Power giant E.ON warns of 'tough' times ahead

German power giant E.ON on Wednesday said it booked a 7.0-billion-euro ($7.7-billion) net loss in 2015 and warned that "the course ahead will be tougher and longer than anticipated."

E.ON, Germany's number one power supplier, said in a statement that massive writedowns on its electricity plants pushed it deep into the red, but at an underlying level, operating profit was "in line with expectations."

"We posted solid operating results in a very difficult market environment," said chief executive Johannes Teyssen.

"Our numbers reflect the far-reaching structural transformation that our industry is experiencing and that continues unabated in the current year.

German power utilities have complained that the country's transition from conventional carbon fuels to greener, cleaner sources of energy is squeezing their margins. The cost of having to close down their nuclear power plants and the heavy subsidies afforded to renewable energy have pushed them deeply into the red, the companies argue.