Quick take
Estimate heat demand first, then divide by realistic COP—not the highest brochure COP. Cold weather lowers efficiency and increases runtime.
COP is the key efficiency metric
A COP of 5 means roughly five units of heat are delivered for one unit of electrical energy at the stated test condition. COP falls as conditions become more difficult.
Do not use warm-weather brochure COP in winter
Heat-pump listings often show impressive COP at warm ambient air. A winter spa in near-freezing conditions will operate less efficiently.
Insulation changes the bill
A well-insulated cover, shell and plumbing reduce heat loss, which reduces both heater runtime and heat-pump sizing needs.