The rate of transactions per second does not affect Cardano's power consumption significantly, but implementation of smart contracts, scaling mechanisms, etc. may have a measurable impact in the future.
Cardano is largely as eco-friendly as all proof-of-stake blockchains. Its actual energy consumption is roughly the power consumption of a typical staking pool node times the number of staking pool nodes. The power consumption per transaction is millions of times more energy efficient than Bitcoin.
- Proof-of-work (PoW) blockchains rely on the miner's ability to produce the answer to a question whose answer must be guessed, then proven to be correct. The more guesses and proofs that the miner can make per second, the higher their odds of winning the next reward.
- Proof of stake (PoS) blockchains rely on the quantity of a cryptocurrency owned or delegated to the miner to raise their odds in a fair lottery to win the next reward
PoW blockchains favor miners with the most computing power and PoS blockchains favor miners with the most coins (owned or delegated).
Over time this has led to the two largest PoW cryptocurrencies (Bitcoin and Ethereum) using millions of times more power than PoS blockchains.
Cambridge Bitcoin Electricity Consumption Index