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Undersize panels power due to north facing.

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  • Undersize panels power due to north facing.

    Hi!
    ​​​​​​Im gonna install my home system soon.
    South facing roof is full of shades but my NNW (340 degrees) facing roof is perfect for it.
    I know the lose of north facing system, but anyway my question is this:
    Should I buy lower power panels because of the north facing installation?

    Thank you!

  • #2
    Don't waste your money until you do a lot of research. It would be very helpful if you gave us an approximate location where you plan to install. DO yourself a favor and get a copy of Solar Power Your Home for Dummies. Also look around on the web for PV Watts which will allow you to find out how much of hit you are going to take installing at such a potentially poor angle.

    Ultimately unless you live below the equator its highly likely that if you can free up a southerly exposure on your roof or a pole mount that you just shouldn't waste your money.

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    • #3
      If you do what you describe, do it for reasons that are not economic. Provided you are in the northern hemisphere, you will be wasting your money if you do what you describe. Most likely solar will not be anywhere near an economically viable option for you. Follow Peakbagger's advice and do your homework. If you do, and if your goal in getting PV is to lower your electric bill, you will see why you will be better off financially using other measures to achieve that goal. PV in this application will be a drain on your finances for not much benefit.

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      • #4
        Thanks for your answers.
        Please have a look on the image attached.
        Information was calculated from PVWatts.

        According to the expenses estimated for the system - I will break even after 5.6 years.
        Attached Files

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        • #5
          Originally posted by solero View Post
          Thanks for your answers.
          Please have a look on the image attached.
          Information was calculated from PVWatts.

          According to the expenses estimated for the system - I will break even after 5.6 years.
          1.) How big is the array in STC kW?
          2.) If you are in the U.S., What's your zip ?
          3.) What is the proposed array tilt from horizontal ?
          4.) What are you paying for materials, freight, engineering, permitting ?
          5.) What price or value do you place on your time and labor ?
          6.) What is your annual usage in kWh ?
          7.) Do you understand how you are charged for electricity by your utility provider and so have some means to estimate what your differential savings might be as you vary the array size ?

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          • #6
            Originally posted by J.P.M. View Post

            1.) How big is the array in STC kW? I don't know what's STC is but the system is 10kW.
            2.) If you are in the U.S., What's your zip ? I'm not in the US, I'm from Israel.
            3.) What is the proposed array tilt from horizontal? 17 degrees.
            4.) What are you paying for materials, freight, engineering, permitting ? Total payment is gonna be 8300$.
            5.) What price or value do you place on your time and labor ? Negligible.
            6.) What is your annual usage in kWh ? It's Feed in tariff not net metering but annual usage is around 15 kWh.
            7.) Do you understand how you are charged for electricity by your utility provider and so have some means to estimate what your differential savings might be as you vary the array size ? I do.

            Please check above answers.
            I checked the internet for north facing installation and it doesn't seem so bad. Around 30% less depending on your array angle (in my case it's pretty good). Do you think 340 degrees is really a no-go?
            And again my question - can I save some money on lower-power panels because of the north facing?

            Thank you so much!

            Comment


            • #7
              Wow. A 10kw system installed for $8300. That is an incredibly low price below $1/watt. Based on an annual savings of $1400 it will pay for itself pretty quick even with a 340 degree direction.

              Comment


              • #8
                Originally posted by solero View Post


                Please check above answers.
                I checked the internet for north facing installation and it doesn't seem so bad. Around 30% less depending on your array angle (in my case it's pretty good). Do you think 340 degrees is really a no-go?
                And again my question - can I save some money on lower-power panels because of the north facing?

                Thank you so much!
                Shalom. You're welcome.

                The short answer to your question about lower power (smaller STC size) panels being more cost effective for this application is most likely no, and somewhat moot. If you have a 10 STC kW array, it matters little if you get to that 10 kW array size by using, say, 250 W panels or, say, 300 W panels. The annual output per installed STC W will most likely be about the same either way. The annual output of each 250 w panel will be pretty close to (250/300) of the output of the 300 W panel.

                As a crude analogy, it will it take about the same amount of fuel/km to transport 7,500 kg of 30 ea., 250 kg. rocks as it will to transport 7,500 kg. of 25 ea., 300 kg. rocks over the same road for the same distance.

                Also, smaller (electrical size) panels will require more connections and all that goes with more connections to get to the 10 kW array size.

                As you note, a 340 deg. az. is simply not as good in terms of annual output as a more southerly azimuth, even at an ~~ 30+ deg. N. latitude of Israel. To a first approx., if your incremental cost /kWh for power purchased from the utility is constant, the 340 azimuth will, to a first approx. be ~~ 30 % less cost effective as a south facing azimuth, maybe less, depending on your utilitie's rate structure. The cost effectiveness of a 340 az. array may or may not be acceptable, depending on your definition of what is an acceptable cost effectiveness, but whatever it is, it'll be not as good as a more southerly array azimuth would be.

                BTW, your annual usage is probably not 15 kWh. Do you mean 15,000 kWr ? If so, that's kind of high for a residence in Israel ain't it ? You runnin' a hotel or a kibbutz something ?

                Comment


                • #9
                  Thank you for your answer!
                  I think I didn't explain myself the right way:
                  I have place for 31 panels on my roof.
                  I thought buying 345W panels which will give me 10695W in total.
                  I guess that I don't need 345W in north facing roof, so I thought buying cheaper ones (let's say 300W or even less) as long as it won't hurt my total outcome. Am I getting it right?

                  Comment


                  • #10
                    Originally posted by solero View Post
                    Thank you for your answer!
                    I think I didn't explain myself the right way:
                    I have place for 31 panels on my roof.
                    I thought buying 345W panels which will give me 10695W in total.
                    I guess that I don't need 345W in north facing roof, so I thought buying cheaper ones (let's say 300W or even less) as long as it won't hurt my total outcome. Am I getting it right?
                    Panel size doesn't matter. Efficiency doesn't matter unless you are short on roof space (and more efficient panels reduce your racking costs slightly.) What matters is $/watt. That's what you want to mimimize.

                    Comment


                    • #11
                      Originally posted by jflorey2 View Post

                      Panel size doesn't matter. Efficiency doesn't matter unless you are short on roof space (and more efficient panels reduce your racking costs slightly.) What matters is $/watt. That's what you want to mimimize.
                      Thats regarding the installation cost.
                      I have place for 31 panels, and lets say I want the highest Watts producing, does it matter if I buy 345W or 300W? (because I think that anyway I wont reach the panel maximal capacity due to north facing panels)
                      ​​​
                      Last edited by solero; 09-05-2018, 01:16 PM.

                      Comment


                      • #12
                        Originally posted by solero View Post
                        I have place for 31 panels, and lets say I want the highest Watts producing, does it matter if I buy 345W or 300W? (because I think that anyway I wont reach the panel maximal capacity due to north facing panels)
                        Ah, I think I see where you are going here.

                        First some background. If you have a choice of several "standard size" panels (i.e. roughly 78 by 39 inches) and some are 345 and some are 300 watts, you are really talking about efficiency. Solar panels are rated by power per area. The standard flux is approximately 1000 watts per square meter, so if a 1 square meter panel produces 200 watts under standard temperature and flux conditions, that's 20% efficient. If the same size panel produces 150 watts, that's 15%.

                        So your question above is really about efficiency. If you want to get more power for a given roof area, you do need higher efficiency panels.

                        I think your question is really asking "since it's north facing, will it be a waste of money to get 345 watt panels since they will never hit 345 watts?." Answer is no. If you get 345 watt panels and you are only getting half the STC insolation because they are facing north, you'll get 173 watts. If you get 300 watt panels, under the same conditions you will get 150 watts. So even with poor siting the more efficient panels will produce more.

                        Comment


                        • #13
                          Originally posted by solero View Post

                          thats regarding the installation cost.
                          I have place for 31 panels, and lets say i want the highest watts producing, does it matter if i buy 345w or 300w? (because i think that anyway i wont reach the panel maximal capacity due to north facing panels)
                          ​​​
                          1.) Forget about the wattage per panel !!! Think in terms of the total array size, not per panel !!! That goes for both electrical size and physical dimensions.

                          2.) In the same location and orientation and application,the output per STC watt will be mostly the same for any panel regardless of the total stc output of that panel.

                          3.) Put two panels side by side on your roof, one is 300 W. The other is 345 W. The total output of the 300 W panel will be pretty close to (300/345) of the output of the 345 W panel. That ratio will be the essentially constant for all orientations and locations.

                          4.) If you want the highest annual array output for a physically limited set of dimensions for, say, a roof, start with the panels that have the highest efficiency on a per area basis and juggle dimensions until you get something that's safe, fit for purpose with layout that allows access for maintenance. Don't forget to leave space for access and maintenance as well as any building code or safety requirements.

                          Got it ?
                          Last edited by J.P.M.; 09-05-2018, 03:10 PM.

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                          • #14
                            Super clear now!
                            Thank you all very much.
                            Good luck to everyone

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