The dimensionless figure of merit ZT is formed by multiplying Z with the average temperature ,which is a method for comparing the potential efficiency of devices using different materials. A greater ZT indicates a greater thermodynamic efficiency, subject to certain provisions, particularly that the two materials in the couple have similar Z. ZT is therefore a method for comparing the potential efficiency of devices using different materials. Values of 1 are considered good; values in the 3–4 range are essential for thermoelectrics to compete with mechanical devices in efficiency.