E.On has become the fourth of the big six energy companies to put up its gas and electricity prices. Its gas tariffs will rise by 18% from 13 September, while electricity bills will go up by 11%. Like the other firms, E.On blamed the increases on the 30% rise in wholesale energy prices this year. The government recently estimated that rising fuel prices meant a fifth of UK households now had to spend at least 10% of their incomes on keeping warm. "Uncertain times have had a huge effect on wholesale prices, with events in Japan and Libya all having a dramatic effect on gas and power prices in a relatively short period of time," said Graham Bartlett of E.On.