But energy use isn’t as simple as measuring transaction efficiency, there’s a lot more to a currency than storing who transacted with whom. There’s:
security of transactions - fraud and whatnot
coordination costs - international transactions, etc
cyber security, websites for managing money, etc
Or in terms you’ve used, someone needs to maintain that database, that database needs to be in some facility, and someone needs to audit the database. All of that is baked into cryptocurrencies. Yet the comparisons I’ve seen either account for way too much (e.g. bank tech support), or not enough (e.g. only POS and network costs).