing, is it worth it to get a racking system that penetrates into my roof so that I can get that tilt? Seems like the increase in production wouldn't be that great as to offset the chance I would be taking with my roof springing a leak. Am I wrong?

Bill, I live near Albuquerque New Mexico. We don't have that much snow but we do get some wind. The company I am using to install the system is one of the largest Solar names out there. I am more worried about optimizing production than I am about the quality of their work.

Thank you both for responding.

BB.

Re: Asking for performance advice

I would guess that insulation above the garage ceiling with lots of air flow from soffit to peak to keep it cool.

Some people where using roof vents that could be closed (automatically by temperature?) to try and keep home warmer in winter.

I guess the downside with trapping heat is that you also trap moisture and can end up with mold/rot/etc. in the attic with modern "sealed homes" (vs the old homes where you could see light between the wall sheeting).

In the end, I am not sure there any major drawbacks to lots of ceiling insulation and a good amount of attic ventilation (winter or summer), at least from what I can see (I live in a very temperate climate--so take that with a grain of salt)--Other than for those of us who use the attic for storing 1/2 our life--e.g., junk. :roll:

If you are placing solar panels on the roof--that will cut down tremendously on the roof / attic temperatures (sure made mine cooler).

-Bill

solarhours

Re: OT

Bill, I just want to say how much I love your city. I have only visited a few times but I love it more every time I go. I took my family with me last time and they fell in love with it also. You are a lucky man to be living in such a beautiful place. Enjoy the day.

BB.

Re: Asking for performance advice

We live south of San Francisco on the Peninsula... It sure is a nice place to live and take relatives on tours.

Last year we spent a week driving around New Mexico--A very beautiful state in its own right. Carlsbad Caverns are just amazing. We really enjoyed Santa Fe too.

-Bill

drees

Re: Asking for performance advice

solarhours wrote: »

One last thing, is it worth it to get a racking system that penetrates into my roof so that I can get that tilt? Seems like the increase in production wouldn't be that great as to offset the chance I would be taking with my roof springing a leak. Am I wrong?

The vast majority of systems that are installed are installed using penetrations.

Properly done, the work will last as long as the rest of the roof.

Sounds like your roof is (nearly?) flat - a typical flat roof requires more work to properly guard against leaks.

For example, my roof is a rolled bitumen type roof. Each standoff had these steps:

1. Locate/drill lag bolt pilot hole.
2. Install standoff using appropriate sealant under standoff.
3. Heat and scrape off excess rock on surface to prep for flashing install.
4. Apply elastomeric roofing sealant under flashing.
5. Apply sealant on top of flashing w/roofing tape around edge of flashing.
6. Cut and install roofing patch over/around flashing w/additional sealant.
7. Sprinkle sand around edges of roofing patch to reduce sun exposure to any exposed sealant.

That was a very messy and time consuming job.

Some will take a shortcut and just rely on the sealant under the standoff to seal the roof (on tilted roofs, too), but a roofer would never accept that kind of work. Properly installed flashing should be used for any roof penetration.