The conclusion: people (in the UK) could live on sustainable energy sources, but only with massive lifestyle changes.
Or (if I remember the book correctly) nuclear power.
A better summary of the book would be people need to realize that sustainable energy is going to require sacrifices (greater than unplugging a phone charger), that energy sources like nuclear power need to play a role unless a country is willing to import energy. Or as a one-liner: There is no silver bullet, but a shotgun will work!