f97.beERIK SCHMIDT’S WORKSHOP
← All terms

Glossary · Energy industry & energy transition

Immersion heater (Heizstab)

Heating element power-to-heat thermal storage electricity consumption heat photovoltaics Self-Consumption

An immersion heater (Heizstab) is an electrical resistance heating element that converts electricity directly into heat. It is usually located in a hot water tank, a buffer tank or an electric boiler. There it heats water because current flows through a resistive conductor. Technically, it is a very simple component: it needs no refrigerant, compressor, burner or complex hydraulics. Its output is stated in kilowatts (kW), the amount of heat generated in kilowatt-hours (kWh).

This simple technology easily leads to a wrong assessment. At the device level, an immersion heater achieves an efficiency of almost 100 percent, because one kilowatt-hour of electricity yields approximately one kilowatt-hour of heat. That sounds efficient, but compared with a heat pump it is a weak figure. A heat pump uses electricity to lift ambient heat from air, ground or water to a higher temperature level. In a Fraunhofer ISE field test of 77 systems in existing single- to three-family houses, the measured seasonal performance factors ranged from 2.6 to 5.4. Air-to-water heat pumps achieved an average of 3.4, ground-coupled systems 4.3. The actual values depend on temperature lift, system design and mode of operation. An immersion heater, by contrast, does not multiply heat; it merely converts electricity.

This distinction matters because electricity is a high-quality form of energy. It can be used to drive motors, produce light, supply data centres or charge electric vehicles. Low-temperature heat for drinking water or space heating, by contrast, is a low-quality form of energy. When electricity is turned directly into heat, almost nothing is lost in energy terms, but the work value (exergy) of the energy used drops sharply. Anyone arguing only in terms of efficiencies easily overlooks this difference between energy conservation and energy quality.

Output, energy quantity and storage

Immersion heaters are usually described by their electrical output, for example 2, 3, 6 or 9 kW. Smaller models often run on a single phase at 230 volts, larger ones usually on three phases at 400 volts. The output indicates how quickly electricity is converted into heat. The amount of energy results from output times running time. A 3 kW immersion heater running for two hours consumes 6 kWh of electricity and delivers approximately 6 kWh of heat to the tank.

A worked example shows what this means in practice: heating 300 litres of water from 45 to 60 °C takes around 5.2 kWh. This follows from the specific heat capacity of water, about 1.16 watt-hours per litre and kelvin. A 3 kW immersion heater therefore needs just under two hours for this. After that the tank is full, and further electricity can no longer be absorbed in a useful way, even if cheaper or self-generated electricity happens to be available.

The rated output alone is therefore not sufficient for an assessment. Tank size, target temperature, control and operating time are just as important. A large buffer tank offers more room for manoeuvre, but costs money and space and constantly loses heat, the more so the hotter it is. The immersion heater can therefore hardly be assessed on its own. It belongs to a system consisting of power source, control, storage and heat demand.

In photovoltaic systems, the immersion heater is often used to increase self-consumption. Surplus solar power that would otherwise be fed into the grid then heats water in the tank. This can be worthwhile if the feed-in tariff is low and the household actually needs the heat later. It is not automatically the best use of the electricity, however. The same electricity could charge an electric car, drive a heat pump, fill a battery storage unit or be fed into the grid.

Distinction from heat pumps, electric heating and power-to-heat

An immersion heater is a form of direct electric heating. It differs from a heat pump in its physical principle: in a heat pump, electricity drives a cycle that makes additional ambient heat usable. In an immersion heater, the electricity itself is the heat source. In many heat pump systems an immersion heater is installed as a backup or emergency heater. In well-planned systems, however, it rarely runs. In the Fraunhofer field test, only about 1.3 percent of the electricity consumption of air-to-water heat pumps was attributable to the immersion heater.

There are also clear differences from other electric heating systems. Direct heaters and electric underfloor heating release heat to the room immediately. Night storage heaters do store heat in a solid storage core, but are likewise tied to the room. An immersion heater, by contrast, usually heats water in a tank, which is later used as domestic hot water or heating water. This allows its operation to be shifted in time within the limits of the storage. This flexibility is of interest for the power system.

The term power-to-heat is broader. It covers all processes in which electricity generates heat, including large electrode boilers in district heating networks or plants in industry. An immersion heater in a single-family house is a small power-to-heat application, but not every power-to-heat plant is an immersion heater. Large plants differ in connection capacity, market role, control and purpose. At very low electricity prices, for example, they can generate heat for a district heating network.

Why operating time matters

For the power system it makes a difference when an immersion heater runs. If many immersion heaters switch on simultaneously at times of high load, they increase the peak load and place greater strain on the grids. If, on the other hand, they run specifically at times of high wind or solar feed-in, they can use electricity that would otherwise be curtailed or sold very cheaply. The same immersion heater can thus be a burden or a flexible load, depending on its control.

In many debates this distinction is lost. The immersion heater is then either rejected across the board as inefficient or promoted across the board as a solution for surplus electricity. Both ignore the conditions. As permanent space heating, direct electric heat is usually very electricity-intensive. As a rarely used backup heater, as a reserve or for temporary surpluses, it can be a robust and inexpensive complement. This only brings a benefit, however, if controllability, a suitable tank size, suitable tariffs and a heat demand that can be shifted in time come together.

With dynamic electricity tariffs or control based on exchange prices, an immersion heater can run specifically in low-price hours. A low wholesale price does not automatically mean, however, that capacity is free in the local distribution grid. The price shows scarcity or surplus across the entire market area, not the utilisation of a particular local substation. Grid-serving operation therefore requires different information and rules than pure price optimisation.

Legal classification in Germany: Section 14a EnWG

Since 1 January 2024, the determinations of the Federal Network Agency (Bundesnetzagentur) on controllable consumption devices under Section 14a EnWG have applied in Germany. They concern installations with a grid connection capacity of more than 4.2 kW that are connected to the low-voltage grid. The catalogue is exhaustive: it comprises private charging points, heat pump heating systems including backup or emergency heating devices such as immersion heaters, room cooling systems and electricity storage units with regard to their charging capacity. An immersion heater therefore expressly falls under the rule as part of a heat pump heating system. Its output then counts towards the relevant connection capacity of the heat pump.

In practice this means: the grid operator must connect new installations that fall under the rule. If there is a threat of grid overload, it may temporarily curtail their electricity draw, but must guarantee a minimum output of 4.2 kW. In return, grid charges are reduced. Older installations that went into operation before 2024 enjoy grandfathering or transitional rules, depending on the previous agreement. Night storage heaters are expressly excluded. A stand-alone PV immersion heater without a heat pump is not specifically named in the catalogue. Whether it is covered in an individual case should be clarified with the grid operator. What is decisive are the device category and the grid connection capacity, not controllability alone.

Typical applications

  • Backup and emergency heating in heat pump systems: the immersion heater steps in during faults, very low outdoor temperatures or the construction phase.

  • Domestic hot water and Legionella protection: the immersion heater can bring the tank to temperatures that a heat pump should not constantly supply for efficiency reasons. For large systems, i.e. tanks over 400 litres or pipework with a volume of more than 3 litres, the technical rules require a permanent minimum of 60 °C at the outlet of the drinking water heater. The German Environment Agency (Umweltbundesamt) does not consider merely occasional short heating-up a reliable substitute.

  • Use of PV surpluses: an energy manager or a continuously controlled immersion heater absorbs electricity in the buffer or hot water tank that would otherwise be fed into the grid.

  • Special cases of space heating: as the sole heat source, the immersion heater is only plausible in cases of very low heat demand, in rarely used parts of buildings or in emergency operation.

In a poorly insulated building, a permanently running immersion heater would consume a great deal of electricity. One kilowatt-hour of heat then costs about as much as one kilowatt-hour of electricity. A heat pump with a seasonal performance factor of 3.4 would need only about 30 percent of this amount of electricity for the same heat. The immersion heater is financially attractive above all because of its low purchase cost: it is cheap, low-maintenance and easy to retrofit. These low costs can, however, mask the ongoing electricity costs. With infrequent operation this hardly matters. With regular operation, by contrast, what counts is how many kilowatt-hours of electricity are burned directly as heat over the years and which alternative would have been available.

Misunderstandings about self-consumption

The assumption is particularly widespread that higher PV self-consumption automatically means greater efficiency. An immersion heater does increase the self-consumption share, because less electricity is fed into the grid. But if the solar power produces only one kilowatt-hour of heat via the immersion heater, whereas a heat pump would have made a good three kilowatt-hours from it, the benefit per kilowatt-hour of electricity is lower.

Talk of free solar power also often leads to wrong decisions. Self-generated electricity has opportunity costs: it could be fed in, stored or used in another way. These alternatives are not always economically better, but they exist. An immersion heater makes sense when heat demand, tank state, electricity price, feed-in tariff and other possible uses fit together.

An immersion heater is therefore neither a particularly efficient heating system nor fundamentally a mistake. It converts electricity directly into heat, is cheap to buy and technically very reliable, but cannot make more than one kilowatt-hour of heat from one kilowatt-hour of electricity. It makes sense where simplicity, backup function, short running times or the use of otherwise low-value electricity are more important than the greatest possible electrical efficiency. Anyone who uses the term precisely considers device efficiency, electricity demand, storage capability, operating time and alternatives separately.

Sources

Related glossary entries

Edited by eCMS on 12.11.2026 05:05

© 2026 Erik Schmidt. All rights reserved. Quotations with a link to the source are welcome.