During your time at the Department of Energy the deployment of renewable energy in the U.S. doubled. Is the fall in fossil-fuel prices killing the business case for renewables?

The decline in fossil-fuel prices does have some effect, but remember that 78 percent of the economies of the U.S. have state-mandated renewable portfolio standards. They require that a specified fraction of electricity must come from renewable energy. For example, in California the goal is 33 percent renewable energy by 2020.

Right now renewable electricity is roughly 13 percent of total electricity generated in the U.S. Half is hydropower and the other half is mostly wind energy, with some solar, biomass and geothermal. Renewable energy costs have come down significantly. Even if natural gas, which is the cheapest form of electricity generation today, stays at $4 per million Btus [British thermal units], wind without subsidy is almost as inexpensive.

Electrical generation in the sunnier parts of the U.S. is also approaching equality with a new natural gas power plant. The cost of wind and solar is anticipated to decline for at least a decade or two. Perhaps in a decade, renewables will be competitive with any new form of energy in many parts of the U.S. Published on Mar 8, 2015

Jonathon Porritt, is an eminent writer, broadcaster and commentator on sustainable development.  He is Co-Founder of Forum for the Future, the UK's leading sustainable development charity.

He is Co-Director of The Prince of Wales's Business and Sustainability Programme, was formerly Director of Friends of the Earth (1984-90) and co-chair of the Green Party (1980-83). In 2009 he stood down as Chair of the UK Sustainable Development Commission after nine years providing high-level advice to Government Ministers.