Failures of countries to set and achieve renewable energy targets are prevalent, producing uncertainty as to the possibility of renewable energy contributing to a reduction in global emissions. Lack of policy and incorrect modelling analyses are among the sources of these failures and understanding these two sources is crucial for improving confidence in renewables. We assess errors in projections pertaining to the capacity and production of renewable energy in the United States and those countries of the European Union that have strong commitments to green energy supply. Our results show that solar energy has the lowest level of uncertainty as it has the most achievable capacity projections. However, other renewables entail more attractive policies and further research is needed for the advancement of reliable technology and accurate weather predictions. Our findings also provide ranges for the projection uncertainties for six renewable energy technologies, drawing attentions to ways that the dominant errors in these renewable energy projections may be rectified.