x[T_] := (1/40)* (T/300.) (* Handy scaling factor for 1/T, in eV *)

Z[T_] := Exp[-0/x[T]] + Exp[-2/x[T]]

Z[T]

Plot[Z[T], {T, 300, 300000}]

FindRoot[x[T] == 2, {T, 300}] (* At what Temperature is kT = 2 eV? *)

Table[{T, Z[T]}, {T, {300, 3000, 30000, 300000}}] // TableForm