By James McKeigue   America has been here before: in the 1970s, wars and revolution in the Middle East pushed up the oil price and derailed the US economy. Since then, solar and wind energy have struggled to compete on price with fossil fuels. But that looks set to change as they finally achieve ‘grid parity’ – the moment when alternative energy is as cheap as conventional grid power. Solar has already achieved this in sunny islands such as Hawaii, while wind does the same in windy places…