FRANKFURT (Reuters) - Germany's cartel office is examining power prices volatility late in 2024, it said on Thursday, as it looks to ensure energy firms did not hold back supply to take advantage of weather conditions that had already sent prices to their highest levels in months.

Bouts of dark and calm weather in early November and mid-December, known in German as "dunkelflaute", meant wind and solar plants were not producing.

By comparison, renewables accounted for 80% of Germany's energy mix on Wednesday.

"It is an open probe ... there are no signs of abuses of market power (yet)," a spokesperson said in reply to an enquiry by Reuters.

Andreas Mundt, president of the cartel office, told the Rheinische Post newspaper in an interview published on Thursday that prices spikes had been anticipated this winter as fossil fuel curbs put in place under environmental laws come into play in a big way.

"We are monitoring the prices determination in the market continuously and closely," Mundt said. He did not say when the investigation began or when it would end.

As German boosts its renewables supply, it retains coal and gas plants to ensure the market is not undersupplied.

Some market participants had questioned why those plants were not tapped more quickly in November and December.

The cartel office has the power to fine suppliers if it deems there is undue exploitation of a dominant market position.

Germany's expanded renewables supply also means periods of negative prices where buyers are paid for taking power, for example in sunny and windy periods during holidays when there is little demand.

Power grids have to be balanced as storage capacity is not advanced enough yet to store all the power produced in such instances.

(Reporting by Matthias Inverardi; writing by Vera Eckert; editing by Miranda Murray and Jason Neely)