On/Off is defined as “all or nothing (AON type)”, to ensure it can function properly, its power must meet the condition for thermal balance: W1 >= Q1 (water heating) + Q2 (lose from water to air) When the pool is at the right temperature, Q1 is no longer necessary, so the power needed is W2 = Q2 (lose from water to air) Q1 is no longer needed, but AON type can’t change its power, so it keeps function as W1 which makes Q1 a heating waste. As consequence, a waste of energy will cost more on electricity bill, also the temperature is not stable, in fact the temperature will varied in an interval. 2. Inverter can adjust the speed rate to use a minimum power required to maintain the temperature. It can change from W1 to W2 intelligently to avoid unnecessary energy waste. That is why the inverter is an energy saving technology, to avoid the energy waste. And also the temperature is more stable. 3. AON type has a power rate constant. It didn’t consider the change of environment. Therefore, AON usually is not a full season heat pump, its performance will be very different in different condition.Like too hot in summer or not warm enough in autumn or winter. Inverter heat pump can totally avoid this problem by changing the power rate.