Of course, as has been argued above, there are trade-offs to make in terms of economies of scale. There are some renewable energy technologies that can only be used on the relatively large scale – tidal barrages being an obvious example. The barrage at one time proposed on the Severn estuary in the UK would have had an 8.6 GW capacity. Offshore (i.e. deep sea) wave energy generation would be on a similarly large scale. Obviously, there are also possibilities for smaller barrages and smaller onshore or inshore wave energy devices, but it does seem that any viable energy system using renewable energy would have to have a mix of large-scale and small-scale renewable energy technologies. Very roughly speaking, and depending very much on the pattern of society being considered, it might be possible to generate about as much from small-scale local technologies as from large centralised systems. But the precise balance is a matter for debate and negotiation – on the basis of technical, social, economic and environmental concerns.