Ukraine's national energy company has urged citizens to "charge everything" by 07:00 (04:00 GMT) Thursday because of expected power cuts caused by Russian missile strikes.
Energy plants were hit by Russian missiles again on Wednesday - part of a wave of such strikes since 10 October.
Outages of up to four hours at a time will affect the whole country on Thursday, grid operator Ukrenergo said.
It comes as Russia declares martial law in areas of Ukraine it has annexed.
Heightened security measures have also come into force in Russia - mostly areas along the Ukraine border.
In preparation for the blackouts, Ukrenergo has appealed to Ukrainians to stock up with water and ensure they have "warm socks and blankets and hugs for family and friends".
Phones, power banks, torches and batteries need to be charged, it urged.
As much as 40% of Ukraine's energy infrastructure has been seriously damaged, according to Oleksandr Kharchenko, an adviser to the energy minister.
President Volodymyr Zelensky said three energy facilities had been destroyed on Wednesday and energy companies were preparing for "all possible scenarios" for winter. He was due to address a summit of EU leaders, who are trying to reach an agreement on bringing down gas prices.
https://www.bbc.co.uk/news/world-europe-63323263