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Thread: Roof Panels Cont'd

  1. #1
    George Valley Guest

    Default Roof Panels Cont'd


    I know someone who wants to put solar panels on a 20,000 sqft roof. Does anyone know how many panels would roughly fit onto this roof? Assuming it is totally flat.



  2. #2

    Default Re: Roof Panels Cont'd

    The roof measures what by what? Lots of different numbers come out to 20,00 sq. ft. Not all of them would accommodate the same # of panels. Which panel makes a difference too. My Sharp 175's measure about 33" x 62". The Evergreen 210's I was looking at for my porch roof are 36" x 65". Mounts may add some to the over-all dimensions, and positioning so that none of the panels are shaded by other rows will also use space (different amount depending on the angle needed for maximum insolation).

    Look up some specs, and start practicing that geometry you learned in high school!
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3
    Join Date
    Mar 2006
    SF Bay Area (California)

    Default Re: Roof Panels Cont'd

    With a flat roof, you have the question of installing the panels "flat" or pitched with spacing between rows such that the front row does not shade the row behind too badly during winter.

    Use the PV Watts program to work out the best array tilt angle vs flat for installed price.

    If you are in the lower latitudes (near the equator), flat arrays (or 5 degree minimum tilt for self cleaning of leaves and dirt) works out pretty well... If you are in the northern (closer to the north/south poles), then the panels will have to have a significant tilt (typical angle is equal to latitude of installation) and the panel rows need to be spaced further apart to reduce shading.

    Take some numbers--something like $7 per watt installed price ($6-$10 / watt for US turnkey install--very rough pricing)... And run the numbers with the PV Watts program to figure out kWhrs per year:

    (Installation Costs + Maintenance for 20 years - rebates) / (kWhrs per year * 20 years) = $$/kWhr

    Run is for a couple of different configurations and see what you come up with for pricing (or for a system that large, make the installer(s) run different the configurations past you).


    PS: Plus, if you are somewhere around Ottawa Canada--you will have the issue of snow and how to clear the roof without damaging the panels/installation.

    Normally, in areas with significant snow fall--mounting panels up high so that the snow can self-clear to the ground in front is helpful. Also, if you can tilt the panels between summer and winter positions--near vertical for winter helps with clearing too (although, for such a large installation, may not be practical).
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

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