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Formula to Esimate Electricity output in a Solar Power

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  • Formula to Esimate Electricity output in a Solar Power

    What is the formula to caclulate or estimate the electricity generated in output of a solar panel system?

  • #2
    Google "PVWatts", read the help screens/documentation, and come on back here with your results or any questions.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

    Comment


    • #3
      Couple things to assume (starting with Grid tie batteryless systems and then to battery/off-grid systems)
      1) PV panels produce (generally) 80% of their STC wattage
      2) Grid Tie inverters are 95% efficient
      3) Battery charge controllers are
      MPPT 95% efficient when in BULK mode only, otherwise they are -
      PWM efficiency varies with the the ratio of PV array output voltage and battery charge voltage
      4) Flooded batteries start life at 90% efficient, decreasing to 70% at failure
      5) Off Grid inverter efficiency varys from 70-95% depending on loading and brand/model

      Hope this quick guess works for you
      Last edited by Mike90250; 07-14-2016, 06:54 PM. Reason: withe typo
      Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
      || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
      || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

      solar: http://tinyurl.com/LMR-Solar
      gen: http://tinyurl.com/LMR-Lister

      Comment


      • #4
        Hi Mike90250,

        I want to make sure I understand your information correctly - you say PV panels generally produce about 80% of their STC (standard test conditions) wattage. So if I had a 300w panel (module) I should actually expect just 240w in real world conditions, (not really 300w as I might get under optimal conditions in a lab test).

        Then, I should expect to convert only 95% of the panel (module) output from DC power to AC power. This is just because of the way electrical engineering and physics works (?).

        So in summary, at any moment in time, one 300w panel would ordinarily be expected to generate 240w (as DC power?) ... and that 240w would then be converted to AC by the inverter (losing 5% in the process) resulting in something like 228w of AC power? So it is really just the 228w times hours of production that I am calculating to figure out my offset?

        Thanks.

        Comment


        • #5
          Originally posted by entgegnen View Post
          Hi Mike90250,

          I want to make sure I understand your information correctly - you say PV panels generally produce about 80% of their STC (standard test conditions) wattage. So if I had a 300w panel (module) I should actually expect just 240w in real world conditions, (not really 300w as I might get under optimal conditions in a lab test).

          Then, I should expect to convert only 95% of the panel (module) output from DC power to AC power. This is just because of the way electrical engineering and physics works (?).

          So in summary, at any moment in time, one 300w panel would ordinarily be expected to generate 240w (as DC power?) ... and that 240w would then be converted to AC by the inverter (losing 5% in the process) resulting in something like 228w of AC power? So it is really just the 228w times hours of production that I am calculating to figure out my offset?

          Thanks.
          No... that 80% number is more like the peak output you will get near the middle of the day, if you have south facing panels with a tilt about equal to your latitude. Once you get more than an hour or so away from solar noon, that number starts to fall off fairly quickly. In the spring when it is cooler, the peak might be a bit higher into the mid-80's, in the winter, maybe not even 70%, again, depending on the orientation.

          PVWatts is a great tool to estimate how much array is needed to achieve a targeted offset, and the hourly output option can be used for more granular analysis if needed.
          CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

          Comment


          • #6
            Well... except that the sun varies with time, too.

            Take a look at http://pvwatts.nrel.gov, that implements the formula you're looking for, and takes the sun into account (assuming no shading).
            (Watch out though - it gives the output for a typical year with both cloudy and sunny days, so don't take any one day's output too literally.)

            And/or look at a nearby similar system, find out how many kWh it generated last year, divide that by the sum of the faceplate ratings of its panels, and you've got the infamous kWh/kW aka "sun hours" aka "yield" number (see eq 1 in http://www.nrel.gov/docs/fy05osti/37358.pdf ), also called 'efficiency' by pvwatts.org ( see my system's at http://pvoutput.org/aggregate.jsp?id...=44908&v=0&t=y )
            This is roughly the "hours of production" you're talking about, but it already includes the inefficiencies of that system's inverter.
            For my system over a whole year, it's about 4.5, but your mileage will vary.

            17kw. I like science, but I'm no expert.

            Comment


            • #7
              Ok, so it sounds like a general rule of thumb is that any given panel will just give you up to 80% of the rated power at the peak moment in the day... And ordinarily a little less depending on conditions and time of day.

              And that from that number (being generated at the panel) you will lose 3-5% at the inverter.

              So you may build a 7.68Kw system... But don't be surprised if under optimal conditions,the readout from the system just says it is making 6200w AC.

              ?
              Last edited by entgegnen; 07-26-2016, 08:09 PM. Reason: Auto spell should be auto

              Comment


              • #8
                My system's nameplate output is 8950 watts DC. Today, a hot sunny day, the AC output was
                8am 1000W
                9am 2300W
                10am 3800W
                11am 4900W
                noon 5700W
                1pm 6100W
                2pm 6100W
                3pm 5700W
                ...
                Peak today was about 68%, not 80%.
                At no point has it ever gotten anywhere close to 8900W AC. Best I've ever seen it do is 7800W, but that's rare.

                See graph at http://pvoutput.org/intraday.jsp?id=...08&dt=20160726
                17kw. I like science, but I'm no expert.

                Comment


                • entgegnen
                  entgegnen commented
                  Editing a comment
                  So over the course of each day, you are producing well over 35,000 watt/hours! That seems like a pretty good amount!
                  Thanks for this, it is helpful as I watch the performance of my system.

              • #9
                Besides time of day, clouds might knock your output down to as low as 10% of normal. Between clouds and shorter days, my
                winter months may produce only 1/3 as many KWH as summer months. Bruce Roe

                Comment


                • #10
                  Originally posted by entgegnen View Post
                  Ok, so it sounds like a general rule of thumb is that any given panel will just give you up to 80% of the rated power at the peak moment in the day... And ordinarily a little less depending on conditions and time of day.

                  And that from that number (being generated at the panel) you will lose 3-5% at the inverter.

                  So you may build a 7.68Kw system... But don't be surprised if under optimal conditions,the readout from the system just says it is making 6200w AC.

                  ?
                  Rules of thumb are good for measuring thumbs. For estimating long term average solar output use PVWatts as suggested by others. Read the help/info screens before making a few runs. The learning curve of facility with the model vs. time is steep.

                  Comment


                  • #11
                    Originally posted by DanKegel View Post
                    find out how many kWh it generated last year, divide that by the sum of the faceplate ratings of its panels, and you've got the infamous kWh/kW aka "sun hours" aka "yield" number
                    No Dan it isn't. That is an incorrect, misleading and ignorant statement.

                    Sun hours" is generally considered to be the number of kWh/m^2 of solar radiation falling on a horizontal surface per time period (usually per day).

                    The term "sun hours" has nothing to do with system performance or system size. It is an anachronistic term that represents resource availability only. The currently accepted term is Global Horizontal Radiation or GHI.

                    Get it right.

                    Comment


                    • #12
                      Thanks for the info everyone.

                      Comment


                      • #13
                        Originally posted by J.P.M. View Post
                        Get it right.
                        OK, I stand corrected, strike the words "sun hours".
                        17kw. I like science, but I'm no expert.

                        Comment


                        • #14
                          Originally posted by Mike90250 View Post
                          Couple things to assume (starting with Grid tie batteryless systems and then to battery/off-grid systems)
                          1) PV panels produce (generally) 80% of their STC wattage
                          2) Grid Tie inverters are 95% efficient
                          3) Battery charge controllers are
                          MPPT 95% efficient when in BULK mode only, otherwise they are -
                          PWM efficiency varies with the the ratio of PV array output voltage and battery charge voltage
                          4) Flooded batteries start life at 90% efficient, decreasing to 70% at failure
                          5) Off Grid inverter efficiency varys from 70-95% depending on loading and brand/model

                          Hope this quick guess works for you
                          If I only spoke Acronym, I could understand that.

                          Comment


                          • #15
                            Originally posted by Charlie W View Post
                            If I only spoke Acronym, I could understand that.
                            What are you needing to do ? Only 3 acronym's
                            STC = Standard Test Condition
                            PWM = Pulse Width Modulation
                            MPPT = Max Power Point Tracking
                            the rest is english

                            I assume you can understand PV = Photo Voltaic since this is basically a solar power forum,
                            Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                            || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                            || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                            solar: http://tinyurl.com/LMR-Solar
                            gen: http://tinyurl.com/LMR-Lister

                            Comment

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