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. They are shown 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.
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.
- 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.