Transactions are what makes Bitcoin useful (in theory) for "normal people". Thus it is fair to evaluate the entire Bitcoin network in terms of how much energy it takes versus (i.e. divided by) how many transactions it handles.
It means that to revert a 1 hour-old transaction, an attacker would have to consume double the energy the whole bitcoin network expended in that hour.
More energy expended == more security.
Number of TX is totally irrelevant.