<P> Distributed generation is a means of reducing total and transmission losses . </P> <Table> <Tr> <Th> Source </Th> <Th> 2012 production (TWh) </Th> <Th> Percentage of Total </Th> </Tr> <Tr> <Td> Coal </Td> <Td> 1,709 </Td> <Td> 41.5% </Td> </Tr> <Tr> <Td> Petroleum </Td> <Td> 30 </Td> <Td> 0.7% </Td> </Tr> <Tr> <Td> Natural Gas </Td> <Td> 1,066 </Td> <Td> 25.8% </Td> </Tr> <Tr> <Td> Nuclear </Td> <Td> 813 </Td> <Td> 19.7% </Td> </Tr> <Tr> <Td> Renewable sources </Td> <Td> 486 </Td> <Td> 11.8% </Td> </Tr> <Tr> <Td> Other </Td> <Td> 21 </Td> <Td> 0.5% </Td> </Tr> <Tr> <Td> Total </Td> <Td> 4,120 </Td> <Td> 100% </Td> </Tr> </Table> <Tr> <Th> Source </Th> <Th> 2012 production (TWh) </Th> <Th> Percentage of Total </Th> </Tr> <Tr> <Td> Coal </Td> <Td> 1,709 </Td> <Td> 41.5% </Td> </Tr>

When did the usa become an energy exporter