Heat needed
Heating 1,500 L of water from 10 °C to 38 °C takes about 49 kWh of heat: 1,500 kg × 4.19 kJ/(kg·K) × 28 K. That is why a 3 kW electric heater needs about a day to heat a fresh fill, while a 25 kW wood stove does it in two to three hours.
| Tub (≈ 1,500 L, 5 °C outside) | Heat-up from 10 °C | Weekly energy (4 uses) |
|---|---|---|
| Acrylic spa, full foam, thermal cover, 3 kW heater | ≈ 16–20 h | ≈ 30–45 kWh electricity |
| Wood-fired fibreglass-lined tub, wooden lid, 25 kW stove | ≈ 2–3 h | ≈ 30–60 kg firewood (2 uses, fresh water every 5 days) |
What makes the biggest difference
- The cover. An open tub loses heat about ten times faster than a covered one.
- Insulation. Full-foam spas lose far less than wooden tubs, which is why wooden tubs are heated per use.
- Water changes. Every fresh fill costs the full heat-up again.