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Solar power is a very old technology: Socrates explained how to use it to heat a house, while the Romans, the Chinese, the Puebloans all used houses to be shady in summer while trapping sunlight in the winter. All very elegant: but in recent years, solar power has come to mean something altogether more technologically advanced and disruptive to the oil energy order. So just how quickly is solar photovoltaic energy becoming affordable? And how will the world change, asks Tim Harford, if, or when, the cheapest source of power is solar power? Producer: Ben Crighton Editor: Richard Vadon