Electrode boiler · Glossary

An electrode boiler generates heat electrically. The current flows via electrodes through a conductive medium, usually treated water, and heats it through its electrical resistance. It is one of the power-to-heat technologies because it converts electrical energy into usable heat for a district heating network, an industrial plant or a larger heating system. In a classic immersion heater, a metallic resistance element first becomes hot and then heats the water. In an electrode boiler, by contrast, the heat is generated directly in the water between the electrodes. One can think of it as a very large "kettle" that needs no heating coil and works with alternating voltage.

The most important technical parameter is the electrical power, usually stated in kilowatts or megawatts. Individual electrode boilers in district heating are usually built with roughly 5 to 50 megawatts and connected to the medium-voltage grid. Plants in the three-digit megawatt range generally consist of several boilers. One example is the 120-megawatt plant at the Berlin-Mitte combined heat and power station: it consists of three boilers of 40 megawatts each and receives its own transformers stepping down from 110 to 22 kilovolts. The amount of heat generated is measured in kilowatt-hours or megawatt-hours. A boiler with 20 megawatts of electrical power delivers, at full load, a good 20 megawatt-hours of heat in one hour, minus small losses. Electricity is thus converted almost completely into heat. However, this figure describes only the conversion in the boiler. It does not yet follow from it whether the use in the energy system is sensible, inexpensive or good for the climate.

The achievable temperatures depend on the design. The 20-megawatt boiler of Stadtwerke Erfurt, for example, heats water to up to 128 degrees Celsius. This is sufficient for the flow temperature of typical district heating networks. For industrial processes there are also designs that produce steam.

Distinction from immersion heater, heat pump and storage

Electrode boilers are often lumped together with other electric heating technologies. For assessment at system level, however, the distinction matters. An immersion heater or resistance boiler likewise converts electricity directly into heat, but uses solid heating elements to do so. For large capacities, electrode boilers are generally used; for smaller ones, electric instantaneous water heaters are also used. Electrode boilers can be regulated quickly. Depending on the design, this is done via the water level, the effective electrode area or the conductivity of the water. This makes them suitable for district heating networks and industry, where high power is to be absorbed at short notice.

A heat pump works differently. It does not use electricity as the heat source, but lifts ambient heat, waste heat or geothermal heat to a higher temperature level. From one kilowatt-hour of electricity it can therefore produce several kilowatt-hours of heat. This ratio is called the coefficient of performance, and averaged over a whole year the seasonal performance factor. An electrode boiler cannot achieve this multiplication: one kilowatt-hour of electricity becomes roughly one kilowatt-hour of heat. In return, it is technically simpler, cheaper to purchase per kilowatt and reaches high temperatures more easily. Per unit of heat, however, it consumes considerably more electricity than a suitable heat pump.

A heat storage is also not an electrode boiler: the boiler generates heat, the storage shifts it in time. In practice the two are often combined. The boiler can generate heat when electricity is cheap or when there is a lot of wind and solar power in the grid, and a hot-water or steam storage releases it later. Without storage, the heat must be taken off immediately. The flexibility of the boiler is then limited by the current heat demand, in summer for example by the low hot-water demand.

Role in the district heating network and in the power system

Electrode boilers make sense above all where large quantities of heat are in any case generated and distributed centrally: in district heating networks and larger neighbourhood networks, in industrial steam supply and at sites with existing boiler technology. There they can supplement or partly replace fossil peak-load boilers. These boilers today often cover load peaks on cold days, reserve cases or short operating windows. An electrode boiler can take over such tasks if the electricity connection, grid capacity, temperature and economic viability are suitable. Technical literature sees the role of electric boilers in district heating mainly as peak-load boilers, because they require little investment.

For the power system, it is not only the heat generated that counts. An electrode boiler is a controllable load that can draw additional electricity. This pays off when electricity is cheap on the market or when a grid operator, under certain rules, provides incentives for or requests additional consumption. In this way the boiler contributes to flexibility. This is most effective when the electricity consumption does not have to coincide with the heat demand in the same hour. This requires heat storage, suitable schedules and contracts between heat supply, the electricity market and grid operation.

Technically, electrode boilers react quickly and are therefore in principle suitable for flexible load control. Whether this capability is used depends on market rules, grid charges, levies, connection capacity, balancing group responsibility and operational requirements. A technically flexible plant may nevertheless run only rarely in economic terms if high electricity procurement costs or charges make operation unattractive. Conversely, it can become interesting at very low or negative electricity prices, as long as the heat displaces fossil fuels or can be stored.

Legal framework in Germany: use against grid congestion

In Germany there are two rules that specifically use electrode boilers in cases of grid congestion. Both are intended to prevent electricity from renewable plants from being curtailed. The first is Section 13 (6a) of the Energy Industry Act (EnWG). Under it, transmission system operators may conclude contracts with operators of combined heat and power plants: during congestion the CHP plant feeds in less electricity, and the heat supply is maintained with electricity supplied on a balance-sheet basis, for example via an electrode boiler. Prerequisites include: the CHP plant is located outside the legally defined southern region, was commissioned before 14 August 2020 and has more than 500 kilowatts of electrical capacity. The transmission system operator reimburses the investment in the electric heat generation once. The contracts run for at least five years, at most until the end of 2033. In total, no more than 2 gigawatts of electric heat generator capacity may be created in this way (as of the version of the law read).

The second rule is Section 13k EnWG, "Nutzen statt Abregeln" (use instead of curtail), in force since 29 December 2023. The day before, the transmission system operators forecast how much electricity from renewable plants in so-called relief regions would presumably have to be curtailed because of congestion. They allocate portions of these quantities to registered, additionally switchable loads, which receive remuneration for this. Since 1 October 2024, a two-year trial phase with simplified allocation has been running for this purpose. After that, a competitive tendering procedure is envisaged. One example is the 20-megawatt electrode boiler of Stadtwerke Erfurt: it has been in operation since May 2025, is financed by 50Hertz and is operated under Section 13k. The rules apply only to Germany, and which procedure a given plant may use depends on location, registration and contract.

In such projects the transmission system operator often co-finances. In the 120-megawatt project in Berlin-Mitte, the heat supplier BEW builds the plant, and 50Hertz bears a major part of the financing. Commissioning is planned for the end of 2028. Such plants are thus also a tool of grid congestion management and not only of heat generation.

High efficiency, limited effectiveness

The high conversion efficiency often leads to an oversimplified assessment. That almost every kilowatt-hour of electricity becomes heat sounds efficient. Compared with a heat pump, however, direct heating with electricity is electricity-intensive. A heat pump with a seasonal performance factor of three delivers about three times as much heat from the same amount of electricity. The electrode boiler therefore makes sense above all when heat pumps are technically or economically difficult to use: at very high temperatures, short peak loads, lack of a heat source, little space, as a reserve or for few operating hours with very cheap electricity.

The climate balance depends on the timing of operation. An electrode boiler is not automatically climate-neutral just because it runs on electricity. What is decisive is which power plants supply the additional electricity in the respective hour. This is described by the marginal emission factor. If this electricity comes predominantly from fossil power plants, the balance can be unfavourable. If the boiler uses electricity that would otherwise be curtailed and displaces a gas boiler, emissions fall considerably. The term "surplus electricity" therefore requires care. Electricity is not already surplus just because a lot of wind or sun is feeding in. What matters are the market price, the grid situation, line capacity and whether renewable plants would otherwise really be curtailed.

Location also matters. Electricity grids are line-bound. A boiler that draws electricity while wind turbines are being curtailed in another grid region does not necessarily prevent this curtailment. Flexible loads help above all where they are favourably located with regard to the congestion, that is, usually behind it, where the electricity cannot be transported away. This is exactly why the instruments mentioned are tied to relief regions or to congestion in the extra-high-voltage grid. Where such location and grid signals are missing, additional electricity consumption can even increase overall costs.

Economic and institutional conditions

Electrode boilers cost comparatively little per kilowatt of capacity and are technically simple. This makes them attractive even for only a few operating hours per year. The running costs, by contrast, depend strongly on the electricity price, grid charges, taxes and levies, and the price of the displaced fuel. For a gas peak-load boiler, fuel, CO₂ costs and efficiency determine the costs. For the electrode boiler, electricity procurement, connection capacity and regulatory costs are in the foreground. The grid connection in particular, with its own transformers and switchgear, can be a large cost block.

This makes planning more demanding for district heating operators. Besides the heat demand, they must also keep an eye on electricity prices, storage fill levels, grid restrictions, CHP operating mode, reserve requirements and emission costs. The electrode boiler competes with biomass boilers, large heat pumps, waste heat, solar thermal energy, CHP and fossil peak-load boilers. Which plant runs first results from variable costs, minimum loads, temperature requirements and contracts.

Institutionally, several actors are involved. The heat supplier plans the security of supply in the district heating network. The electricity grid operator checks connection capacity and grid compatibility. The electricity market supplies price signals for operating mode and balancing. Regulation determines which costs arise on electricity procurement and whether flexible loads are treated differently. Conflicts arise when a plant would make sense for the heating network but causes high grid costs at the electricity connection or reacts wrongly during congestion.

Typical misinterpretations

"Electrode boilers solve every surplus problem." This overlooks that they depend on heat demand, storage, grid connection and location. Heat can often be stored more cheaply than electricity, but not for arbitrarily long and not without losses or infrastructure costs. Without organised heat offtake, an electrode boiler remains an additional electricity consumer with high connection capacity.
"Power-to-heat is the same as an electrode boiler." Power-to-heat also includes large heat pumps, immersion heaters, electric instantaneous water heaters, industrial electric process heat and hybrid systems. The electrode boiler is the solution within it for direct electric heat with high power and fast controllability, not the one with the lowest electricity demand per unit of heat.
"More electricity consumption means more energy consumption." If an electrode boiler replaces gas, electricity consumption rises while the use of fossil fuel falls. For the assessment, what counts are the displaced fuel, operating hours, the emission factor of the electricity, grid impact and system costs. An electrode boiler reduces primary energy use above all when it uses renewable electricity that would otherwise be curtailed. Because it does not incorporate ambient heat, this effect is weaker than with heat pumps.

In summary, the electrode boiler denotes a powerful, direct electric heat generation. Its strengths are simple technology, fast controllability, low investment per kilowatt and good integration into central district heating networks. Its weakness is the high electricity demand per unit of heat. It makes sense where heat demand, storage, electricity price, grid situation and displaced fossil generation fit together. In Germany this is the case especially at grid congestion points, where it absorbs electricity that would otherwise be curtailed.

Sources

Energiewirtschaftsgesetz (EnWG) § 13 Systemverantwortung der Betreiber von Übertragungsnetzen — Bundesministerium der Justiz / Bundesamt für Justiz (gesetze-im-internet.de); unbekannt (konsolidierte Fassung, abgerufen 2026-11-11). § 13 Abs. 6a
Nutzen statt Abregeln — Netztransparenz.de (Informationsplattform der deutschen Übertragungsnetzbetreiber); unbekannt. Einführungstext zu § 13k EnWG
Power-to-Heat-Anlagen in der Fernwärmeversorgung (Dissertation) — Deutsche Nationalbibliothek (Hochschulschrift); unbekannt. Abschnitt zu E-Heizern / Elektrodenkesseln
Power-to-Heat — SWE Stadtwerke Erfurt; unbekannt. Abschnitte zur Anlage und „Der Elektrodenkessel“
Startschuss für Bau einer der größten Power to Heat Anlagen Europas — BFW Newsroom (Pressemitteilung BEW / 50Hertz / Stromnetz Berlin); unbekannt (ca. Mai 2026). Projektbeschreibung

Related glossary entries

Edited by eCMS on 11.11.2026 19:44