Use excess PV power with a heating element instead of feeding it into the grid.
Those who operate a photovoltaic system often feed more electricity into the grid than needed on sunny days. With the right technology, this surplus can be used directly for hot water preparation. This increases your self-consumption and simultaneously reduces your heating costs.
How to convert your excess PV power into hot water using an immersion heater
A PV surplus heating element converts excess solar power directly into hot water. Whenever your photovoltaic system produces more energy than your household consumes, the heating element activates and heats your domestic hot water. Instead of feeding the electricity into the grid for a few cents, you use it yourself and save money. At PVundSO, our qualified electrical engineers regularly advise customers on this very topic. In this guide, you'll learn how a PV surplus heating element works, what the technology costs, and what you should consider when selecting and installing one.
What happens to the surplus energy from your PV system?
Feed into the grid or consume it yourself
A photovoltaic system on your home often generates significantly more electricity during the day than the household needs. This surplus is automatically fed into the public grid, for which you receive the legally mandated feed-in tariff. Currently, this is around 8 cents per kilowatt-hour.
The electricity you draw from the grid, on the other hand, costs you 30 to 40 cents per kilowatt-hour. Every kilowatt-hour you consume yourself therefore saves you many times more than what you would pay for feeding it back into the grid. This is precisely where the targeted use of surplus PV power comes in.
If you want to increase your self-consumption of energy, several options are available. A battery storage system buffers the surplus for the evening hours. An immersion heater converts it directly into hot water. Both solutions can also be combined.
Why using surplus energy is financially worthwhile
An example illustrates the difference. A 10 kWp photovoltaic system easily generates 50 kWh of electricity on a sunny summer day. If the household only uses 15 kWh of this, 35 kWh flow unused into the grid. This results in a feed-in tariff of around €2,80.
Instead, use 10 to 15 kWh of this energy for hot water preparation with an immersion heater, and you'll save 3 to 6 euros on gas or electricity costs in a single day. Over a whole year, this adds up to several hundred euros. An immersion heater for surplus PV energy is therefore one of the quickest ways to operate your photovoltaic system more economically.
This is how a heating element works with excess PV power
Structure and functional principle
A PV heating element is essentially an electric heating element that is screwed into your existing hot water storage tank or buffer tank. As soon as the photovoltaic system generates more electricity than is consumed in the household, the excess control system detects the available surplus. It then directs the excess energy to the heating element, which converts the electricity into heat.
The principle is similar to an immersion heater. The heating element gradually heats the water in the storage tank as long as solar power is available. As soon as the sun's rays diminish or the household requires more electricity, the control system reduces the heating power or switches off the heating element. In this way, no additional grid electricity is used for hot water preparation.
Stepless control for optimal utilization
Simple heating elements operate on an on/off principle. They only switch on when a certain threshold is reached, for example, a surplus of 2 kW. If the available surplus is below this threshold, the heating element remains off and the energy is fed into the grid.
Continuously variable heating elements like the my-PV AC ELWA 2 solve this problem. They smoothly adjust their output to the available PV surplus. Heating begins even with just a few hundred watts, and the output rises or falls with the solar supply. This allows you to make efficient use of even small surpluses, and the grid feed-in drops to almost zero.
What are the requirements for a PV heating element?
Minimum output of the photovoltaic system
For a heating element to be worthwhile when using excess PV power, your system should have a capacity of at least 3 to 5 kWp. With smaller systems or balcony solar installations, the excess power is usually insufficient to noticeably heat the water. Household consumption further reduces the available surplus.
The size of the system alone is not the deciding factor. Your consumption profile also plays a crucial role. Those who are rarely home during the day have more surplus energy available than a household working from home. Our free PV system checklist will help you realistically assess your own situation.
Hot water storage tank and connection
A hot water storage tank or buffer tank with a free 1,5-inch thread for the heating element is required. Most common tanks with a volume of 150 liters or more have a corresponding socket. For older tanks, it's worth taking a quick look at the existing connections.
The heating element is installed in addition to the existing heating system. It does not replace the boiler or heat pump, but rather complements them effectively. On sunny days, the PV heating element takes over the entire hot water production. During the transitional months, it noticeably reduces the load on the main heating system.
How much does a heating element for excess PV power cost?
Acquisition costs and installation
The investment for an immersion heater with PV surplus control ranges from €300 to €2.000, depending on the features. A simple immersion heater without its own control system costs between €80 and €350. The surplus control system itself adds another €150 to €800 to the cost. Installation costs are approximately €200 to €500.
For those who prefer a complete solution, we offer complete photovoltaic kits with a heating element . The solar modules, heating element, and control system are already perfectly matched. This simplifies planning and can save costs compared to purchasing components separately.
Amortization and annual savings
| household size | Annual savings | Investment | Amortization |
|---|---|---|---|
| 2 people | 200 to 350 Euro | 400 to 800 Euro | 1,5 to 3 years |
| 4 people | 350 to 600 Euro | 500 to 1.200 Euro | 1 to 3 years |
| 6 people | 500 to 800 Euro | 600 to 1.500 Euro | 1 to 2,5 years |
The exact savings depend on your hot water consumption, the size of your PV system, and current energy prices. A typical four-person household saves 1.200 to 2.000 kWh of grid electricity per year, which the immersion heater covers with solar power. At electricity costs of 30 cents per kWh, this results in annual savings of 350 to 600 euros. Most immersion heater solutions therefore pay for themselves within one to three years.
PV heating element or heat pump for hot water.
Efficiency in comparison
An immersion heater converts electricity into heat at a 1:1 ratio. One kilowatt-hour of electricity produces exactly one kilowatt-hour of heat. In contrast, a domestic hot water heat pump achieves a COP of 3 to 4. It generates three to four kilowatt-hours of heat from one kilowatt-hour of electricity by also utilizing ambient heat from the air.
The higher efficiency of the heat pump seems clearly superior at first glance. However, this advantage is less significant if there is a sufficient surplus of free PV power. If the solar power would otherwise flow unused into the grid, the COP (coefficient of performance) plays a subordinate role.
When is which solution worthwhile?
The immersion heater for surplus PV power scores points with its low purchase price, easy installation, and maintenance-free operation. It is particularly suitable if you already have a hot water storage tank and your PV system regularly produces surplus power in the summer.
A domestic hot water heat pump is more worthwhile if the PV yield is limited or if solar power is needed to support hot water production even in winter. The higher costs of €1.500 to €3.500 are offset by the improved efficiency over a longer period. For many homeowners, a PV immersion heater is the ideal entry point because the investment is low and pays for itself quickly.
Choosing the right surplus control and energy management
Continuously variable vs. switching control
The controller is the heart of every immersion heater solution. Switching controllers operate with a simple threshold. If the PV surplus exceeds a defined value, the immersion heater switches on at full power. If the surplus falls below the threshold, it switches off again. This option is inexpensive and simple, but it wastes surplus energy below the switching threshold.
In contrast , stepless controllers like the my-PV AC THOR regulate the power output smoothly from 0 to 3 kW. The AC THOR makes any standard immersion heater intelligent by adjusting the heating output to the available solar surplus with second-by-second precision. In combination with a suitable 3 kW immersion heater for the AC THOR, it forms a coordinated system for domestic hot water heating using solar power.
Compatibility with inverters and smart home systems
Modern excess power controllers communicate with your inverter via Wi-Fi or Modbus. Devices from manufacturers such as Hoymiles, Solis, Sofar, or Deye are generally compatible. When choosing a controller, make sure it's compatible with your existing system.
Those using a smart home system can often integrate the PV heating element. Target temperature, priorities, and time windows can be set via apps or web portals. This allows you to keep track of your self-consumption and hot water production at all times.
Properly dimensioning the PV heating element
Performance based on household size and hot water demand
| household size | Hot water demand per day | Recommended heating element power | Recommended storage volume |
|---|---|---|---|
| 1 to 2 persons | 60 to 100 liters | 2 kW | 150 to 200 liters |
| 3 to 4 persons | 120 to 200 liters | 2 to 3 kW | 200 to 300 liters |
| 5 to 6 persons | 200 to 300 liters | 3 to 3,5 kW | 300 to 400 liters |
For smaller households with moderate hot water needs, the my-PV DC ELWA with 2 kW output is suitable . It operates directly from the solar modules using direct current and does not require an inverter. For larger households, heating elements with 3 to 3,5 kW output are recommended, such as the AC ELWA 2 with stepless control.
The correct sizing depends not only on the number of people in the household. Showering habits, the presence of a bathtub, and the installed PV capacity on the roof also play a role. If you are unsure, we would be happy to advise you personally on the right solution for your home.
Storage volume and temperature settings
The storage volume should be appropriate for the heating element and the daily hot water demand. A storage tank that is too small limits the usable excess energy because the water reaches the target temperature too quickly. Conversely, a storage tank that is too large requires correspondingly more energy to reach the desired temperature.
A target temperature of 55 to 60 degrees Celsius is recommended for daily operation. This temperature is sufficient for all household applications. The storage tank should be heated to 65 degrees at least once a week to reliably kill Legionella bacteria. Many control systems perform this Legionella protection function automatically.
Common mistakes when using excess PV power for hot water
Avoidable errors frequently occur during the installation and operation of a PV heating element. You should be aware of the following points before putting your system into operation.
- Heating element is too large A 6 kW heating element connected to a 5 kWp system can rarely operate at full capacity. As soon as the PV system doesn't generate enough excess power, the heating element draws the missing electricity from the grid. Choose the heating output to match your system and your typical excess power output.
- Legionella protection neglected Hot water below 55 degrees Celsius promotes the growth of Legionella bacteria in the storage tank. Ensure that your controller regularly heats the water to at least 65 degrees Celsius. Modern devices like the AC ELWA 2 offer this function as standard.
- Heating element and battery storage compete Without clear prioritization in energy management, the heating element can be used to Power storage Remove the excess. Establish a sensible order. Generally, battery storage takes priority, as stored electricity is more flexible in its use than hot water.
- Missing or inappropriate excess detection Without a smart meter or suitable control system, the heating element doesn't know when there's actually excess energy available. In the worst case, it will run on expensive grid electricity. Make sure that the excess energy detection is configured correctly.
Frequently asked questions about the PV heating element
Can I operate a heating element with a balcony power plant?
With a balcony solar power system generating 800 watts of feed-in power, very little surplus energy remains after household consumption for use with an immersion heater. In most cases, the available power is insufficient to significantly heat the water. Using an immersion heater to utilize excess PV power becomes worthwhile with a PV system of at least 3 kWp.
Is it possible to retrofit an existing heating element?
Yes, with an external excess power controller like the my-PV AC THOR, an existing heating element in the hot water storage tank can also be intelligently controlled. Retrofitting requires no structural modifications to the tank. The controller is connected between the energy management system and the heating element and automatically detects the available solar surplus.
Will a PV heating element also work in winter?
In winter, PV yield is lower and the hours of sunshine are shorter. The heating element therefore runs less frequently and doesn't always reach the full target temperature using solar power alone. However, on clear winter days, it can still make a noticeable contribution to hot water production. Its main use is from March to October, when solar yields are significantly higher.
Do I also need a battery storage system?
An immersion heater and a battery storage system are two different ways to utilize surplus PV power. The immersion heater converts electricity into heat, while the battery storage system stores the electricity for later use. Both solutions complement each other well, provided the energy management system prioritizes them correctly.
Conclusion
A PV surplus heating element is one of the simplest and most economical ways to increase the self-consumption of your photovoltaic system. With investment costs starting at €300 and a payback period of one to three years, the technology quickly pays for itself. Whether continuously adjustable or as a simple switching solution, there is a suitable option for almost every system size and every household. If you are interested in a heating element to utilize your solar surplus, you will find various products from my-PV in our range. We are happy to assist you personally with any questions regarding correct sizing or compatibility with your existing system.
