All forms of low carbon electricity generation will need to grow significantly if the world is to control anthropogenic greenhouse gas emissions. Renewables, in particular solar and wind, will play an important role, but are not the whole solution. Two key considerations for energy policy makers are energy density and intermittency:

1.Energy density. Solar and wind are inately diffuse sources of energy. Powering a modern, and increasingly urban society with renewable energy alone would require many hundreds of times more space than doing so by either fossil fuels or nuclear.

2.Intermittency. Solar and wind are intermittent sources of energy that require backup. Calculating the additional costs of integrating intermittent renewable electricity sources into an energy system is complicated. Integrating low percentages of renewable energy incurs low costs, but the expense increases non-linearly as penetration grows and very significant backup or storage solutions are required.

At present the only potential complement for a system with high renewables penetration that is low carbon, continuous and scalable, is nuclear.