Hot water preparation: electricity vs. heat pump

 

Solutions for hot water preparation are often offered in combination with PV systems. The hot water is usually heated using a heating rod, i.e. purely electrically. The use of a heat pump would be much more efficient. To illustrate the difference, I have put together a small online calculator:

simple calculator

Temperature / water volume

Increase the temperature by:
°C
Amount of water to be heated:
Liters
Assumption heat pump coefficient of performance:

Result

Electric heating, amount of energy required:
-
kWh
Heat pump, amount of energy required:
-
kWh

extended practice calculator:

In practice, the water temperature of the boiler is predominantly heated from a lower threshold value (start temperature) to the target temperature. If the extracted water is topped up with cold water, the amount of water available until the boiler has to be reheated from the start temperature to the target temperature can be determined (result: "Amount of water extracted until the boiler has to be recharged").

Temperature / water quantity

Boiler size Water volume:
Liter
Ø Boiler start temperature:
°C
Ø Boiler target temperature:
°C
Cold water temperature:
°C
Assumption heat pump coefficient of performance:

Result

Electric heating, amount of energy required:
-
kWh
Heat pump, amount of energy required:
-
kWh

Amount of water
that can be withdrawn until the average water temperature drops from - °C to - °C.
-
liters
(The amount of water taken from the  - liter boiler is topped up with - °C cold water).

The calculator is based on the simplified assumption that the water temperature in the entire boiler is the same
and does not take heat losses into account.

COP

The COP, or coefficient of performance, is a parameter for the efficiency of the heat pump. A COP of 3 means that three times the amount of energy is required for heating than a heat pump would require.

To consider

  • Depending on the temperature difference, there is less hot water available than the storage volume, especially at a low target temperature: Heated to 50°C, a 500 liter boiler cannot provide 500 liters of hot water, see result: Amount of water.
  • Although the efficiency of solar thermal systems is higher than that of a PV system, the calculation looks completely different when combined with a heat pump.

See also: Control heating: PV surplus > ESP32 & Home Assistant

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