Here are some guidelines for choosing an MOV (metal oxide varistor):

• We recommend a 250 mA fuse, so that the fuse blows before the varistor
• Expected surges, i.e.: bigger surges = higher rated MOV
• Choose one that has a higher rated voltage than your working voltage
• Physically dimensions, i.e.: fits in with the wiring at the back of the meter
• Environmental considerations, i.e.: is the unit in a location prone to lightning strikes?