One of the most common questions we get from customers planning a tub installation is some version of "how much energy does this actually take?" Fair question. Whether you are sizing a heater, deciding between electric and propane, or trying to figure out how many bags of ice it takes to drop a cold plunge to 55°F, the answer comes down to a small handful of numbers.
This post walks through that math for our two standard sizes — using real Pacific Northwest tap water temperatures — so you can plan your setup with eyes open.
The Two Tubs We're Using
These numbers are for our two most common builds, filled to a comfortable soaking depth (not all the way to the brim):
Standard Cedar Hot Tub: 48"L × 30"W × 27"H, filled to 22" deep = about 137 gallons, target temperature 104°F.
Small Cold Soak Tub: 32"L × 32"W × 27"H, filled to 22" deep = about 98 gallons, target temperature 55°F.
The Math, In Brief
A few constants drive everything:
- Water weighs 8.34 pounds per gallon.
- It takes 1 BTU to raise one pound of water by 1°F.
- 1 kWh of electricity = 3,412 BTU.
- Western red cedar walls (1.5" thick) have a thermal resistance of about R-1.5, plus a bit of outdoor air-film resistance = roughly R-2.0 effective when the tub is lidded and sheltered.
Pacific Northwest hose-spigot water in western Washington and Oregon runs roughly 55–68°F in the warm months and 38–48°F in the cold months, depending on how exposed your supply line is.
Heating the Hot Tub to 104°F
This is the energy you need to bring 137 gallons of cold tap water up to soaking temperature.
| Spigot temperature | Temperature rise | BTU needed | kWh (electric) |
|---|---|---|---|
| 68°F (summer high) | 36°F | 41,100 | 12.1 |
| 55°F (summer low) | 49°F | 56,000 | 16.4 |
| 48°F (winter high) | 56°F | 64,000 | 18.8 |
| 38°F (winter low) | 66°F | 75,400 | 22.1 |
What That Means in Time
The BTU number is just energy — how long it actually takes depends on how much power your heater puts out.
| Heater type | Output (BTU/hr) | From 55°F | From 38°F |
|---|---|---|---|
| 1,500 W standard plug-in (120V) | 5,120 | ~11 hours | ~15 hours |
| 4,500 W spa element (240V) | 15,350 | ~3.6 hours | ~4.9 hours |
| 5,500 W spa element (240V) | 18,770 | ~3.0 hours | ~4.0 hours |
| 40,000 BTU propane tankless | 40,000 | ~1.4 hours | ~1.9 hours |
| 150,000 BTU gas tankless | 150,000 | ~22 minutes | ~30 minutes |
Real-world heat-up times run 10–20% longer than these theoretical minimums, because the tub itself is losing some heat to the surroundings while it heats up.
Chilling the Cold Soak Tub to 55°F
Here the picture flips. In the cold months, your spigot water already arrives below 55°F — the tub may need a touch of warming, not cooling. In the warm months, you've got 13°F or so to remove.
| Spigot temperature | What's needed | BTU to remove |
|---|---|---|
| 68°F (summer high) | Cool by 13°F | 10,600 |
| 55°F (summer low) | Already there | 0 |
| 48°F (winter high) | Warm slightly | — |
| 38°F (winter low) | Warm by 17°F | 13,900 to add |
How Many Bags of Ice?
For a no-equipment cold plunge approach, ice does the work. Each pound of ice absorbs about 174 BTU as it melts and warms to soak temperature (144 BTU to melt + ~30 BTU as the meltwater rises from 32°F to ~55°F).
With 817 lb of water in the small tub, one pound of ice drops the tub about 0.21°F:
| Ice added | Temperature drop |
|---|---|
| 10 lb bag | ~2°F |
| 20 lb bag | ~4°F |
| 40 lb (two 20 lb bags) | ~8.5°F |
| ~60 lb | ~13°F |
So a typical summer fill at 65–68°F needs roughly 5 to 6 ten-pound bags of ice to hit 55°F. A small water chiller (1/3 to 1/2 HP) handles the same job in a couple of hours with no trips to the gas station.
Holding Temperature: The Standing Heat Loss
Filling and heating is one cost. Holding the tub at temperature day after day is another. This is where insulation and the lid earn their keep.
The formula is straightforward: heat loss per hour = (surface area × temperature difference) ÷ R-value.
With a lid on and a sheltered location, our standard hot tub has about 44 square feet of heat-losing surface area (walls below waterline + bottom + lid). The cold soak has about 34 square feet.
Hot Tub at 104°F — Heat You Have to Replace
| Ambient air | Temperature difference | Heat loss/hr | Per day | kWh/day |
|---|---|---|---|---|
| 30°F (cold winter night) | 74°F | 1,630 BTU | 39,100 BTU | 11.4 |
| 50°F (typical PNW winter) | 54°F | 1,190 BTU | 28,500 BTU | 8.4 |
| 70°F (mild day) | 34°F | 750 BTU | 18,000 BTU | 5.3 |
| 90°F (summer heat wave) | 14°F | 310 BTU | 7,400 BTU | 2.2 |
Cold Soak at 55°F — Heat Lost or Gained
| Ambient air | Temp difference | Net flow | Per day |
|---|---|---|---|
| 30°F | –25°F | Tub loses 420 BTU/hr | Cools on its own |
| 50°F | –5°F | Tub loses 85 BTU/hr | Holds easily |
| 70°F | +15°F | Tub gains 250 BTU/hr | ~35 lb ice/day to offset |
| 90°F | +35°F | Tub gains 590 BTU/hr | ~80 lb ice/day to offset |
The cold tub becomes work in the summer — that 80 lb/day number is why anyone running a year-round cold plunge in a sunny spot ends up buying a chiller.
A Few Practical Caveats
These tables paint a clean picture, but a few real-world factors will move the numbers around.
The lid matters more than anything. Roughly 70% of heat loss from an uncovered hot tub happens at the water surface through evaporation. A cedar lid does not just slow conduction — it shuts off evaporation almost entirely. Without a lid, expect to roughly double the heat-loss numbers above. The same evaporation works in reverse for cold tubs: an uncovered cold tub on a dry, breezy day will cool itself faster than the table suggests.
Wind exposure cuts your R-value. A sheltered tub on a covered porch holds heat substantially better than the same tub in an open yard during a windstorm. Wind can knock 25–35% off your effective insulation.
Plumbing and pumps are heat-loss paths. If your tub has jets, every jet is a small chimney that pushes hot water through cold pipes. Circulation pumps run warm and add a bit of heat back, but jet systems on the whole are net losers.
Cold-weather upgrades are available. If you are running a tub year-round in a particularly cold climate and want to bring down holding costs further, get in touch — we can talk through insulation options.
What This Means for Sizing Your Setup
A few rules of thumb that fall out of the numbers above:
- A 120V plug-in heater can hold temperature in summer easily, and even in cold weather with a good lid. The 1,500 W output (5,120 BTU/hr) comfortably exceeds the 1,200–1,600 BTU/hr loss you'd see on a typical PNW winter night.
- For fast heat-up from cold, gas wins. A 40,000 BTU propane tankless brings the hot tub up in about 90 minutes regardless of season. Electric heaters get you there too, just on a much longer timeline.
- A cold plunge is easier to keep cold in the PNW than people expect. From October through May, ambient temperatures do most of the work for you. The hot months are when ice or a chiller starts to pay for itself.
- Lids are non-negotiable if you care about operating cost. We make 6"-wide cedar lid planks that drop on and double as a tray.
If you are mapping out a tub installation and want to think through the heating side, get in touch. We are happy to walk through what makes sense for your setup, location, and budget.