FRANKFURT—Electrical transmission networks in Germany are preparing for a solar eclipse this week that could trigger a sudden fall of around 1 gigawatt in solar power output, grid firm Amprion said on Monday. The event will take place between 09:20 GMT and 11:40 GMT on Thursday in northern Europe, an Amprion background paper said. Although it is expected to have a more modest impact than 2015’s solar eclipse across Europe—which reduced output by 15 GW in a country that can need up to 80 GW of power on winter days—Germany’s four high-voltage grid operators, or TSOs, must keep the grids in balance because large amounts of power cannot be stored. “The network frequency has to be stable and swings in production must be balanced,” said an Amprion spokeswoman. The TSOs can draw on alternative energy sources such as coal, gas, nuclear, and hydroelectric energy. One GW is equivalent to the …