Failures of countries in setting and achieving renewable energy targets are prevalent, raising uncertainty about the overall contribution of renewable energy to global emission reductions. Lack of policy and incorrect modelling analysis are among the sources of the failures. Thus understanding these two sources is crucial to improve confidence about renewables. We assess errors in the projections of renewable energy capacity and production in the United States and European Union countries, which have high commitments to green energy supply. Our results show that solar energy has the lowest uncertainty due to having the most achievable projections of capacity and production. On the other hand, other renewables may entail attractive policies, and further research is needed related to advancing reliable technology and accurate weather predictions. Our findings also provide ranges of projection uncertainty of six renewable energy technologies and, at the same time, draw attention to ways to rectify the dominant errors in the renewable energy projections.