The price of electricity changes every 30 minutes. It’s most expensive in a “peak” time during the day - in the morning when people are getting ready for work, and in the evening when people arrive home from work.
During “peak overload” times, such as during a cold snap in winter the ‘peak load’ gets too much for the day-to-day national infrastructure. Everyone wants to use power at the same time as it might be really cold, but not enough electricity can be generated to support us all. So a standby electricity generation plant has to start to prevent the national grid from shutting down. It costs lots of money to start and keep these standby generation plants maintained - only to be started a handful of times a year.