 Household electricity bills could be pushed up by around ?300 a year by 2020 as a result of a continued reliance on fossil fuels to provide energy, environmentalists claimed today.  Friends of the Earth hit back at claims that "green taxes" were causing energy bills to rise with a report suggesting investment in fossil fuel plants rather than renewables would leave UK households paying the increasing costs of coal and gas to make electricity.  according to the report, electricity bills rose by 30 per cent between 2000 and 2010, while gas bills rose 78 per cent. The rises were largely due to increased costs of coal, which rose by 71 per cent, and natural gas, which rose 90 per cent, in the decade.  If gas and coal costs rise in the coming decade by the same amount as they did between 2000 and 2010, an additional ?8bn a year would be needed by 2020 to generate electricity, costing the average householder an extra ?300 compared with lower costs if the UK met its targets to boost renewables, Friends of the Earth claimed.