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Solar Energy Panels – System sizing and design – Example

For example, if you run a 25 Watt light bulbs in your bedroom for 2 hours a day, then this will consume 25 x 2 = 50 Wh of energy.



Note: Please note that this procedure is a basic guide only. Your installer should be able to assist you with more detail and advice.


The following step-by-step guide will help work out how to design your Solar Energy Panels system:

solar energy panels

  1. Working out how much power you use

    Fill out the table on the last page to work out how much energy you consume in your own household. In the following example, extra appliances have been added to give you an idea of how much energy some items use. Sum all the values in the last column to work out your total energy requirement per day. The maximum power required per day is the total power rating of all appliances that would be running at any one time.

    solar energy panels

  2. Working out how much energy you need

    When designing a system it is important to remember to take into account inefficiencies in the system. Batteries and inverters have efficiencies of around 80-90% and 85-95% respectively.

    In our example, if we use a battery and inverter rated at 85% and 90%, you need to divide the total energy requirement by 0.85×0.9=0.765.

    Thus, the adjusted total energy requirement (ie the amount of energy you need to produce to run all the equipment in the table) is:
    6023/0.765 = 7873 watt hours.

  3. Choosing a System Voltage

    The next step is to determine the voltage required for your system. The following diagram can be used as a rough guide, though the best design selection may be influenced by the inverters available on the market, how much power you need, and what appliances you have drawing power at the same time.

    solar energy panels

  4. Choosing an Inverter

    Inverters are rated at continuous, half hour and surge ratings. For example a 1200 watt inverter may be able to provide up to 3600 watts for short periods (surge).
    Many appliances will draw many times their rated wattage during start-up. It is important to choose an inverter which has enough spare capacity (or surge rating) to allow equipment to start.

    One school of thought is to work out maximum amount of power that would be running at any one time and rate the inverter using this figure. For smaller installations, the general rule of thumb is to add the watt ratings for all appliances in the house and take around 60% of the total figure. Then use an inverter with the equivalent continuous rating. In our example the equipment that would be running continuously for long periods totals around 2300 W and the most running for short periods is around 4000 watts. Thus, choosing an inverter which is safely above these figures say around 2600 watt continuous rating (with a half hour rating over 4000 watts) should be reasonable.

  5. Sizing the Battery Bank

    To size your battery bank, you will need to work out the number of amp-hours you require. To do this, divide the your total energy requirement by the system voltage.

    In our example: 7873/48 = 164 Ah.

    You will need to have enough battery storage capacity to ensure they are not discharged beyond 20% on a daily basis. As well as this, you need to account for times of bad weather when the batteries may not be fully charged, and that batteries should never be discharged beyond about 80%. The usual practice is to multiply your daily consumption (in amp-hours) by 5. So assuming you do not discharge your batteries beyond the 80% level, this offers enough capacity for 4 days storage.

    For our example: 164 x 5 = 820 Ah.

    You should also ensure that the battery can supply the maximum demand current. If the demand current is too high, the batteries may overheat and be damaged. A solar power system installer should be able to help you with this.

  6. Sizing the Photovoltaic Array

    The size of array you need will depend on:

    • Where you are located.

      The amount of sunlight that will shine on your panels will vary depending on where you live.
      The peak sun hours is the amount of time in a day that the sun will shine at a
      peak value of 1kW/m2. The peak sun hours is an expression used by solar
      designers to quantify the daily solar energy at your site. It is important to
      remember that this value will be influenced by the orientation and inclination
      (tilt) of the panels. When you design your system, you should also realise that
      the winter solar resource is significantly less than that in summer. This means
      your solar array will produce less power in winter than in summer.

      So add the values for summer and winter and divide by 2: (7.5 + 2.5)/2 = 5 hours

    • What type of panels you use ?

      Photovoltaic panels can be obtained in a wide range types and sizes, with varying outputs. For our example we will use an 80 watt solar panel (around half a square metre in area) which will provide about 4.96 amps in full sun.

      Thus to work out the number of panels you need, divide the total consumption (in watthours) by the rating of the panel and by the number of peak sun hours:

      7873 / (80 x 5) = 19.7 = 20 solar energy panels.


      Note: The maps describe the solar resource falling on a flat surface, on an angled surface the value will be different. Most solar panels are installed at an optimum angle to the horizontal, and the
      peak sun hours for the tilted surface will be greater. For example, a panel installed at an optimum angle of 47° in Adelaide actually receives an average 6.19 peak sun hours per day. So the number of
      panels required will be less = 16 solar energy panel


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