Taking the United Kingdom as a case study, this paper describes current energy use and a range of sustainable energy options for the future, including solar power and other renewables. I focus on the the area involved in collecting, converting, and delivering sustainable energy, looking in particular detail at the potential role of solar power. Britain consumes energy at a rate of about 5000 watts per person, and its population density is about 250 people per square kilometre. If we multiply the per-capita energy consumption by the population density, we obtain the average primary energy consumption per unit area, which for Britain is 1.25 watts per square metre. This areal power density is uncomfortably similar to the average power density that could be supplied by many renewables: the gravitational potential energy of rainfall in Scottish highlands has a raw power per unit area of roughly 0.24 watts per square metre; energy crops in Europe deliver about 0.5 watts per square metre; wind farms deliver roughly 2.5 watts per square metre; solar photovoltaic farms in Bavaria and Vermont deliver 4 watts per square metre; in sunnier locations, solar photovoltaic farms can deliver 10 watts per square metre; concentrating solar power stations in deserts might deliver 20 watts per square metre. In a decarbonized world that is renewable-powered, the land area required to maintain today's British energy consumption would have to be similar to the area of Britain. Several other high-density, high-consuming countries are in the same boat as Britain, and many other countries are rushing to join us. Decarbonizing such countries will only be possible through some combination of the following options: the embracing of country-sized renewable power generation facilities; large-scale energy imports from country-sized renewable facilities in other countries; population reduction; radical efficiency improvements and lifestyle changes; and the growth of non-renewable low-carbon sources, namely ``clean'' coal, ``clean'' gas, and nuclear power. If solar is to play a large role in the future energy system, we need new methods for energy storage; very-large-scale solar would either need to be combined with electricity stores, or it would need to serve a large flexible demand for energy that effectively stores useful energy in the form of chemicals, heat, or cold.
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