Cryptocurrencies are an energy hog, and are mostly powered by fossil fuel generators, depending on location:

Buildings used to house cryptocurrency mining can create a massive strain on local electricity grids, with a single crypto transaction consuming more energy than that required to power 6 houses for a day in the U.S. The estimated global annual energy consumption of the current cryptocurrency market is over 68 TWh, equivalent to more than 19 coal fired power plants operating continuously. Due to the technical nature of blockchain, this number is projected to grow to 100 TWh annually

Source: The Energy Cost of Cryptocurrency | ENERGY STAR

It is now estimated that one bitcoin transaction uses 1.5 MWH of electricity to process.

Not only due they use huge amounts of electricity for the CPU and GPUs, they then use even more electricity to run coolers to remove the excess heat.

And bitcoin is just one of thousands of digital currencies, some better, some worse, when it comes to energy use.