Methodology
How the number gets made.
Calculation chain
- 1
Volume
Length × width × average depth gives water volume in litres. One litre of water is treated as one kilogram.
- 2
Heat-up energy
Pool volume in cubic metres × 1.16 kWh per °C × the rise from starting to target temperature × the workbook's 1.3 overhead. Available heating time accounts for the daily run window, the elapsed deadline and ongoing maintenance.
- 3
Daily losses
The workbook estimates daily maintenance as 30% of the energy needed to heat from monthly ambient temperature to the target, including its 1.3 overhead. A cover reduces this by 40% at 24 hours covered, scaled by hours used. All cover types share this generic factor; separate cover performance data was not supplied. Wind is reported, not numerically modelled.
- 4
Heater fit
The performance workbook adjusts rated output for air temperature, target water temperature and humidity: rated kW × [1 + 0.034(air − 27)] × [1 + 0.034(26 − water)] × [1 + 0.004(RH − 80)]. The multiplier is bounded to 0–1.3. Sizing uses the coldest selected monthly mean; cool-condition catalogue figures are reference values, not an interpolation curve. Both maintenance and warm-up requirements must be met.
- 5
Running cost
Thermal kWh divided by the assumed COP gives electrical kWh, multiplied by your electricity rate. COP defaults to 8 as a stated assumption; change it to match the model and conditions.
What is not included
- Filtration pump electricity
- Equipment purchase and installation
- Tariff time-of-use variation
- Site-specific shade, wind exposure and cold extremes
- Spa jets, water features and heavy bather load
- Any guarantee of achieved temperature
Results are estimates for planning and client conversations. If no catalogue model fits, the tool says so rather than recommending the largest unit; that is the point to involve a specialist.
