Sauna heater sizing and running cost
A heater is sized by the volume of air it has to heat, plus allowances for surfaces that soak up heat. Enter the inside dimensions of the room; the heaters rated for it, and what a session costs, follow.
Heater size
6 kW
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| Heater | Rated room | Current at 240 V | Cable or breaker |
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Per session
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| Period | Energy | Cost |
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How the numbers are worked out
Room volume is length × width × height. Harvia's manuals size heaters at about 1 kW per cubic metre (35 cubic feet) of air, and count each square metre of glass, stone, tile or other uninsulated surface as 1.2 cubic metres of extra room, because those surfaces absorb heat that wood over insulation reflects back. A room with solid log walls, which take a long time to warm through, is multiplied by 1.5. The result is the volume the heater has to behave as though it heats.
Heater size picks the Harvia band whose middle is nearest that volume: 3 kW for 2–4 m³, 4.5 kW for 3–6, 6 kW for 5–8, 8 kW for 7–12, 9 kW for 8–14 and 10.5 kW for 9–15. The bands overlap, and two sizes often both fit.
The heater table lists every heater on this site whose US manual rates it for the adjusted volume, with the manual's current and cable. Makers set their own ranges, so a room can fit one maker's 6 kW and another's 4.5 kW. Only HUUM prints a breaker size; for the others the electrician sizes the breaker from the current and the cable.
Running cost counts the warm-up at full power, then the time in the sauna at the share you set. A 6 kW heater with a 45-minute warm-up and a 30-minute session at half duty uses 4.5 + 1.5 = 6 kWh. The running cost guide goes through the assumptions and how to measure your own.
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