Have you ever wondered how energy efficient a Softub is, or how long it takes to warm the water up? Unlike most hot tubs, Softub has no dedicated heating element. Instead, it uses waste heat from the pump motor to warm the water, which is in a sense very clever design. On the other hand, since the pump (hydromate) consumes roughly 900 W, the warm-up process takes a bit longer.
How long will it take?
Use the calculator below to estimate warm-up time and energy for a Softub Portico (1100 L) based on outdoor temperature, starting water temperature, and your target temperature. The model is calibrated using my real measurements, explained below.
The modeling is based on my own measurements, which I did just recently and I am going to describe in more details here below.
Real measurements, real numbers, and what they mean
When I switched my Softub Portico back from the biguanide experiment to dichlor, I had to fully drain the tub again.
- Fresh refill
- Cold tap water (~8 °C)
- Empty thermal mass
That gave me the opportunity to do a measurable warm-up cycle from scratch, executed under winter outdoor conditions.
Instead of just restarting the chemistry, I decided to measure something else:
How efficient is Softub really?
How much energy does it actually take to heat 1100 liters from cold to 39–40 °C?
And more importantly:
Does the insulation really isolate the tub from outside weather, or is that marketing?
Why Softub is different
Unlike most acrylic hot tubs, Softub:
- has no dedicated heating element
- heats water using waste heat from the circulation pump
- relies heavily on insulation and low-flow design
- is built around efficiency rather than brute heating power
The hydromate consumes roughly ~0.88–0.90 kW in operation. Almost all of that electrical energy ends up as heat in the water.
That means:
It behaves less like a traditional electric heater and more like a slow, steady, insulated thermal system. If you’re interested in the technical background and installation details, I wrote about that separately here:
👉 https://smarthome.exposed/softub/
Measurement Setup
To make the experiment meaningful, I logged:
- Electrical power (kW) – I used my Modbus meter connected to Loxone
- Cumulative energy (kWh) – Calculated
- Outside temperature – Weather station via Modbus to Loxone
- Wind (to see if insulation leaks heat) – Ultrasonic wind speed meter
- Manual water temperature readings at key milestones – Manual from the display
All of the figures above were automatically captured and I monitor these anyway in the smart home, the only manual readings were the hot tub water as there is no sensor one could connect to for logging the data automatically. Since I did not want to open the cover regularly to check the temperature, I relied on the temperature reading on the hydromate. That for sure is not the best thermometer in the world, given the resolution of just 0.5 C, but for this exercise it should be good enough.
So on one sunny February winter day I decided to empty the Softub, cleaned it, rinsed, put a new filter cartridge, and filled with tap water fully.
Fresh water temperature measured with thermometer was: 8.0 °C (46 F)
The hydromate ran continuously with the desired temperature set to 39 C (102 F) for the next two full days finishing the third early morning with water temperature 40.0 °C (104 F)
Target setting: 39 °C, Softub overshoots slightly due to hysteresis. Total heating duration: ≈ 65 hours.
Raw Results
From 8 °C → 40 °C: Temperature rise: 32 °C
Total electrical energy consumed: ≈ 57 kWh
Average heating rate: ~0.5 °C per hour
Hydromate power during heating: ~0.88–0.90 kW (very stable)
Theoretical energy vs. reality
Theoretical energy needed to heat 1100 L by 32 °C would look like this:
- Water heat capacity: 4.186 kJ/kg°C
- Ideal energy: ≈ 40.9 kWh
- Measured consumption: ≈ 57 kWh
- Difference: ≈ 16 kWh lost to environment during warm-up
That means roughly:
- 72% of input energy increased water temperature
- ~28% was lost to ambient environment during heating
For an outdoor 1100 L tub in winter conditions this is actually extremely good. The outdoor temperature was between -8 °C to +5 °C (18 F – 41 F).
Did outside temperature or wind matter?
Surprisingly little. I intentionally logged ambient temperature and also wind changes. The warm-up curve remained almost linear.
The only visible effect was that as water temperature increased,
losses increased slightly (as expected due to ΔT). But wind spikes and small temperature fluctuations did not visibly distort the heating slope.
This strongly indicates that Softub insulation is doing its job. Heat loss behaves like a smooth thermal gradient, not like a drafty exposed tank.
What this tells us about insulation
We derived an approximate heat-loss coefficient:
~0.004–0.007 kW per °C difference (water vs ambient)
In simple terms: If water is 30 °C warmer than outside,
heat loss is roughly: 0.15–0.20 kW
Given heating input of ~0.88 kW, that still leaves plenty of net heating power. For steady-state winter operation at 39 °C,
daily energy demand would roughly be: ~6–9 kWh per day
(depending on outside temperature)
Which aligns very closely with real-world user reports. For a permanently heated 1100 L outdoor tub in winter, this is solid performance.
Efficiency perspective (COP equivalent)
Softub doesn’t use a heat pump, so COP > 1 is not possible. But we can define a “COP-equivalent”: Useful heat into water / electrical energy consumed.
During warm-up: ~0.72
That sounds “low” compared to a heat pump but this includes full environmental losses in winter.
What matters more, Softub achieves this with:
- No dedicated heater
- No high-power spikes
- Only constant ~0.9 kW draw
- And excellent insulation
It is slow, but very predictable and controlled.
Data in chart
The red line shows the steady increase in water temperature over time. The blue line is the Hydromate’s electrical consumption. Pump load is influenced by water temperature (viscosity), motor temperature, hydraulic resistance, jet configuration (Poseidon vs. standard), and air injection (Venturi). Even without changing any settings during the test, small variations are visible. The yellow line shows outdoor temperature.
Final thoughts
This experiment started as a chemistry reset. It turned into something more valuable: A quantified understanding of how Softub behaves thermally, with the following outcomes:
- The insulation is genuinely effective
- Warm-up is slow but consistent
- Energy consumption aligns with theoretical expectations
- Weather impact is surprisingly small
- Efficiency is not marketing hype, it’s physics
The bottom line: Softub is not designed to heat fast. It is designed to heat gently, stay warm efficiently, and maintain temperature with minimal draw. And in that context, it performs very well. Measured in real winter conditions, Softub’s efficiency holds up remarkably well.
