The problem of energy transfer in W UMa contact binaries is discussed. sEnergy transfer by a large-scale circulation is unlikely. A small-scale sphenomenological model similar to the mixing-length theory is proposed. sExpressions for the scale, velocity and flux are derived. A minimum spressure gradient between the components exists. The implications for scontact binaries are investigated. Equal entropy systems need deep scontact. Shallow contact unequal entropy systems might not be stable.