Always check your local building codes
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.
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.
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:
- Jump to How To Calculate Maximum String Size
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.
Steps to Find Out Minimum String Size
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))]
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.
Steps to Find Out Maximum String Size
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
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
Is your head spinning from all that math? Don’t worry, here’s a list of some free solar string sizing tools:
Avila Solar String Sizing (Coming Soon!)
Fronius String Sizing
US String Sizing
This Fronius String Sizing Configuration Tool allows you to design your system as efficiently as possible, with software that can handle 1,000 Voc designs and bifacial modules.
The software also allows you to skip manually calculating the temperature data, and it also provides guidance on rapid shutdown and fusing source circuits.
SolarEdge String Sizing
SolarEdge String Sizing & Design tool offers a free web-based tool to help you design your solar energy system using SolarEdge equipment.
Simply create a free account, and then you can get started using a satellite image of your home to plan out the system. It will automatically calculate string sizes for you.
This tool is specifically designed to help you determine how many solar panels are necessary for your system, how many should be on each string, and how to connect them to your inverter.
It offers a simple and fast design tool and has the data of many popular solar panels and inverters built into the software. In the next update of this program, you will be able to plan for a system with or without battery backup.
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.