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Calculating minimum available solar energy

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  • Calculating minimum available solar energy


    I'm currently developing a solar battery package to operate a range of valves and actuators as well as associated control devices and telemetry in remote applications.

    Below is the calculation procedure I have used to determine the amount of solar energy available from a single panel.

    A ‘Peak Sun Hour’ is an hour in which 1kW/m2 hits the solar panel. Thus, if the average solar irradiance (kWh/m2) per day for a particular location is known, the number of ‘Peak Sun Hours’ can be found by dividing this value by 1kW/m2.

    Panels are rated for a cell temperature of 25oC. For every degree above this, the maximum power output from the column reduces by a small percentage. This seems to be 0.4%/oC for Jinko’s Eagle range which is what I have looked at so far.

    The least amount of sunlight is going to be received during winter. Thus a winter average (June, July & August) solar irradiance is going to represent an average minimum value.

    The warmest conditions are going to occur during summer. Thus a summer average (December, January & February) maximum temperature is going to represent an average maximum value.

    Combining this together, the minimum average output energy from the solar array would be:

    Output Energy (Wh) = (Max Output Power (W) – Max Output Power (W) * [Temperature Coefficient of Pmax (%/oC) * (Average Max Summer Temperature (oC) – 25oC)]) * Peak Sun Hours (hrs)

    Would this be a reasonable method to ensure I receive sufficient energy? There will be multiple days of battery capacity as a backup, but I want to make sure I can satisfy my load and fully charge these backup batteries in a day if sunlight is available.

    I appreciate any advice you maybe able to provide.

  • #2
    Hello ajhayes and welcome to Solar Panel Talk

    IMO going through the calculation that you have is probably a waste of time. I believe this because there will be some days that you will not have any useful sunlight to produce any power.

    Therefore you need to size you batteries based on your load running all day without any sunlight. From there you can determine your solar panel wattage and charge control size. You need to figure not discharging your batteries more than 25% to get the most cycles out of them. You can discharge them more but based on the manufacture you will see a drastic drop in life cycles.

    So you can probably forget your formula and work on your daily watt hour needs. From there you size your battery and then your panel wattage and charge controller.


    • #3
      Before you do that, I'd suggest you peruse something called PVWatts, including the info and help screens. You're not wrong, but there's a lot better ways to do resource assessment and preliminary system design. PVWatts is one helpful, easy to master tool. See what's available.

      After studying that stuff for a bit, you may come to see that the methods you have in mind can be done, and have been done a lot better, cheaper, faster.

      Welcome to the neighborhood.


      • #4
        On a cloudy, rainy day, my 5kw of PV might produce 200w peak, that I can use to charge the batteries. See Dec 29th !!

        Attached Files
        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



        • #5
          I agree with @SunEagle that your first step should be to asses your loads which will determine you battery needs. This can be done simultaneously with investigating options with PV Watts. Once you provide the basic input about azimuth and tilt, it is not hard to scale up the numbers. The only constraint is the roof or site area available. Both steps in resource assessment are important to get an initial system design.
          Last edited by Ampster; 02-10-2020, 04:47 PM.
          9 kW solar. Driving EVs since 2012