A 3 kW geyser element converts one unit of electricity into one unit of heat. A heat pump moves heat instead of creating it, delivering roughly three units of heat per unit of electricity — a coefficient of performance around 3.
For a four-person household using 150 litres of hot water a day, that difference is significant enough that most installations pay for themselves within two to four years, depending on tariff and usage pattern.
The catch is installation quality. Undersized or uninsulated pipework, a poorly placed circulation pump, a missing non-return valve or a badly set controller will erode the savings and generate 'no hot water' complaints on a healthy unit.
Annual servicing matters too: a dirty evaporator drops the COP, and a drifting temperature probe can quietly run the element back-up instead of the compressor, which defeats the entire purpose.