How efficient are Thermomelectric generators
Reasonably well answered here 

The upper absolute maximum limit is the Carnot efficiency
= Delta_T / Thot.  

Where   


Delta_T is (Temperature on hot side - temperature on cold side) and 
Thot is the ABSOLUTE temperature of the hot side
=~ 273 + Thot_Celsius.   


So for eg A 100 C Delta T and hot side of 120C
Z_Carnot = Carnot efficiency = 100/(273 + 120) =~ 25%.
Actual efficiency will be a modest fraction of Z_Carnot
eg 10% absolute (= 10/25 = 40% of Carnot) would be good
and 5% absolute would be more likely .  

Manufacturers data sheets for TEGs invariably provide performance data for various Thot and delta T values. 

Waste heat TEG generation paper

Useful

Some value

Many leads here

Numerous images - relevance varies. Each linked to a web page.