This paper describes the presumptive technique of power generation of amorphous silicon module considering various factors. The influences exerted by solar spectrum, spectral response, angle of incidence and efficiency deviation are evaluated. Then, various factors are formulated. The presumptive technique of power generation including these factors is examined. As a result, the proposal technique is able to presume with a high accuracy as compared with the conventional technique. (C) 2009 Published by Elsevier B.V.