PV String Sizing — Method
A string is a series chain of modules; its voltage must stay inside the inverter window in all weather. The worst case for high voltage is the coldest temperature, which raises open-circuit voltage: Voc_cold = Voc × [1 + (temp-coefficient ÷ 100) × (T_min − 25)]. The maximum modules per string is floor(inverter max DC ÷ Voc_cold). The minimum is set so the string still sits above the MPPT lower voltage when hot: ceil(MPPT min ÷ Vmp). Design between these two limits.
Frequently Asked Questions
Maximum = floor(inverter maximum DC voltage / Voc at the coldest site temperature). Minimum = ceil(MPPT minimum voltage / module Vmp). Design the string length between these limits.
Open-circuit voltage rises as temperature falls (negative temp coefficient). At the record low, a string can exceed the inverter maximum DC voltage and damage it, so the max length is set at the coldest temperature.
Voc_cold = Voc x [1 + (temp coefficient %/C / 100) x (Tmin - 25)]. With Voc 49.5 V, coefficient -0.27%/C and Tmin -10 C, Voc_cold is about 54.2 V.
The inverter MPPT lower voltage. When modules are hot their voltage drops, so the string must have enough modules to stay above the MPPT minimum and keep tracking.
Use the site record low (ASHRAE extreme minimum) for the maximum-voltage check and a high cell temperature (e.g. 70 C) for the MPPT-minimum check, per the inverter manufacturer guidance.
PV String Sizing Explained
Modules wired in series form a string whose voltage must live inside the inverter MPPT window across the full temperature range of the site. Two limits define the valid string length.
The cold limit (maximum)
Open-circuit voltage rises as it gets colder. At the record low temperature the string Voc must stay below the inverter maximum DC input, or the inverter can be damaged. That fixes the maximum modules per string.
The hot limit (minimum)
When panels are hot their operating voltage falls; the string must keep enough modules to stay above the MPPT lower voltage so the inverter can still track. Combine with inverter sizing and array sizing for the full design.
Related: Inverter Sizing, Panel Sizing, Battery Bank.