The energy needed to maintain a certain pool temperature varies greatly with a change in ambient temperature and with air movement (wind) over the pool surface.
A fixed output heat pump can only be on or off. When turned on, the fixed output heat pump works at 100% capacity to meet the heating demand of the pool. It will continue to do this until the desired pool temperature is met and will then cycle between off and on to try and maintain this temperature.
An inverter heat pump, however, uses a variable speed compressor which modulates its output, increasing or decreasing its output power to match the exact heat demand requirements of the pool.
When the demand is low the heat pump will reduce its output, limiting the electricity usage and the exertion placed on the heat pump’s components and limiting the start-up cycles.