A report in German magazine Der Spiegel has alleged that E.ON, RWE, EnBW and Vattenfall Europe all overcharged their customers by hundreds of millions of euros in 2006 and 2007.
The German electricity market regulator, BNE, has opened an investigation into allegations that customers have been overcharged by up to E800 million. E.ON and RWE have both denied any wrongdoing.
According to Der Spiegel, the investigation alleges that electricity reserves made available to compensate for fluctuations across networks due to demand or production were manipulated.
The complaint against the four energy providers was brought by Lichtblick, which claims that the companies involved did not co-operate to compensate for fluctuations and caused unnecessary costs to be passed onto customers. The small energy provider has asked that these costs be reimbursed.
BNE will come to a decision within two months.