Sustainable development requires methods and tools to measure and compare the environmental impacts of human activities for various products (goods and services). Providing society with goods and services contributes to a wide range of environmental impacts. Environmental impacts include emissions into the environment and the consumption of resources as well as other interventions such as land use, etc. Life cycle assessment (LCA) is a technique for assessing environmental loads of a product or a system. The aim of this paper is to review existing energy and CO2 life cycle analyses of renewable sources based electricity generation systems. The paper points out that carbon emission from renewable energy (RE) systems are not nil, as is generally assumed while evaluating carbon credits. Further the range of carbon emissions from RE systems have been found out from existing literature and compared with those from fossil fuel based systems, so as to assist in a rational choice of energy supply systems.