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The current-voltage characteristics of various compositions of nonlinear varistor ceramics based on ZnO (ZOC) and SnO2 (TOC) are studied in a wide range of electric current. On the basis of obtained data were determined the parameters that allow finding the coefficients kprs and kprl, which define the protection  levels against the switching surge and the lightning surge for surge suppressors on the basis of ZOC and TOC varistors. The lower the value of these coefficients are, the significantly higher the efficiency of surge protection is. A relationship between kprs, kprl, and the effective linear resistivity ρlin limiting the current of intergranular barriers in varistor ceramics are established. It is shown that the values of kprs and kprl increase with increasing ρlin, which value is determined by the bulk resistance of the ceramic grains. The studied TOC compositions have 2-3 times larger values of ρlin and more high values of kprs and kprl than ZOC. ZOC-based varistors with lower kprs and kprl values provide more effective surge protection than TOC-based varistors.