 SIR - a senior Downing Street adviser has told the Prime Minister that energy bills will rise by more than ?300 a year (report, September 5).  Scottish Power increased my annual electricity bill by 41.5 per cent on august 1, including a 53 per cent increase on the Economy 7 night tariff, despite its claim that prices were rising by 10 per cent.  Is this a record?  Robert Hood-Wright  Bodmin, Cornwall  SIR - Your report highlights the unexpectedly sharp rises in energy bills that have shocked households this year. This is a result of rising international fossil fuel prices. Renewables will add ?19 to average energy bills of ?1,232 this year, according to Ofgem - a fraction of the price of our fossil fuel dependency.  This dependency will be increasingly costly; 75 per cent of gas prices are linked to oil prices and the International Energy agency warned this year "the era of cheap oil is over". almost a third of Britain's ageing electricity generation infrastructure requires replacement by 2020, so investment cannot be avoided. Our gas reserves are in decline. The case for investing in homegrown renewable energy is overwhelming even before you get to climate change.  Leonie Greene  Renewable Energy association London SE1