Three types of solar  cookers have been developed: solar heat 
  boxes, which must be oriented manually and are composed of an 
  insulated  box  with  double  glazing;  parabolic  disk- type 
  reflectors  that concentrate the direct beams on a focal zone 
  where  the  pot  is  placed;  and  a  system  in  which solar 
  collection  is  separated  from the cooking stove and heat is 
  transported by a working fluid.  The latter makes it possible 
  to cook inside the house and to install a heat storage system 
  for  cooking  when the sun is not shifting.  It is well known 
  that a number of promotional  demonstrations of solar cookers 
  failed.  Such  factors  as  high  unit costs, lack of storage 
  facilities for supplying heat to cook the food during evening 
  hours,  unreliable  units  which  could  not  withstand rural 
  conditions and inadequate social acceptability was responsible 
  for the failure. In the last two years, however,it seems that 
  the insulated box cooker in certain parts of India(especially 
  Gujarat  state) has been  introduced with a reasonable amount 
  of  success.  The collector  surface and, therefore, costs of 
  solar  cookers  not  only  depend  on  average  overall solar 
  radiation, types  of fuel and cooking habits, but also on the 
  thermal efficiency of the cooker.  These thermal efficiencies 
  are  again  related  to  design ,  materials  and   state  of 
  maintenance  of  the  equipment.  Devices  and  corresponding  
  efficiencies are as follows.
  Flat plate solar cooker :   10-20 per cent (of the total with 
                              heat transport solar radiation)
  Parabolic disk reflector:   60-70 per cent (of  direct cooker 
                              beam  radiation)
  Insulated box cooker    :   35-45 per cent ( of  direct  beam 
                              radiation)