It’s an overestimate. Right now the total cost per transaction is around 82 US dollars, which at current exchange rates is 75 Euros. That cost covers everything - electricity, rent for the building, salary for staff, taxes, depreciation of mining equipment, and whatever profit is required to keep the miners in business. I don’t know what proportion of miner costs actually goes to electricity but I expect it’ll likely be much less than 70 Euros.

Perhaps someone got that 700 kWh figure by doing the reverse calculation - looking at how much a transaction cost and then assuming that all goes toward electricity.