The French electricity grid operator warned Friday of a "high risk" of network strain due to ongoing nuclear power plant outages, which could see businesses and households forced to curb usage to avoid outright power cuts.

In a winter outlook update, RTE said it expects France's network of nuclear plants will be operating at just 65 percent of capacity at the beginning of next year, producing around 40 gigawatts.

That forecast is well below the 48 gigawatts the state-owned electricity group EDF says will be available on January 1, when demand usually soars as winter settles in.

Around 25 of the 56 reactors operated by EDF across France are shut down for maintenance or to evaluate and repair tiny cracks that have been discovered in cooling pipes.

The company has mobilised more than 600 people, including around 100 specialised welders and other workers brought in from the United States and Canada, but it has acknowledged delays in finishing the work.

Last week, EDF said it would have 46 reactors online in January.

RTE has rolled out an Ecowatt application and website to alert when grid usage is too high, requiring it to ask clients to cut consumption or risk voltage drops ("brownouts") and targeted electricity cuts.

Much will depend on the weather and "a possible cold wave, even a moderate one," RTE said.

The government is already urging people to show "restraint" with electricity use, including advertisements showing tips such as turning down thermostats, taking shorter showers and running appliances at night.