|There is widespread popular support for using renewable energy, particularly solar and wind energy, which provide electricity without giving rise to any carbon dioxide emissions. Harnessing these for electricity depends on the cost and efficiency of the technology, which is constantly improving, thus reducing costs per peak kilowatt. Utilising electricity from solar and wind in a grid requires some back-up generating capacity due to their intermittent nature. Policy settings to support renewables are also generally required to confer priority in grid systems and also subsidise them, and some 50 countries have these. Utilising solar and wind-generated electricity in a stand-alone system requires corresponding battery or other storage capacity. The possibility of large-scale use of hydrogen in the future as a transport fuel increases the potential for both renewables and base-load electricity supply.