ENERGY EFFICIENCY, what environmentalists call the ‘fifth fuel’, after coal, gas, oil, and nuclear. ‘The best hope of avoiding that much warming [4ºC by 2100] is a huge increase in energy efficiency. One big component of that task will be to adapt the existing stock of buildings. Amory Lovins, one of the foremost prophets of energy efficiency and founder of Rocky Mountain Institute, a think-tank and consultancy based in Colorado, believes that the scope for improvement remains huge. He has long preached that proper building [homes, factories] design and energy storage can eliminate the need for air-conditioning and space heating in most climates, and illustrates this by growing bananas in his own house, on a windswept mountainside in Colorado where winter temperatures can drop to –44˚C. Eliminating the heating system for his house, he says, saved more money than he spent on insulation and fancy windows. His optimism is slowly winning converts’1‘Let there be light’. The Economist Special Report–Energy & Technology January 17th 2015, p.3-6. Amory Lovins ‘also sees clean-energy batteries in electric vehicles displacing oil demand, as petroleum did whale oil in the 19th century.’2‘At what cost?’ The Economist July 15th 2017, p.53.