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  • Sun number

    I just noticed that zillow has added a 'sun number' feature projecting solar potential based on location and home details. Not sure when they added that. My score was 79 I think.

  • #2
    Google: Energy.gov. + sun number

    Probably a pretty harmless idea, but of limited use. Get informed about PV and you'll have a better probability of not getting screwed by your own solar ignorance.


    • #3
      For my house in Poway, CA the sun number is 87. The solar I put in earlier this year is outperforming the estimate my bid was based on. Perhaps we are having an above average amount of sunny days this year in the San Diego area.


      • #4
        Originally posted by RichardCullip View Post
        For my house in Poway, CA the sun number is 87. The solar I put in earlier this year is outperforming the estimate my bid was based on. Perhaps we are having an above average amount of sunny days this year in the San Diego area.
        I believe you may be experiencing some of what I've got about 15 or so miles north of you.

        Also, if your vendor is like most every vendor I've encounters over the last 11 years around here, and that's where your estimate came from, I'd bet money that estimate was done to produce intentionally low output to sell a larger system. No peddler ever got fired for selling too much. Besides, oversizing keeps a lot of solar ignorant folks happy when the think their system is more efficient than they bought.

        Any model can be manipulated just the same as I do. All the output is whatever you want it to be. To my experience and as a SWAG, the oversurfacing of arrays around here seems to run ~ 10 % or so and often more if sold to senior citizens.

        But, as an example of what it's been like around here lately as far as solar weather is concerned, I've got array weather input including irradiance data in 1 minute increments, and array output data in 5 minute increments since my array got PTO 10/17/2013.
        I've been sorting, slicing and dicing all of it in various ways ever since.

        I've got running 31 day totals for a lot of stuff including actual daily output.

        To your comment about the recent past being sunnier than usual, I believe it has been.

        For the last 31 days, my array has produced 121 % of the PVWatts modeled output for the same period. That PVWatts annual output has been adjusted using the PVWatts system loss parameter to match SAM's annual output as much as possible.

        Matching this year's immediately prior 31 days' actual system output (Note: not modeled output),10/8/19 - 11/8/19 inclusive to prior years' actual outputs for the same calendar period:

        2019/2018 = 108.2 %
        2019/2017 = 121.7 %
        2019/2016 = 114.6 %
        2019/2015 = 109.6%

        I don't have the 2013 data for the same period (yet) because the array didn't go live until 10/17/2013. Thus, the 2019/2013 comparison for the period 10/08/ to 11/08 won't be filled until after 11/17/2019.

        I'm looking at the data and thinking that the TMY data used for the Miramar MCAS for the autumn fall around here may just be a bit pessimistic.

        FWIW, since 10/17/2013 the array has produced 55,220 kWh over 2,212 days; 9,112 kWh/365 days, with that number adjusted down by 0.0051 to account for a calibrated difference between what the array monitor shows and what the SDG & E meter shows. The way I've got the PVWatts model set up, it pukes out 9,161kWh/yr.

        System size, 5.232 STC kW.
        Array tilt: 18.75 deg.
        Array azimuth 195.75 deg.
        Measured output per STC kW = 9,112 kWh/yr./5.232 STC kW = 1,742 STC kWh/yr per installed STC kW.

        Take what you want of the above. scrap the rest.,

        Lots more data. too little space and pretty boring anyway.


        • #5
          My Zillow Sun Number is 62.06 and I live in sunny San Diego Country with a roof which faces almost due South with little shading. Go figure.


          • #6
            My sun number: 94.78. Saratoga, Ca. And every neighbor around me is exact same number, even those facing 45 or 90 degree different direction. So not sure what that number really does for anyone.

            From Zillow site:
            The Sun Number Score technology uses a detailed roof analysis to determine how much roof area is suitable for solar based on the pitch, orientation, and size of each roof plane, as well as the amount of sunlight the roof receives based on surrounding obstructions like trees or taller buildings.
            Based on comparing to houses with very different situations around me who all have the same number, I call BS on their description above!
            Last edited by blueman2; 11-08-2019, 07:11 PM.


            • #7
              How do they know what the pitch of my roof is? Are they going around measuring everyone's roof pitch? You can't see that on a map. I say Sun Number is BS just like Zillow's Zestimate which is also BS.


              • #8
                It is possible that Zillow is using a LiDAR app that measures the solar reflective energy values coming off of a roof.

                I believe Butch is familiar with that app.


                • #9
                  I just compared my roof and my neighbors roof with Zillow Sun Number and Google Sunroof. My neighbor has a big Oak tree shading 2/3 of his south facing roof and Google Sunroof was more accurate. Sun Number shows both houses at 90 and Sunroof shows my neighbor has less sun hours and less square footage available. Overall the houses actually have similar roof areas.
                  Last edited by Ampster; 11-09-2019, 10:39 AM.
                  9 kW solar. Driving EVs since 2012