The Basic Calculation
Pool heat pump sizing depends on: pool volume (length × width × average depth), desired temperature rise, whether the pool is indoor or outdoor, whether a cover is used, and local climate conditions.
As a starting point: an outdoor pool in southern England with a cover, targeting 28°C, needs approximately 1.2–1.5 kW of heat pump output per cubic metre of water. A 50m³ pool (approximately 10m × 5m × 1m) needs a 60–75 kW heat pump.
Covered vs Uncovered Pools
A pool cover reduces heat loss by 50–70%. This is the single most impactful factor in sizing and running costs. An uncovered outdoor pool may need a heat pump twice the size of a covered one — and will cost twice as much to run.
If budget is a concern, investing in a quality thermal cover and fitting a correctly-sized heat pump will always beat installing an oversized unit for an uncovered pool.
Common Sizing Mistakes
The most frequent mistake we see is undersizing. Installers often quote the minimum heat pump size that can theoretically maintain temperature in ideal conditions. In practice, you need headroom for cold spells, heat-up from cold, and periods when the cover is off.
Oversizing is less common but still problematic. An oversized heat pump will short-cycle — starting and stopping frequently — which reduces efficiency and increases wear on the compressor. The ideal system runs for extended periods at partial load.