  We have evaluated the power factor of transition metal oxides at high
temperatures using the Heikes formula and the Ioffe-Regel conductivity. The
evaluated power factor is found to be nearly independent of carrier
concentration in a wide range of doping, and well explains the experimental
data for cobalt oxides. This suggests that the same power factor can be
obtained with a thermopower larger than $2k_B/e$, and also suggests a
reasonably high value of the dimensionless figure of merit $ZT$. We propose an
oxide thermoelectric power generator by using materials having a thermopower
larger than 300 $\mu$V/K.
