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  • Best roof pitch?

    Hello. I'm in the design phase of my home in Iowa and have wondered, if I install solar panels its there a certain pitch roof that would be best? Or as long as the roof is walk-able like a 5:12 pitch, I should be fine in the future?

    Thanks

  • #2
    Run PV watts for your location.enter your data and then vary the tilt angles.

    Comment


    • #3
      Originally posted by peakbagger View Post
      Run PV watts for your location.enter your data and then vary the tilt angles.
      Thanks.. The PV watts calc says tilt in degrees should be 20 for Iowa City Iowa..

      Would there be a advantage building a 4:12 (18.5 degrees) or 5:12 (22 degrees) roof?

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      • #4
        If you have snow I would err on the steep side. On the other hand a shallower pitch saves a bit of plywood and shingles plus its easier to walk on.

        A BTW, do yourself a favor and put ice and water shield from the edge of the roof all the way to the top of the panels. The area under the panels is not the place for roof leaks.

        Comment


        • #5
          Originally posted by peakbagger View Post
          If you have snow I would err on the steep side. On the other hand a shallower pitch saves a bit of plywood and shingles plus its easier to walk on.

          A BTW, do yourself a favor and put ice and water shield from the edge of the roof all the way to the top of the panels. The area under the panels is not the place for roof leaks.
          To bad no one makes a standing seam panel with PV built in.

          So when the PV watts calc says tilt in degrees should be 20 for Iowa City Iowa.. Is that a average angle between winter and summer?

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          • #6
            PV Watts looks at long term weather conditions for specific areas and calculates what a possible yearly output is for a fixed array. If you want to put in an adjustable angle array its a different story.

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            • #7
              The best angle for summer production is likely very poor at rejecting snow. That may
              be fine, because roof mounting is difficult to clean, write off winter production. A
              summer and a winter angle is much better, but quite difficult on a roof. Bruce Roe

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              • #8
                Originally posted by rebar View Post

                Thanks.. The PV watts calc says tilt in degrees should be 20 for Iowa City Iowa.
                Not questioning your results, but IIRC for maximizing annual production at 1 angle, that surprises me. USUALLY, an angle approximating your latitude is optimum ... about 40 degrees. Recheck your input data.

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                • #9
                  Because of the long days and clear weather in the summer, I generally have a daily surplus, and I run my array at winter angle all year round, to harvest whatever sun there is in the winter.
                  (off - grid)
                  Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                  || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                  || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                  solar: http://tinyurl.com/LMR-Solar
                  gen: http://tinyurl.com/LMR-Lister

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                  • #10
                    Originally posted by gbynum View Post

                    Not questioning your results, but IIRC for maximizing annual production at 1 angle, that surprises me. USUALLY, an angle approximating your latitude is optimum ... about 40 degrees. Recheck your input data.
                    But I'm questioning the results of pvwatts.. 20 degree's sounds to flat for Iowa City. Is there another non profit calc I can use?

                    Comment


                    • #11
                      Originally posted by rebar View Post

                      Thanks.. The PV watts calc says tilt in degrees should be 20 for Iowa City Iowa..

                      Would there be a advantage building a 4:12 (18.5 degrees) or 5:12 (22 degrees) roof?
                      PVWatts says no such thing. That's a default value.

                      Before you go any further, download a free but slightly dated version of "Solar Power Your Home for Dummies, or get a new hardcopy for ~ $25 at bookstores/Amazon. You need more information to make more informed/better decisions.

                      The array azimuth and tilt PVWatts uses are user inputs. You vary both along with the other user inputs for optimum annual production or what ever you want to optimize.

                      Reads to me like you may not know how to use the model.

                      I'd SWAG your best orientation for a residential situation with net metering will be at an array tilt of approx. the local latitude and an array azimuth about true (not magnetic) south.

                      But, there are lots of other considerations that will modify the optimum orientation/placement and also the optimum system sizing.

                      Read the book. and come back with questions to fill in any gaps your knowledge acquisition creates.

                      If you are building, know that your energy $$'s will go a lot farther and be much better spent on conservation measures before any sexy solar devices.

                      Getting your building heat gain/losses/energy use as low as possible before PV or other active solar devices is a whole lot cheaper and so more cost effective (and easier) that throwing what's about the most expensive and so close to the least cost effective method of supplying energy to a larger than necessary energy load.

                      Welcome to the forum of few(er) illusions.

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                      • #12
                        Optimal solar inclinations, rough approximation, by latitude
                        https://www.solarpaneltilt.com/

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                        • #13
                          Latitude of Iowa City, IA is 41.6611° N

                          41.6611 * 0.76, plus 3.1 degrees = 34.76 degrees for optimal (~71% of 2 axis yield) year round harvest.

                          Is your PoCo Net Metering? How frequently due they true up (annual)? Can you set/change your anniversary/commissioning date?

                          Do you get much snow?

                          If I had the option to build a new house from scratch, I would definitely opt for a steeper pitch so that I might be able to walk upright in the attic or perhaps turn it into a spare room. Similarly, I would not use pre-fab trusses for the same reason.

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                          • #14
                            Try the Solar Tilt app on your phone. Really cool and a keeper.

                            https://play.google.com/store/apps/d...olartilt&hl=en
                            2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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                            • #15
                              All the above is good advice. I agree that a steeper pitch is better for best production. Also see if you can design plumbing vents to be on the north pitch to maximize your South pitch for simplest installation and future proofing.
                              9 kW solar, 42kWh LFP storage. EV owner since 2012

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