A curable epoxy resin formulation composition useful as insulation for an electrical apparatus including (a) at least one liquid epoxy resin; (b) at least one liquid cyclic anhydride hardener; (c) at least one thermally conducting and electrically insulating filler, wherein the filler includes an epoxy-silane treated filler; and (d) at least one cure catalyst with no amine hydrogens; wherein the epoxy resin formulation composition upon curing provides a cured product with a requisite balance of electrical, mechanical, and thermal properties such as Tg, tensile strength, dielectric strength, and volume resistivity such that the cured product can be used in applications operated at a temperature of greater than or equal to 120° C.