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facing PV arrays: East/West, North or South

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  • facing PV arrays: East/West, North or South

    i constructed an initial south facing pv array this year and it is operational.

    the next array is scheduled for next year. it can be faced south, but there is a fair amount of sunlight on the property at sunrise and sunset, which brings an east/west array to mind. a north facing array is also possible.

    any ideas?

  • #2
    You are grid-tied, right? All esle being equal, south facing will probably produce the most for you, but you can double-check with PVWatts. Once you max out the inverter rating that your service can accept, additional panels at less optimal orientations can help stretch out the production day.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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    • #3
      Clearly South will give you the best bang (kWh) for your buck as you get the longest and most intense sun hours per meter squared. But if you have time of use metering (in CA for example) a lot of folks seem to like to add Westerly or SW facing strings as well to spread some of their production into the late afternoon/early evening (when they get home from work) especially if their net metering terms aren't favorable with the TOU.

      My raised ranch roof has one face that faces due South (175 degrees) and the other faces due North. But I find that I could potentially get a lot of sun on my Northern face (Only 20 degree pitch) in the mid to late afternoon. I don't have any panels there currently (and thus no data) but just from the impact of the solar radiation (heating) and duration, it seems SW/W/NW would be much better for me (in NJ) than Easterly facing.

      But, if you are in a time of use market, I would do a differential cost analysis of adding storage vs. adding an additional string / array. To see which might give a better ROI.

      PVWatts and/or the SolarEdge site designer (both free) should be able to give you fairly good production estimates for each of your various orientations.

      -Jonathan


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      • #4
        Any northerly facing panels may produce in the summer season but in the winter would be pretty weak unless you are within the tropics. The tropics begin where on the summer solstice the midday sun is actually north of overhead, shining onto the north facing surfaces of vertical structures. Of course the opposite is true in the southern hemisphere.
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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        • #5
          Ha! I always wondered the same thing. So I decided to see for myself (attached).

          For my current 12.2kW system:

          Facing South: 16.754 MWh
          Facing East or West: 14.609 MWh (12.8% less than South)
          Facing North: 11.119 MWh (33.6% less than South)

          NOTE: this is for a tilt of 20 degrees

          Not as bad (for East/West) as I thought. But North takes a pretty substantial hit -- though its still "usable" if your install/add-on cost per Watt is low. But, given the cost per Watt, and these reduced yields, what impact does this have on your time to break even for this add on?

          -Jonathan
          Attached Files

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          • #6
            Originally posted by sensij View Post
            You are grid-tied, right? All esle being equal, south facing will probably produce the most for you, but you can double-check with PVWatts. Once you max out the inverter rating that your service can accept, additional panels at less optimal orientations can help stretch out the production day.
            yes, grid tied.

            what do you mean by 'max out inverter rating'?

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            • #7
              Originally posted by Jest Waitn View Post

              yes, grid tied.

              what do you mean by 'max out inverter rating'?
              At some point, more south facing panels will start to be less cost-effective because you will begin to lose power to clipping mid-day. Eventually, the loss from clipping will exceed the loss you'd see from orienting the panels east or west. At that point, you could consider adding another inverter, but eventually, you'll max out the amount of backfeed your service panel can take. It isn't a scenario you are likely to run into, but I wanted to mention it as a qualifier just in case.
              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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              • #8
                Originally posted by sensij View Post

                At some point, more south facing panels will start to be less cost-effective because you will begin to lose power to clipping mid-day. Eventually, the loss from clipping will exceed the loss you'd see from orienting the panels east or west. At that point, you could consider adding another inverter, but eventually, you'll max out the amount of backfeed your service panel can take. It isn't a scenario you are likely to run into, but I wanted to mention it as a qualifier just in case.
                ok, inverter capacity. no this isn't a problem.

                the issue arose because a poco rep told me that the scaling is based on annual usage. the formula is ~ .86 * annual total = max allowable system size in kwdc. this has not been stated anywhere previously, so i want to add the final array for the best ROI.

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                • #9
                  image_9538.jpg
                  Originally posted by Jest Waitn View Post
                  i constructed an initial south facing pv array this year and it is operational.
                  the next array is scheduled for next year. it can be faced south, but there is a fair amount of sunlight on the property at sunrise and sunset, which brings an east/west array to mind. a north facing array is also possible. any ideas?
                  If you are running a string system, you can use those extra E-W panels to extend your hours of
                  production, without increasing the size of your AC inverter plant. Extras would also help under
                  clouds, since orientation is not critical under the dispersed light. This array might produce this
                  curve on a sunny day. The inverters are pegged out for 7 hours straight, but they may exceed 8
                  once I clear some shade problems. Bruce Roe
                  NSnview.jpg
                  Last edited by bcroe; 07-10-2017, 05:39 PM.

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                  • #10
                    Originally posted by bcroe View Post
                    image_9538.jpg


                    If you are running a string system, you can use those extra E-W panels to extend your hours of
                    production, without increasing the size of your AC inverter plant. Extras would also help under
                    clouds, since orientation is not critical under the dispersed light. This array might produce this
                    curve on a sunny day. The inverters are pegged out for 7 hours straight, but they may exceed 8
                    once I clear some shade problems. Bruce Roe
                    NSnview.jpg
                    that's real good output. good installation.

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                    • #11
                      Originally posted by Jest Waitn View Post

                      ok, inverter capacity. no this isn't a problem.

                      the issue arose because a poco rep told me that the scaling is based on annual usage. the formula is ~ .86 * annual total = max allowable system size in kwdc. this has not been stated anywhere previously, so i want to add the final array for the best ROI.
                      Since you are limited in total size based on DC capacity your best return is going to be south facing unless there is some time of use.
                      OutBack FP1 w/ CS6P-250P http://bit.ly/1Sg5VNH

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                      • #12
                        Only 86% of annual usage? What state/Poco? Here in NJ (JCP&L / PSEG) ~100-105% (sometimes even 110%) seems to be permissible for residential (aka current use plus future plans for an electric car, for example). Depending on the productivity estimates, my system is ~15 MWh Est Production / ~14 MWh average annual consumption = ~107% I have good numbers for consumption (as I've lived here for 11 yrs) production is still TBD. Estimates seem to range from approx 14-16 MWh / yr depending on software package and assumptions.

                        What is the current DC size of your current array? How big is your inverter? Are you planning to add additional strings to your existing inverter (to broaden its power curve or approach clipping levels for maximum AC yield?) or are you planning to add a second inverter? What is the financial incentive? Time of use production shifting? Have you already paid off your current system? What % of your annual use does your current system provide that you're looking to upgrade to 86%?

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                        • #13
                          Originally posted by ButchDeal View Post

                          Since you are limited in total size based on DC capacity your best return is going to be south facing unless there is some time of use.
                          ok. that's what i thought - south facing.

                          i have found no proof that the poco sets the maximum pv scaling level, so i have challenged this because i believe that the rep is either misinformed or is trying to 'cap' system sizes. the PSC sets the standards and i found nothing anywhere, including the administrative code (law) that states caps on the production, bar that the system '[SIZE=2]Does not exceed 90% of the customer

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                          • #14
                            Originally posted by JSchnee21 View Post
                            Only 86% of annual usage? What state/Poco? Here in NJ (JCP&L / PSEG) ~100-105% (sometimes even 110%) seems to be permissible for residential (aka current use plus future plans for an electric car, for example). Depending on the productivity estimates, my system is ~15 MWh Est Production / ~14 MWh average annual consumption = ~107% I have good numbers for consumption (as I've lived here for 11 yrs) production is still TBD. Estimates seem to range from approx 14-16 MWh / yr depending on software package and assumptions.

                            What is the current DC size of your current array? How big is your inverter? Are you planning to add additional strings to your existing inverter (to broaden its power curve or approach clipping levels for maximum AC yield?) or are you planning to add a second inverter? What is the financial incentive? Time of use production shifting? Have you already paid off your current system? What % of your annual use does your current system provide that you're looking to upgrade to 86%?
                            its NOT 86% of usage. the scaling (kwdc) = ~.86 * total annual consumption in kwh.

                            a method is actually used, that's not a %, but that arrives at a %. florida/florida power. the calculation used was based on 8269 kwhac. so the result is 7.1 kwdc = (85.9% * 8269). but for this to be 'true' it has to be stated and i can't find it, so i have asked to see this in print.

                            the method is to take annual kwh, multiply it by 115 and then divide the result by 1135. which yields 7.1kwdc for the maximum array. the present array size is 3460wdc.

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                            • #15
                              Here there is no restriction on plant size, just varying regs. The E-S-W panels allow a smaller inverter plant
                              if that matters. Facing S only will allow somewhat fewer panels at the expense of a larger inverter plant,
                              since your energy all comes in a big lump. But clouds are a big incentive here; your results may vary. Bruce Roe

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