The guests turn on the sauna and leave it running

A sauna of 4.5 kW that draws power 5 hours per day costs around 90 kWh over four nights, which at 0.30 to 0.40 euros per kWh comes to 27 to 36 euros per stay. With twelve rentals per year, that quickly adds up to 150 to 300 euros, purely because guests leave the sauna heating without paying for it themselves. By measuring the sauna's consumption separately and having it paid for in advance, that cost item no longer comes out of your pocket. With few rentals or a small sauna, the investment in such a split may not be worth the effort; that depends on your own figures.