Quick Answer
To calculate the ideal string length for your solar setup, determine the maximum allowed voltage drop per string, then divide your solar panel array's total voltage by this number. This will give you the number of panels per string.
Determining Maximum Allowed Voltage Drop
The maximum allowed voltage drop per string is typically 3% of the total system voltage. For a 48-volt system, this would be 1.44 volts (48 x 0.03). When wiring series-connected solar panels, the total voltage of the array is the sum of the individual panel voltages. For a string of 20 panels with 250-volt maximum power point tracking (MPPT) voltage each, the total string voltage would be 5000 volts (20 x 250). Using the 3% rule, the maximum allowed voltage drop for this string would be 150 volts (5000 x 0.03).
Calculating Ideal String Length
To calculate the ideal string length, divide the total array voltage by the maximum allowed voltage drop per string. For the example above, with a 5000-volt total string voltage and a 150-volt maximum allowed voltage drop, the ideal string length would be 33.33 panels (5000 / 150). You cannot have a fraction of a panel, so to be safe, round down to 33 panels per string.
Practical Considerations
In practice, you may need to round down the ideal string length to ensure that the voltage drop remains within the allowed limit. It’s also essential to consider the inverter’s maximum allowed string length and the maximum allowed cable length for each string. Additionally, keep in mind that the voltage drop will vary depending on the current flowing through the string, so it’s crucial to monitor and adjust the string length accordingly.
Find more answers
Browse the full Q&A library by topic, or jump back to the topic this question belongs to.