Comment by ZeroGravitas

Home hot water heating in the UK with heat pumps is about 250-300% efficient (slightly lower than the efficiency of home heating but still much better than resistive).

No one is storing 1000C water at home.

It is true that the temperature deltas affects efficiency. You can use the thermocline to draw from the cooler lower portion of the storage tank to push this further. Or less technically, just a bigger tank, though this has some tradeoffs.

In warmer countries they are set up differently can act as free air conditioning by extracting heat from indoor air at the same time as heating water.