Heating element (auxiliary electric heater)
The heating element is an auxiliary electric heater in the cylinder or appliance. It kicks in on very cold days, but only produces 1 kWh of heat from 1 kWh of electricity.
- Heating load of the house: the colder it is outside, the more heat the house needs.
- Heat pump output: it drops slightly in cold weather.
- Bivalence point: this is where the lines intersect.
- To the left of this, a heating element or a second heat generator helps out.
A heating element works like an immersion heater: electricity flows through a resistor, which heats up. Many air-to-water heat pumps have one built-in or as an accessory in the cylinder. Unlike the heat pump, it does not use environmental heat. One kilowatt-hour of electricity only produces one kilowatt-hour of heat.
What it is intended for
According to the Bundesverband Wärmepumpe (German Heat Pump Association), the heating element covers peak loads during the few very cold hours of the year. This is planned deliberately: a heat pump that has to manage everything on its own even on the coldest day would be oversized most of the time and would cycle frequently. Heating elements can usually be switched in stages.
When it becomes a problem
If the heat pump is significantly undersized, the heating element runs frequently. According to the association, this leads to poor efficiency and high operating costs. Its output also counts towards the registration with the grid operator, as the maximum power consumption of the heat pump and heating element combined must be reported.
Heating element and photovoltaics
klimaaktiv describes the direct use of solar power via a heating element as a simple way to convert surplus energy into hot water. If a heating element is already installed, all that is needed is a suitable control system. However, it is more efficient to use the solar power via the heat pump.
Tip: Ask to be shown where you can read the electricity consumption of the heating element. A high proportion is a reason to have the system checked.
Related terms
Bivalence pointMonovalent, monoenergetic, bivalentPhotovoltaic self-consumption with a heat pumpHeating load
Sources
- Bundesverband Wärmepumpe (BWP): Leitfaden Wärmepumpendimensionierung (German Heat Pump Association: Heat Pump Dimensioning Guideline) (accessed on 4 October 2026)
- klimaaktiv Ratgeber (guide) Hot Water for Single- and Two-Family Houses (BMK, 2021) (accessed on 4 October 2026)