Pointing east is only a 15% drop

I've been trying to reconcile PVWatts and RETScreen (you guys in the US know about RETScreen? Google it!) outputs with what my febrile brain is telling me. My brain says that pointing my panels east will cause a lot of power to be lost, everything from about 2 PM on, but PWatts and RETScreen are telling me it's closer to 15%.

I can sort of figure this out... I get better angles in the morning and noonish. But can that really make up for being shadowed for a significant portion of the day? And I don't know how to factor in minimal power curves on the panels and inverters...

Does anyone out there actually have an array that's significantly off-south that might be able to give me some hard numbers?

Maury

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Pointing east is only a 15% drop

    PVWatts (at least) is based on ~20 years of measured solar data... So, depending on what your (typically summer 1/2 of the year) is like (morning fog vs afternoon thunder showers) can bias your results.

    Also, the hot afternoons can reduce solar power output by 10-20% vs cooler morning power collection (I don't think PV Watts takes panel temperature into account).

    Solar Guppy (Florida) has lots of data that shows easterly biased arrays collect a lot more power for him than south or south west arrays (for him and his weather--Hot afternoons, summer afternoon thunder storms).

    For me, I have seasonal Time of Use billing... So, during summer peak time period (noon-6pm Monday thru Friday)--It would makes sense to bias my system to the west to optimize afternoon power production where I get 3x the $$$/kWH for my afternoon power production/usage offsets ($0.27 vs $0.09 per kWHr)... For "winter" 6 months of the year, the difference is much less ($0.12 vs $0.09)--So I "don't care" as much about winter production/usage.

    But, practically, because of my site (house faces south east, heavy oak trees to west on neighbor's property)--I just had to go with what worked best for my site.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AntronXAntronX Solar Expert Posts: 462 ✭✭
    Re: Pointing east is only a 15% drop
    BB. wrote: »
    ...Solar Guppy (Florida) has lots of data that shows easterly biased arrays collect a lot more power for him than south or south west arrays (for him and his weather--Hot afternoons, summer afternoon thunder storms)..

    Interesting. Where can I see his report? I often noticed myself that mornings tend to be sunny here and heavy clouds form by 2pm.
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Pointing east is only a 15% drop

    It was a post from him about some corporate investors (?) that visited his home (which is also his development lab) and from SG's records, he showed them that easterly biased panels generated more power overall... However, the utility wanted westerly biased panels to generate power in the afternoons instead (when their loads were the heaviest)...

    At least that is what I recall--I did a quick look for the post and could not find it.

    Then those questions lead to the tariffs and which would offer a better return on investment (flat rate vs time of use with higher payments in the afternoon). And then, if you have reliable cloud cover (PM thunderstorms)--then the westerly biased panels don't generate much afternoon power anyway.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Maury MarkowitzMaury Markowitz Registered Users Posts: 9
    Re: Pointing east is only a 15% drop
    BB. wrote: »
    But, practically, because of my site (house faces south east, heavy oak trees to west on neighbor's property)--I just had to go with what worked best for my site.

    If I'm parsing this properly, your array is facing south east? If so, have you ever tried to calculate the "pointing error" for your system?

    Maury
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: Pointing east is only a 15% drop

    No, South Easterly--Roughly 165 degrees...

    I did not worry about the error--it was not that much and I (or my wife) was not going to let them make an ugly or ground install in a suburban area.

    In the mean time, we have done a lot of conservation (around 200-250 kWH per month) and with Time of Use billing--the current 3,500 watt array generates way more power than we use at the moment... Was thinking about some version of electric car. But those always seem to be a year or two out (for the last 5 years).

    If you use PV watts for San Francisco, and defaults for everything else (plus 165 facing array)--You can see what the difference would be.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 4,994 ✭✭✭✭
    Re: Pointing east is only a 15% drop

    I use to live in Tallahassee FL and regularly had afternoon clouds and rain due to a weather cycle called sea breeze. I'm Sure The Guppy's right for many areas.

    If I lived in northern coastal Florida, fall and winter fog as the gulf is warmer than the land might be a big worry.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do more...lol
  • dreesdrees Solar Expert Posts: 481 ✭✭✭
    Re: Pointing east is only a 15% drop

    Around here we get marine layer mostly in the morning which burns off by noon if it's going to burn off.

    So I would guess that aiming a bit west would improve output around here (should check PVwatts hourly data).
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: Pointing east is only a 15% drop

    Here's the post:

    http://forum.solar-electric.com/showpost.php?p=29449&postcount=6


    To find it, use the forums search, choose advanced search, in the keywords put "east array" (no quotes) and in the user name put "Solar Guppy" no quotes.
  • DaveH9DaveH9 Registered Users Posts: 13
    Re: Pointing east is only a 15% drop

    It really depends on your local conditions, some places are foggy in the morning. Generally East is good as long as it's not a steep pitch. If it is too steep, then it will be shaded in the afternoon by itself. At 165 that should not be a problem. I like to use this tool http://www1.solmetric.com/cgi/insolation_lookup/lookup.cgi
    to show my customers that their not so south facing roof is really not so bad.
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