When sourcing square varistors, you must prioritize Varistor Voltage (Vn), which is the voltage at which the device begins to conduct. For industrial stability, ensure the Maximum Allowable Voltage (AC/DC) exceeds your system's peak operating voltage by at least 10-20%. Additionally, evaluate the Peak Surge Current (8/20μs waveform); square varistors typically offer higher energy absorption (measured in Joules) compared to round versions, making them ideal for heavy-duty surge protection in power supplies and telecommunications.