Because blocksize is fixed and blocks are consistently full, it's possible to estimate the next block's energy usage per transaction. All blocks are currently full as miners have an incentive to collect all transactions available, ordered by their attached transaction fee.

If you mean that the same amount of power would be used whether the blocks are full or empty, you are technically correct but in practice it's not relevant until blocks are consistently not full.

Does BTC makes sense if there would be no transactions?

If transactions are critical for everyone, wouldn't that mean that keeping bitcoin mining alive is a fundamental part of transactions?

We could argue that fiat keeps databases running (if we ignore physical money) and bitcoin is keeping blocks mining active.

I don't think it is wrong to say megatwatts per transactions. We could ignore this completly and say 'the baseload of just keeping btc running is x megawatts per hour' and that would just ring the same bells.

I'm pondering if we could also say something like "btc itself as a cryptosystem motivates actors to consume megawatts per hour due to the interest in btc and the current fiat<>btc exchange value"?