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How-To Determing Solar String Size (Examples + Calculator)

Determine your solar string size by considering panel & inverter specs, temperature effects, and calculating maximum string size. Consult a professional for accuracy.

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    Solar Plan Set Example Page 7

    5 Steps to Find Out Your String Size

    The size of a solar string, or the number of panels you can have in a series, is determined by the specifications of your solar panels and the inverter you’re using, and the climate conditions where the panels are installed. Here are the steps:

    1. Find Your Panel and Inverter Specs

    • Check the spec sheets for your solar panels and inverters. Key numbers are the panels’ open circuit voltage (Voc), maximum power voltage (Vmp), and the inverter’s maximum and minimum DC input voltage.

    2. Consider Temperature Effects

    Solar panels’ voltage decreases as temperature increases. The opposite is also true: as temperature drops, the voltage rises. You need to be aware of this because the inverter might get damaged if the voltage is too high. To account for this, use the “temperature coefficient of Voc” listed in your panels’ spec sheets. This tells you how much the voltage changes for each degree Celsius change in temperature.

    Then find out the lowest expected ambient temperature in your area. This is when your panels will produce the most voltage. Adjust your panels’ Voc for temperature. For instance, if your panels have a Voc of 40V at 25 degrees Celsius, a temperature coefficient of -0.33%/degree, and your lowest temperature is -10 degrees Celsius, you would calculate:

    1. Change in temperature = -10 – 25 = -35 degrees
    2. Change in voltage = 40V -0.33%/degree -35 degrees = 4.62V
    3. Adjusted Voc = 40V + 4.62V = 44.62V

    3. Calculate the Maximum String Size

    Take your inverter’s maximum DC input voltage. Divide it by your adjusted Voc. This gives you the maximum number of panels you can have in a string. For instance, if your inverter’s max input is 1000V:

    • String size = 1000V / 44.62V = 22.4

    You can’t have a part of a panel, so round down to the nearest whole panel. In this case, you could have up to 22 panels in a string.

    4. Verify Minimum String Size

    You also need to make sure your string voltage isn’t too low for your inverter. To check this, multiply your panels’ Vmp by the number of panels in your string and check if this is higher than your inverter’s minimum DC input voltage.

    5. Check Inverter’s Maximum DC Input Current

    Finally, you need to ensure that the total current of your string (which is the same as the short circuit current, Isc, of one panel, since panels in a series have the same current) does not exceed the inverter’s maximum DC input current.

    Real World Example

    Let’s say we’re using a specific solar panel model and a particular inverter, under specific climatic conditions. Here are the specifications:

    Solar Panel:

    • Open Circuit Voltage (Voc): 45.6V
    • Maximum Power Voltage (Vmp): 37.6V
    • Short Circuit Current (Isc): 9.4A
    • Temperature Coefficient of Voc: -0.33%/degree Celsius

    Inverter:

    • Maximum DC Input Voltage: 1000V
    • Minimum DC Input Voltage: 200V
    • Maximum DC Input Current: 25A

    The lowest expected ambient temperature for the area where the panels will be installed is -10 degrees Celsius.

    Step 1: Adjust the Panel Voc for Temperature

    Change in temperature = -10 – 25 = -35 degrees

    Change in voltage = 45.6V -0.33%/degree -35 degrees = 5.27V

    Adjusted Voc = 45.6V + 5.27V = 50.87V

    Step 2: Calculate the Maximum String Size

    String size = 1000V / 50.87V = ~19.6

    So you could have up to 19 panels in a string (rounding down to the nearest whole panel).

    Step 3: Verify Minimum String Size

    String voltage = 37.6V * 19 panels = 714.4V

    This is higher than the inverter’s minimum DC input voltage (200V), so it’s fine.

    Step 4: Check Inverter’s Maximum DC Input Current

    The total string current is the same as the Isc of one panel, 9.4A, which does not exceed the inverter’s maximum DC input current (25A).

    So, based on these calculations, for this specific scenario, you could have a solar string of 19 panels.

    Online Calculator

    There are online calculators available for string sizing, such as the one found at AltEstore. These calculators can make it easier and more accurate to determine the appropriate string size for your specific set of conditions. Always verify any calculations with a solar professional to ensure they meet your specific system’s safety and performance requirements.

    Takeaway

    Calculating solar string size involves several steps that require an understanding of specific solar panel and inverter specifications, as well as the impact of temperature on solar panel performance. Ensuring the correct sizing is essential for both the performance and safety of your solar system.

    Now that you have a general idea of the process, we strongly recommend you leave this complex task to professionals. We offer comprehensive plan sets, tailored specifically for your project’s unique needs. Our experts will ensure your solar system is designed for optimal performance. Order your customized solar plan sets through us today!

    Please remember that these are just general guidelines. Solar power design can be complex, and it’s important to consult with a professional, especially for larger systems.

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