We assess the impact of the existence of a costly energy substitute (like wind, solar) for a non-renewable resource (like oil, coal) on the sustainability of consumption. The prospects for sustainability depend crucially on the costs of this substitute. If one can reduce the costs of the resource substitute via learning-by-using then we find that still this does not guarantee sustainability. Also, the poorer a country the less it will take the learning-by-using effect into account and the more likely it will be unsustainable.