String Sizing: How to Calculate Solar String Size

Table of Contents
    Add a header to begin generating the table of contents

    How To Calculate Solar String Size

    Solar string sizing refers to the amount of PV modules in series within your solar array. It’s critical to calculate the minimum and the maximum number of modules that can be included in one string in order to keep your system functioning safely and efficiently. 

    Solar panels produce higher voltages when it is the cold and less operating voltage when it’s hot, so that variance needs to be taken into account. If maximum system voltage is produced for too long, it can damage your inverter or even start a fire. 

    While too little voltage can cause the inverter to shut off entirely in hot weather, which means you will be missing out on the energy generated on long, sunny summer days.

    one line electrical drawing showing string sizing

    String Sizing Online Tools

    At Avila Solar, we want to make the solar installation process as easy as possible for you, which is why we are developing an online tool to help you calculate your ideal solar string size and generate one-lines with ease! We expect to have the tool available to use by the end of 2020. 

    Of course, with any of our solar permitting or design packages, our team of experts will perform detailed calculations accounting for all variables in order to confirm the appropriate string size for your project.

    Manual String Sizing Calculation

    If you enjoy doing math and want to perform the calculations for minimum and maximum solar string size for yourself using equations, here’s how:

    Step 1: Calculate Minimum Module Voltage

    Again, the minimum string size is the number of photovoltaic modules connected in series that are required to keep the inverter running during warm summer months when system voltage output is less.

    The return on your investment is highest during these months due to the plentiful sunshine and longer days, so this is a critical consideration.

    Total Time Needed: 5 minutes

    Required Tools:

    - Calculator

    Things Needed?

    - Min String Sizing Formula

    Steps to Find Out Minimum String Size

    Step 1 : Enter the HowTo Schema title you want

    First, you’ll need to calculate the minimum output voltage (Module Vmp_min) for each module, accounting for the summertime high temperatures of your site. This can be calculated with the following equation:

    Module Vmp_min = Vmp x [1 + ((T_max + T_add - T_stc) x (Tk_vmp / 100))]
    Step 2: Plug Answer into Following Formula

    Then, once you’ve determined your Module Vmp_min, you can plug it into this equation to find your minimum string size, rounding up to the nearest whole number:

    Minimum String Size = Inverter V_min / Module Vmp_min

    With this value:

    Inverter V_min = the minimum MPPT voltage of your inverter

    How To Calculate Maximum String Size​

    The maximum string size is dictated by the highest allowed voltage input for your inverter.

    Total Time Needed: 5 minutes

    Required Tools:

    - Calculator

    Things Needed?

    - Min String Sizing Formula

    Steps to Find Out Maximum String Size

    Step 1: Calculate Maximum Module Voltage

    First, calculate the maximum module voltage, adjusted for the site’s low ambient temperature (Module Voc_max) using the following formula:

    Module Voc_max = Voc x [1 + (T_min - T_stc) x (Tk_voc / 100)]

    With these values:

    Voc =the open current voltage rating for your modules found on the datasheet

    T_min = the lowest expected ambient temperature for your site in degrees Celsius

    T_stc = temperature at standard test conditions (25 degrees Celsius)

    Tk_voc =the module open current voltage temperature coefficient, which can be found on the datasheet and is always expressed as a negative value in a %/degrees Celsius format

    Step 2: Plug Answer into Following Formula

    Then, plug that answer into the following formula and round the final answer down to the nearest whole number:

    Maximum String Size = Inverter V_max / Module Voc_max

    With this value:

    Inverter V_max = the maximum allowable voltage for your inverter found on the datasheet

    Free Solar String Sizing Tools

    Is your head spinning from all that math? Don’t worry, here’s a list of some free solar string sizing tools:

    Are you ready to get started with your solar energy system? Contact us today to get started with a permitting package that will include our professional services to calculate your string size, assemble all the necessary paperwork and documentation, and submit applications for all required permits.

    Read More

    • All
    • Guides
    • How-To
    • Incentives
    • Jobs & Career
    • Permit Process
    • Regulation
    • Uncategorized
    color coded electrical cables

    NEC Solar and Storage Regulations Explained

    Read More →
    curved pv design

    NEC 2017 690.1 & PV Disconnect Placement

    Read More →

    How May We Help?

    Add your name, basic contact info, and service(s) you're interested in. Thank you!