The booster element is a 1.5kw and is very small compared to the average 2kw(180L) / 4kw(270L) heating capacity provided by the compressor. From an energy use perspective, the compressor is 4 times more efficient so it is always the preferred heating method. From a recovery perspective, the element can deliver approx. 20 litres per hr compared to the compressor delivery on average 50-60L/hr(180L) / 70-80L/hr(270L).
So as to when you should use the booster, only under very high demand or recovery needs, and in an emergency situation.