A synthesis of the conditions that optimise accelerator driven subcritical reactors (ADSR) for energy production is presented. A core with low enrichment, effective multiplication factor keff 0.985-0.988 maintained during exploitation with control rods, ensure energy gain G higher than 15 and deep actinide burning in one cycle (15-20 % from the initial mass of the actinides, in comparison with 6-7% achieved in a fast reactor). The use of light ion beams, especially 7Li and 9Be at low energy (0.25-0.3 GeV/n) instead of 1-1.5 GeV protons allows the reduction of the accelerator dimensions ~2 times, correspondingly diminishing the building and maintenance costs. The high G achieved recommends ADSR as more efficient source of energy than fusion power plants. Based on the analysis of possible experiments meant to check the efficiency of different beams, the design of an extended lead target LETASUR (LEad Target for the study of Accelerator driven SUbcritical Reactors) is presented.