There will be in a few decades when the block subsidy dwindles into insignificance and the network security is mostly funded by transaction fees.
The fact that the system will be dependent on transaction fees doesn't mean that you can measure transactions in energy that is spent effectively. A block can contain 4,000 transactions, or it can only contain 1 (e.g., mined right after the previous). Furthermore, a block that took very little time to be mined likely used less energy than one that took longer, while both blocks can have the exact same number of transactions.
Then, we have off-chain transactions, whose number is unknown as far as anyone can tell. Measuring transaction per energy spent is fundamentally prone to error.