Thermoelectric generators (TEG) convert the thermal energy flowing through them into electrical energy in a quantity dependent on the temperature gradient across the thermocouple between the TEG ceramic substrates. Electrical outputs generated by the TEG shown on technical sheets provided by the manufacturer are usually a mismatch to the actual energy generated. This is due to the measurement methods, which are usually conducted under laboratory conditions, often omitting the environmental parameters that the TEG is or will be operating in. Hence the mismatch is of relevant importance to simulating the evolution of thermoelectric systems during thermal transients. In this paper, a finite volume method for three dimensional diffusions to derive the surface temperature on the TEG ceramic substrate is used in conjunction with the environmental parameters and construction material to predict the temperature gradient with high accuracy which will therefore reflect a more precise output voltage generated by the TEG.