In general, renewable energy sources are less energy dense then fossil sources. This has two practical implications: more space to store or extract the energy source is required and it takes longer to harness an equivalent amount of useful energy.
Fossil fuels have limited reserves estimated to last between 50 and 120 years based on their current and projected consumption. Renewable energy is limitless and, as its name suggests, is renewable and at least one of its forms is available anywhere in the world.
In the long-term, the on-going cost of renewable energy based systems is likely to be lower than those based on fossil sources with fossil fuels widely expected to increase in cost as global demand increases. In addition, as the use of renewable systems increase, economies of scale will mean installation costs will become more competitive with traditional sources.