Selecting the correct hot water storage tank with heating element for PV
Choosing the right storage tank determines how efficiently your solar thermal system provides hot water. Not every unit in the basement can be easily retrofitted, and not every new tank is compatible with every electrical add-on. This guide shows you what to look for in terms of hardware.
Which hot water storage tank with heating element is suitable for your PV system?
A hot water storage tank with an immersion heater converts your own solar power into hot water, bringing excess PV energy directly into your home. Choosing the right hardware allows you to significantly reduce your hot water bill during the sunny months and lower your reliance on gas or district heating. Three things are crucial when selecting a system: storage type, capacity, and electrical connection. This guide explains the details, which storage tanks can be retrofitted, and which immersion heater is compatible with your PV system. We share our experience from our installations at PVundSO and provide clear purchasing recommendations.
What distinguishes a hot water storage tank with heating element for PV
A hot water storage tank with an electric heating element is virtually indistinguishable from a conventional model. The key difference lies in the additional socket on the side wall, through which the electric heating element is screwed. An integrated thermostat switches it off once the target temperature is reached, preventing overheating. The housing itself is made of enamelled steel or stainless steel, and a thick layer of insulation retains the heat for many hours.
The additional electrical connection in the storage
The heating element operates in parallel with the existing heating system. When your PV system is running at full capacity, the electric auxiliary heater takes over the heating and uses the solar power directly in the house. If the solar power is insufficient, the gas or oil heating system kicks in as usual. This dual-fuel operation protects against supply shortages during changeable weather and in winter, without requiring you to replace your existing heating system.
Difference to conventional hot water storage tanks
Anyone looking for a complete set with storage tank and heating element will find pre-configured solutions in our category. Photovoltaics with heating elementUnlike a simple electricity storage system, this setup converts solar power directly into heat, thus relieving the strain on both the grid and the heating system. This solution is particularly advantageous in existing buildings because no major renovation of the utility room is required.
Which storage types are suitable for operation with the heating element?
Monovalent storage for pure PV operation
Monovalent storage systems are supplied by only one heat source. In combination with PV, this means that the immersion heater is the sole energy source for hot water. This option is suitable for new buildings with large solar thermal systems or for vacation homes with low hot water demand. However, a backup such as an instantaneous water heater or electric boiler should be available in winter, as sunlight hours become scarce.
Bivalent storage tank with heating and heating element
The dual-energy storage tank is the most common option for retrofitting. It features a heat exchanger for the existing gas or oil heating system and a separate connection for the immersion heater. During the warmer months, solar power heats the water, while in winter the boiler takes over. This setup significantly reduces gas consumption without requiring a complete replacement of your heating system.
Trivalent storage for solar thermal and PV
Trivalent storage tanks have two heat exchangers and a connection for an immersion heater. They combine existing solar thermal systems, conventional boilers, and PV immersion heaters in a single tank. For older buildings with existing solar thermal systems, this is often the most sensible solution, as the entire solar infrastructure can continue to be used.
Typical areas of application at a glance:
- Monovalent: New building with large PV system or holiday home
- Bivalent: Retrofitting with existing gas or oil heating system
- Trivalent: Old building with solar thermal energy plus PV heating element
Volume and dimensions according to household size
Guidelines for households of one to six people
The correct size of a hot water storage tank with an immersion heater depends on the number of people in the household and their daily hot water consumption. A tank that is too small will cool down quickly and prevent it from heating up properly, while a tank that is too large will increase heat loss through radiation. Finding the right size saves energy and protects the components from continuous strain.
Our experience from numerous installations shows that the following guidelines are sufficient in most cases. The table provides a rough orientation before you begin with the specific product selection.
| Persons | storage volume | Heating element power |
|---|---|---|
| 1 – 2 | 150 to 200 liters | 2 kW |
| 3 – 4 | 200 to 300 liters | 2 to 3 kW |
| 5 – 6 | 300 to 400 liters | 3 to 3,5 kW |
Buffer logic for changeable weather
Plan for an additional buffer tank if your system delivers more than 5 kWp or if you have a large surplus on many days. A larger storage tank absorbs this energy and keeps the heat available even on cloudy days. You can find an overview of available storage tank sizes at [link to overview]. PV hot water storage in our shop.
Overview of threads and installation height of sockets
Standard threads from common manufacturers
The socket is the crucial mechanical connection between the hot water storage tank and the heating element. Common models have an internal thread of 1,5 inches, less frequently 1 inch or 2 inches. Before purchasing, always check your storage tank's data sheet to ensure the chosen heating element will fit without an adapter. Most manufacturers specify the socket size directly on the nameplate or in the installation instructions.
Installation position and its effect on energy yield
The installation position affects how much water is actually heated. A heating element in the lower half heats almost the entire contents and is suitable for households with high hot water demand. A central position heats only the upper part but remains more economical if primarily small amounts of water are drawn.
The installation depth is also important, as the heating element must not come into contact with heat exchangers or anodes. Pay attention to the manufacturer's specifications regarding the free length inside the tank. An element that is too long can cause mechanical damage, while one that is too short heats inefficiently. If you are unsure, we can help you match the storage tank and heating element.
Materials and corrosion protection for hot water storage tanks with heating element
Enamel with magnesium anode
The most common design is the enamelled steel storage tank with a magnesium anode. The enamel forms a smooth inner coating that protects the metal from water. The anode is susceptible to galvanic corrosion and should be inspected every two to three years. This maintenance is particularly important with intensive use of the heating element, as the electrical process intensifies the galvanic reactions.
Stainless steel for low-maintenance solutions
Stainless steel storage tanks are the low-maintenance alternative. They don't require an anode, are significantly more expensive to purchase, but often last 20 years or more with less upkeep. For households that plan for the long term or have hard water in their plumbing system, the price difference often pays for itself over the tank's lifespan.
Regardless of the material, regular descaling protects the heating element and maintains its efficiency. Limescale acts like an insulating layer on the heating element and increases energy consumption. In areas with hard water, an annual visual inspection is recommended. The anode and heating element can be replaced individually if necessary, without having to replace the entire storage tank.
Retrofit existing hot water storage tanks or buy a new one
When retrofitting is worthwhile
Many existing storage tanks can be successfully retrofitted, provided they have a suitable 1,5-inch female threaded connection and are no more than about ten years old. The tank's volume should also be appropriate for the current household size. In our consultations, we regularly see that storage tanks less than ten years old can still be operated with an immersion heater for many years without any problems.
When a new storage device is the better choice
A new purchase is advisable if the coupling is missing or blocked, the storage tank already shows signs of corrosion, or its volume has been too small for years. Replacement is also worthwhile for older units with worn insulation, as standby losses drastically reduce the PV system's efficiency. A modern storage tank with better insulation pays for itself within just a few years.
Details on the practical use of excess PV power, feed-in tariffs and control technology can be found in our supplementary article. Use excess PV power with a heating elementThis guide focuses specifically on the hardware aspects of the selection process. Whether you're upgrading or buying new, if in doubt, seek personal advice.
Suitable heating elements for various hot water storage tanks
DC heating element my PV ELWA for pure PV solutions
Three heating element solutions from the Austrian company my-PV have become established on the market. Which one is right for you depends on whether the system already includes an inverter and the size of the storage tank. Our category Heating element for hot water storage tank bundles all three variants with their data sheets. my-PV DC ELWA It works directly with solar modules using direct current, without the need for an inverter. With a power output of 2 kW, it is particularly suitable for dedicated PV hot water solutions or smaller systems.
AC heating element ELWA 2 for existing systems
The AC ELWA 2 With a 3,5 kW output, it connects to an existing inverter and automatically utilizes excess energy. The device is compatible with many common inverters, including models from Hoymiles, Deye, Solis, and Sofar from our product range. The power output modulates continuously between zero and maximum, ensuring that only the actually available solar power flows into the hot water supply. For households with an existing PV system, this is the simplest way to utilize surplus energy.
External heating element with AC THOR for buffer tank
For larger buffer storage tanks or systems with a high surplus, the my-PV AC-THOR Power Manager It controls external heating elements with 3 or 9 kW steplessly and doesn't simply switch on and off, but modulates the power precisely to the available solar power. This precise control noticeably reduces the feed-in to the grid and significantly increases self-consumption in households with fluctuating demand.
Frequently asked questions about hot water storage tanks with heating elements
Which heating element is compatible with my old storage tank?
The crucial factors are the socket size and the free installation length. Check your storage tank's datasheet or measure the thread with calipers. Most my-PV models fit 1,5-inch sockets without an adapter. If you're unsure, we can assist you with the check using photos and the datasheet.
Is it worth replacing the entire memory module?
Replacing your existing tank is worthwhile if it's older than 15 years, shows signs of corrosion, or if its volume is no longer suitable for your household size. Worn insulation is another reason. Modern storage tanks have significantly lower standby losses, which pays off financially over the years.
What compatibility with my PV system do I need to check?
For AC immersion heaters, the inverter is crucial. The AC ELWA 2 works with over 70 brands, and the AC-THOR also supports the most common manufacturers. For DC immersion heaters, the module voltage and power must match the device. PV system checklist helps with systematic testing.
How long does a hot water storage tank with an immersion heater last?
Enamelled water heaters with a well-maintained anode last 15 to 20 years, stainless steel heaters often over 20 years. The heating element itself is a wear part that lasts five to ten years depending on water hardness and can then be replaced individually. If you have any questions about the right solution, we will be happy to advise you personally from our collection point in Hildesheim.
Conclusion
A hot water storage tank with an immersion heater is a practical solution for utilizing excess PV electricity in your home. Crucial to its success are choosing the right type of storage tank, the correct volume, and ensuring proper compatibility between the connection and the immersion heater. By inspecting your existing system and selecting the right hardware before purchasing, you can achieve savings of several hundred euros year after year from solar power. If you are unsure about the right setup for your situation, please don't hesitate to contact us. As electrical engineers, we are happy to advise you directly from our specialist shop in Hildesheim.
