Not the ideal orientation

solemio
solemio Registered Users Posts: 16 ✭
in Solar Beginners Corner #1
We have an eastern facing roof-30 degrees pitch, that is in unshaded sun for at least 10 hours a day from 0630 to around 1630. We signed a contract to install a 8.5kw ac cec / 9.9 kwdc. Does anyone else have a similar exposure (eastern) with success?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #2
    Re: Not the ideal orientation

    My roof faces south eastern and works fine (I have trees due west that knockout late afternoon sun too).

    For some people that live in hot areas (like Florida)--you can even generate more kWhrs in a day because solar panels tend to output less power in the heat of the late afternoon.

    For me--facing east of south is a bit of a drawback because I have a 1 year net metered Time of Use plan (the old E7) which pays me for noon-6pm power generation ($0.30 per kWhr peak, vs $0.09 per kWhr off peak in summer). So, I would generate a bit more "offset" "cash" for my account if I had a more westerly exposure.

    The PV Watts program does a good job of predicting your average output based on array orientation (probably does not account for thermal losses from hot arrays).

    If you live in an area with heavy morning marine layers and afternoon sun--an easterly biased array could cost you power. If you have afternoon/evening clouds--then it will be a help.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #3
    Re: Not the ideal orientation

    if you are facing due east you will not reap even as much as a southeastern one would and that 10hrs of sun will not be 10hrs of the rated intensity the pvs are rated for at full power. you may get 3 or 4 hrs of that full sun intensity over the course of the day and does vary for location and especially for the time of the year for the winter solstice will show a much reduced amount of power as compared to one facing southerly.
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  • solemio
    solemio Registered Users Posts: 16 ✭
    #4
    Re: Not the ideal orientation
    BB. wrote: »
    My roof faces south eastern and works fine (I have trees due west that knockout late afternoon sun too).

    For some people that live in hot areas (like Florida)--you can even generate more kWhrs in a day because solar panels tend to output less power in the heat of the late afternoon.

    For me--facing east of south is a bit of a drawback because I have a 1 year net metered Time of Use plan (the old E7) which pays me for noon-6pm power generation ($0.30 per kWhr peak, vs $0.09 per kWhr off peak in summer). So, I would generate a bit more "offset" "cash" for my account if I had a more westerly exposure.

    The PV Watts program does a good job of predicting your average output based on array orientation (probably does not account for thermal losses from hot arrays).

    If you live in an area with heavy morning marine layers and afternoon sun--an easterly biased array could cost you power. If you have afternoon/evening clouds--then it will be a help.

    -Bill

    We live on an Eastern facing slope at 1800' altitude. We are above the marine layer and therefore morning and early afternoon is prime time for us. I hope this works as well as promised.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #5
    Re: Not the ideal orientation

    The PV Watts program has bee pretty accurate for me... There is a detailed output where you can see how much power you lose after, for example, 4pm.

    You can run multiple orientations/options and see how they compare.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solemio
    solemio Registered Users Posts: 16 ✭
    #6
    Re: Not the ideal orientation

    Our contractor used the CPI EPBB caclulator and convinced us that the system would produce 13800 KW per year. I guess we'll find out in September when the system is installed how well it actually works. In the mean time I feel like we are wasting sunlight until the installation is complete and running.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #7
    Re: Not the ideal orientation

    Here is a guess based on what you have said here... Assuming you are near Tonopah NV (or at least similar weather), we would expect from a south facing system:
    [FONT=Fixedsys]Results 9.9 kW [/FONT][SIZE=3]Tonopah [/SIZE][FONT=Fixedsys]NV 30 degree roof, South

Month
Solar Radiation (kWh/m2/day)
AC Energy (kWh)
Energy Value ($ at[SIZE=3] 9.7 ¢/kWh[/SIZE][/FONT][FONT=Fixedsys])

1      4.19           995         96.52   
2      5.12          1074        104.18   
3      6.01          1392        135.02   
4      6.80          1480        143.56   
5      6.75          1494        144.92   
6      7.48          1522        147.63   
7      7.53          1562        151.51   
8      7.43          1547        150.06   
9      7.09          1443        139.97   
10     6.24          1374        133.28   
11     4.79          1047        101.56   
12     4.25           995         96.52 
========================================  
Year   6.14         15925       $1544.73    
[/FONT]
    and for your setup facing due East 
    Results 9.9 kW [/FONT][SIZE=3]Tonopah [/SIZE][FONT=Fixedsys]NV 30 degree East
[/FONT][FONT=Fixedsys]
Month
Solar Radiation (kWh/m2/day)
AC Energy (kWh)
Energy Value ($ at[SIZE=3] 9.7 ¢/kWh[/SIZE][/FONT][FONT=Fixedsys])

[/FONT][FONT=Fixedsys]1      2.55           578         56.07   
2      3.47           709         68.77   
3      4.56          1056        102.43   
4      5.90          1286        124.74   
5      6.62          1472        142.78   
6      7.58          1551        150.45   
7      7.53          1575        152.78   
8      6.82          1423        138.03   
9      5.76          1178        114.27   
10     4.37           956         92.73   
11     2.95           625         60.62   
12     2.40           532         51.60 
=======================================  
Year   5.05         12941      $1255.28   [/FONT]
    Relatively close to your contractor's predictions -- given the tools we have available on-line and not seeing your installation.

    Roughly a 20% reduction in output because of orientation... If you had the option (and area) for ground mount facing due south--would that be acceptable?

    Of course, I cannot estimate the issues of cool before noon temperatures vs hot evening temperatures on your system output (which may close the power gap significantly)-- and the whole issue of if you have Time of Use metering or not (which favors afternoon/evening generation--especially in summer).

    -Bill

    PS: Again, my estimates are just that -- given that they are within 10% of your contractor's estimates is as close as we could ever expect with the simplifications we have made here. I am not / cannot making judgment on his output predictions--just the question of East vs South orientation.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • solemio
    solemio Registered Users Posts: 16 ✭
    #8
    Re: Not the ideal orientation

    Thank you BB for all the info. Ground mount due South is not an option unfortunately. We are counting on the relatively cooler morning and early afternoon sun to close the gap a little and intend on keeping our E8 rate plan unless we are forced to switch to the TOU meter. We just signed the contract w/ Solarcraft middle of July so install will be sometime in September.
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