A method for conceptual design of Solar Powered Unmanned Aircraft System (UAS) is presented. This method is based on traditional design methodology - wing loading estimation for preliminary sizing - modified for Solar Powered UAS case. Based on past works on Solar Powered UAS design, proposes a method that considers payload power consumption and therefore its impact on battery sizing. This battery sizing composes vehicle conceptual sizing equation. This method is useful for an assessment of Solar Powered UAS use in specific missions and serving as a start point for a more detailed design.A user interface was developed to automate the design process based on this method proposed.Solar Powered UAS is mainly considered for missions that require a long endurance flight with technological advances in areas such energy storages, solar cells and electronics miniaturization, Solar Powered UAS is already reality and has being mentioned in many Market Forecasts related to UAS nowadays.In addition, this method was validated with some built Solar Powered UAS presenting high accuracy between built UAS (its realization) and the method proposed.