Here is where Bitcoin's wasteful algorithm comes in. The vast majority of the energy expenditure comes from bitcoin miners burning compute cycles to bid for the right to add the next round of transactions to the ledger, which happens every 10 minutes. The more energy that the miner spends on computing SHA1 hashes, the higher the chance that they'll find a lucky hash that entitles them to a monetary reward from the transaction fees, plus freshly minted bitcoins.

The idea behind this proof-of-work scheme is that creating an alternative blockchain history becomes prohibitively expensive, pushing the network to achive a distributed consensus. However, it's tremendously wasteful because the energy isn't actually being spent on "useful" work.