Right now PoW blockchain doesn't require energy consumption to scale with # of tx or even total value tx, but rather with value tx * time spent waiting for confirmations * probability of double spend attempt.

Unless we start seeing cartels that would aggregate tx that they'd like to see double spend having a long tail of users and tx means that the likelyhood that you'll be targeted for a double spend attack decreases.

All that to say that you don't necessarily need to increase the electricity spend to extend the same security to more people.