Air source heat pump

It probably depends on how the system is configured. What make & model is your system

I've got a Daikin and the hot water immersion heater will only kick in when the hot water temperature is set for over about 50-55 degrees. However as we can manage quite happily on a water temperature of 45 degrees then it doesn't come on.

Higher water and heating temperatures reduce the efficiency of the heatpump and will increase the cost of running it so it's well worth trying to optimise the the temperatures downwards as far as possible and avoid using the immersion.

What sort of electricity tariff are you on, single rate or E7. If it's E7 then using the immersion overnight will be quite cost effective although not as cheap as heating the water by heatpump alone.

Learn how the heatpump operates and don't try to run it like a conventional boiler, it's better to run it for longer at lower temperatures rather than on and off for short periods at high temps.

As I said it's worth while understanding how your heat pump works and how to use it properly.

Ours is set to heat the water for two hours a day, every day and the set temp is 45degrees - in the summer it takes a around an hour or so and uses about 2kwh.
In the winter it takes longer and uses more energy, just over 3kwh.

Tha'ts because the incoming water is colder so it has to raise the temperature more and because it's colder outside so the units efficiency is reduced. The water temperature in the tank is still 45 degrees.

Most ASHP have their efficiency (called COP) rated at 7 degrees outside with a water flow temperature of 35 degrees. When it gets colder the unit has to work harder, likewise if you want the flow temperature hotter then it also has to work harder. Ideally you want to keep the flow temperature as low as possible.

It's reckoned that an ASHP uses about 2.5% more energy for every degree above 35degrees so running it at 55 degrees will cost you 50% more than running it at 35 degrees

The average COP of a heat pump should be around 3 under specified conditions (7/35), ie you get 3kwh out for every 1 kwh that you put in. An immersion heater has a COP of 1, so costs three times as much to run.

Check your documentation, if you don't have it then find out the make and model number and get the info from the web and download it.

As I said, check what tariff you are on, we are on a single rate tariff. E7 would not suit us as we run our virtually continuously during the winter at a low flow temperature of between 30 and 40 degrees as it has weather compensation and adjusts it's flow temp to suit.