The physics in two lines
- Pull-down: water mass × 4.19 kJ/(kg·K) × the temperature drop. 100 US gallons (379 L) from 24 °C to 10 °C (75 → 50 °F) is about 22,000 kJ, or 21,000 BTU.
- Holding: heat that leaks in through the shell and lid (U-value × area × temperature difference) + about 700 W of body heat during a plunge + the circulation pump's heat.
A chiller rated at 4,500 BTU/h (1.3 kW) pulls that tub down in about 4 hours and then runs a small share of the time, using roughly 2 kWh a day including a 50 W pump in an insulated tub.
Why horsepower is not enough
"1/2 HP" describes the compressor, not the cooling it delivers. Compare chillers by their BTU/h at a stated water temperature (cooling output drops as the water gets colder and the room gets hotter). In a hot garage or outdoors in summer, an uninsulated stock tank can gain heat faster than a small chiller removes it.
| Tub and lid | Heat leaking in at 24 °C room, 10 °C water (≈ 380 L) |
|---|---|
| Insulated tub, insulated lid | ≈ 25 W |
| Inflatable tub, basic lid | ≈ 120 W |
| Stock tank, basic lid | ≈ 250 W |
| Stock tank, no lid | ≈ 310 W |
Safety
Use a GFCI/RCD-protected circuit, never enter a converted chest freezer while it is plugged in, and don't plunge alone. People with heart conditions should ask a doctor first.