As a refrigeration engineer and owner of a company for 25 years plus, I would just like to say that the wrong people are marketing and selling heat pumps without understanding the actual concept and principal of how it works. It is based on the refrigeration cycle and not direct heat , the refrigeration cycle removes heat from one space and moves it to another the energy required to do this for every 1kw in you get 3 Kw out which is one third of your 2kw element therefore a heat pump will heat your water far quicker than an electric element also what people forget is the cold side of the refrigeration cycle can also be used for cooling in air conditioning or refrigeration doubling the efficiency. To explain this if you have a heat pump there is a figure rating the COP of efficiency Google it. I have a heat pump installed at home and never run out of hot water. I also have air conditioning for dehumidifier and cooling.