The basic maths
A commercial sauna's running cost comes down to three numbers: heater power in kW, hours at temperature, and your electricity tariff. A 9 kW cabin does not draw 9 kW continuously — once it reaches temperature the elements cycle, typically averaging 35-50% of rated power depending on insulation, glass area and door traffic.
So a 9 kW cabin held for ten hours at a 40% duty cycle uses roughly 36 kWh. At a €0.35/kWh commercial tariff that is about €12.60 a day, plus the heat-up charge before opening. Across a seven-day week that is a meaningful line item — and it is the line item most operators have never actually measured.
- Heater sizing: roughly 1 kW per cubic metre of effective cabin volume.
- Log walls, glass doors and uninsulated stone all increase effective volume.
- Heat-up: a healthy 8 kW cabin should reach 80 °C in 35-45 minutes.
- Wood-fired: budget by firewood kg/day rather than kWh — usually cheaper per session, more labour per session.
Where the money actually leaks
In every pilot we have run, the same three patterns show up. First, cabins heated far earlier than they need to be — an hour of unnecessary pre-heat every morning is roughly a month of free heat given away each year. Second, cabins left on after close, usually because nobody could confirm from the desk whether it was off. Third, silent performance drift: an ageing element takes 70 minutes to do a 40-minute job, and the cost of that is invisible until someone plots the heat-up curve.
- Pre-heating on habit rather than on booking demand.
- Cabins left running overnight or through quiet mid-week afternoons.
- Degraded elements and failing thermostats extending heat-up time.
- Doors propped open during cleaning while the heater fights the room.
What monitoring changes
You cannot manage a cost you cannot see. Continuous temperature logging gives you the heat-up curve, the time at temperature and the drift — which converts directly into kWh and euro. It also tells staff when the cabin is genuinely Ready, so pre-heat can be tightened to the real number rather than a padded guess.
For multi-site operators the comparison is the useful part: two identical cabins on two sites with a 20% cost gap is a maintenance ticket, not a mystery.