The energy expense of Bitcoin is dominated by its Proof-of-Work algorithm, the cost of processing transactions is negligible compared to that. And the PoW operates over the root of the last block's Merkle tree, which is a hash of all the transactions in the block. Being a fixed-size hash it doesn't matter if the block contains 1,000, 1 million or 1 billion transactions.

Therefore Bitcoin could scale to handle millions of transactions per second with a sublinear increase in electricity spent. [0]

[0] http://blog.vermorel.com/journal/2017/12/17/terabyte-blocks-...