Sources and assumptions
Some of the numbers here are physics. Others are trade conventions that vary by manufacturer. It matters which is which, so this page says.
Physical constants
| Value | Used for |
|---|---|
| Specific heat of water, 4.186 kJ/kg·K | Every heating and reheating figure. This one is not negotiable. |
| Electric resistance heating ≈ 100% efficient | Hot tub and sauna energy. Essentially all the input ends up as heat. |
| 52.1775 weeks per year | Keeping weekly, monthly and annual figures consistent. |
Trade conventions
These are widely used sizing rules, not laws of physics. We show them because different manufacturers use different numbers, and you should be able to see which one we picked.
| Convention | Value we use | Adjustable? |
|---|---|---|
| Sauna heater sizing | 1 kW per m³ of effective volume | Yes, on the page |
| Glass equivalent volume | +1.2 m³ per m² | Fixed, documented |
| Masonry equivalent volume | +1.5 m³ per m² | Fixed, documented |
| Sauna hold duty | 35% of rated power, default | Yes, a slider |
| Hot tub standby loss | 7–26 W/°C by cover quality | Yes, on the page |
Where a convention produces a band rather than a single figure — heater sizing especially — we show the band. A single decimal would imply precision nobody has.
Infrared sauna figures
The infrared mode of the sauna cost calculator uses these. Each row says what kind of figure it is. A maker's spec is a fact about one model. Maker guidance is advice that differs by brand. Our convention is a choice we made, and we say so.
| Figure | Value | Type | Source |
|---|---|---|---|
| Sunlighten Signature I, 1 person | 1,350 W, 120 V, 20 A | Maker's spec | Sunlighten product page |
| Sunlighten mPulse Aspire, 1 person | 2,160 W, 120 V, 20 A | Maker's spec | Sunlighten spec sheet (PDF) |
| Finnmark FD-2, 2 person | 1,750 W, 120 V, 15 A | Maker's spec, from a seller's listing | Sun Valley Saunas listing |
| Clearlight Sanctuary 2, 2 person | 2,260 W, 120 V, 20 A outlet | Maker's spec | Clearlight product page |
| Clearlight Sanctuary 3, 3 person | 2,680 W, 240 V | Maker's spec | Clearlight product page |
| Warm-up time | 10–15 min | Maker guidance | Clearlight FAQ |
| Warm-up time | 15–20 min | Maker guidance | Health Mate manual (PDF) |
| Session length | 20–45 min | Maker guidance | Clearlight FAQ |
| Heaters turn off and on once warm | Cycling | Maker guidance | Health Mate manual (PDF) |
| Set the thermostat to the top during a session | Heaters run all session | Maker guidance | Clearlight FAQ |
| Session draw | 60–100% of rated power, shown as a range | Our convention | Fixed, documented |
| Infrared power warning | Below 1 kW or above 5 kW | Our convention | Fixed, documented |
We left out the running costs some makers quote per session. They don't say what rate or session length they assume, so we can't check them.
Wiring and hot tub electrical figures
The sauna heater wiring guide and the two winter hot tub guides use these. Each figure comes from one maker's manual or listing and applies to that maker's products. Your own manual wins where it differs.
| Figure | Value | Type | Source |
|---|---|---|---|
| Harvia KIP heaters, 240 V single phase | 3 kW: 12.5 A, #12 4.5 kW: 18.8 A, #10 6 kW: 25 A, #10 8 kW: 33.3 A, #8 | Maker's spec | Harvia KIP manual (PDF) |
| Harvia: no GFI breaker, and 90 °C copper wire in the sauna walls | Install rule | Maker guidance | Harvia KIP manual (PDF) |
| Finnleo Designer heaters, 240 V single phase | 4.5 kW: 18.8 A, 10-2 6 kW: 25 A, 10-2 8 kW: 33.3 A, 8-2 | Maker's spec | Finnleo Designer manual (PDF) |
| Fixed electric heat sized at 125% of load | NEC 424.3(B), 210.19(A)(1), 210.20(A) | Code summary | Mike Holt, EC&M |
| Balboa easyPak heater ratings | 4 kW at 240 V is 1 kW at 120 V 5.5 kW at 240 V is 1.375 kW at 120 V | Maker's spec, from a seller's listing | Spa Depot listing |
| Coast Spas circuits | 120 V: isolated 15 A. 240 V: 40 or 50 A GFCI | Maker's spec | Coast Spas manual (PDF) |
| 110 V tubs can heat or run jets, not both | Heater off while jets run | Maker guidance | Coast Spas manual (PDF) |
| Hudson Bay Spas circuits | 120 V: dedicated 20 A. 240 V: 40 or 50 A GFCI | Maker's spec | Hudson Bay Spas manual (PDF) |
| Keep the tub running in winter. Freeze protection starts at 44 °F water | Winter care | Maker guidance | Hudson Bay Spas manual (PDF) |
What is not modelled
- Wind. An exposed hot tub loses noticeably more than these figures suggest.
- Heat pump heaters. Three to five times more efficient than the resistance heating assumed here.
- Humidity and air changes in a sauna, which affect perceived heat more than energy use.
- Infrared cabin air that stays cool. The air in an infrared cabin often stays below the setting, and Health Mate's manual warns the sauna may not reach its top setting. When that happens the heaters never switch off, so the high end of the range is the better guess.
- Time-of-use tariffs. Enter the rate for the hours you would actually run.
Images
Every photograph is CC0, public domain, or Creative Commons with the credit shown beneath the image. The full list, including sources and licences for images that need no attribution, is in credits.md.
The logo, tool illustrations and the room drawing are original work made for this site.
Corrections
If a figure here is wrong, tell us. Conventions change and rules of thumb deserve challenging.