Retrofitting PV heating element: How to integrate the 1 kW solar heating element into heat pumps, gas and oil heating systems
Energy costs are rising, hot water is becoming increasingly expensive, and many heating systems – whether heat pumps, gas or oil boilers – are inefficient, especially during the summer months. Many homeowners are therefore looking for a simple way to generate their hot water directly with solar power using a PV heating element or solar heating element , without costly renovations.
The 1 kW solar heating element from fothermo offers exactly that: a self-sufficient, highly economical retrofit for solar hot water that functions independently of the existing heating system. With just a few PV modules, hot water can be generated cheaply, sustainably, and almost free of charge.
Why a PV heating element? – Maximum hot water savings thanks to direct solar power
A solar heating element (also called an immersion heater or electric immersion heater) is screwed directly into the hot water storage tank and heats the water using PV electricity . The fothermo heating element uses direct current (DC) directly from photovoltaic modules – entirely without an inverter. This eliminates unnecessary losses, grid fees, and feed-in restrictions.
For homeowners, this means: solar hot water at minimal cost, maximum efficiency and a significant increase in self-consumption.
Especially during the summer months – from April to October – the 1 kW solar heating element unfolds its full potential. With just four PV modules, it can generate almost all the hot water for a typical two-person household. The heat pump, gas, or oil heating system remains largely switched off during this time, which reduces energy costs and protects the existing system.
Many users report up to 185 days of hot water per year solely from the solar heating element – documented in real-world practical experience.
Why is the 1 kW solar heating element ideal for retrofitting?
✔ Compatible with almost all storage devices
The 1 kW heating element fits most hot water and buffer storage tanks thanks to its standardized 1½" thread. Whether 150, 300 or 800 liters – retrofitting usually works without any additional modifications.
✔ No changes to the existing heating system
Gas boilers, oil boilers, or heat pumps remain fully functional. The PV heating element operates independently and automatically, supplementing hot water production with inexpensive solar power.
✔ Operation without an inverter
The PV modules are connected directly (DC). This reduces costs, avoids conversion losses, and increases efficiency.
Integration of the PV heating element into various heating systems
Integration in heat pumps
Heat pumps are generally efficient – but not always cost-effective. The COP drops particularly at low outside temperatures, and many heat pumps operate outside their optimal range when producing hot water above 50–55 °C.
Here, the 1 kW solar heating element handles the base load in the storage tank: It supplies solar-heated water before the heat pump starts up. This reduces electricity consumption and wear and tear.
For example: While a heat pump often achieves a COP of 3–4 in heating mode, this sometimes drops to 1.5 when heating domestic hot water in winter. The compressor has to work significantly harder, which increases operating costs.
In contrast, the PV heating element uses pure DC solar power – ideal for the hot water phase with high energy demand.
Advantages:
✔ Relieves the strain on the compressor (fewer starts, longer service life)
✔ Higher self-consumption thanks to direct PV power
✔ Noticeably lower operating costs, especially in summer and during transitional periods
Integration into gas heating systems
Gas heating systems are reliable, but – depending on the gas price – can result in high hot water costs. The 1 kW solar heating element takes over a large part of the hot water preparation and significantly reduces gas consumption.
While the gas boiler starts several times a day to keep the storage tank at temperature, the PV heating element ensures that the water is already heated by solar energy. The gas boiler remains inactive because its temperature sensor reports "hot water available"—without any changes to the installation.
Since the heat is generated directly in the storage unit and there are no losses via the combustion chamber or chimney, efficiency increases noticeably.
Advantages:
✔ Solar power replaces expensive gas energy
✔ Fewer burner starts = less wear and tear
✔ Ideal for existing buildings, as no modifications are necessary
Integration into oil heating systems
The effect is particularly pronounced with oil heating systems, as hot water preparation accounts for a large proportion of annual consumption. The solar heating element significantly reduces oil consumption – especially during the summer months.
Because the hot water is generated directly in the storage tank, chimney and standby losses are completely eliminated. Many older oil boilers run constantly in standby mode during the summer and consume unnecessarily large amounts of oil – thanks to the PV heating element, the system can often be switched off completely.
Advantages:
✔ Significantly lower oil consumption
✔ Higher efficiency without chimney and standby losses
✔ Extended lifespan due to fewer operating hours
Step-by-step: Installation of the 1 kW solar heating element
1. Prepare the storage compartment opening and screw in the heating element.
A free storage opening (1½" or compatible) is prepared. If none is available, a blanking flange or an existing heating element opening can be used.
The heating element is inserted with a gasket, screwed in by hand, and then tightened slightly further. Afterwards, the storage tank is completely filled.
2. Connect PV modules directly
The PV heating element works ideally with 3-4 PV modules between 350-600 Wp. The modules are plugged directly into the heating element via MC4 connectors – without any inverter or additional control system.
3. Set desired temperature
The desired hot water temperature is set directly on the solar heating element.
Maximum: 85 °C (with lime-free water).
In calcareous regions, 65 °C is recommended to reduce limescale buildup and for a longer service life.
4. Optional: Activate surplus function
Once the storage tank is at temperature, excess solar energy can be transferred – e.g. to:
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a second solar heating element (cascade circuit)
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a micro-inverter for feeding into the house grid
This allows excess PV power to be used even more efficiently and increases self-consumption.
To-do list for a successful installation
✔ Check storage opening (1½" or alternative)
✔ Insert and seal the solar heating element
✔ Fill hot water storage tank
✔ Connect 3-4 PV modules (MC4)
✔ Set target temperature
✔ (Optional) Connect surplus box
For further details, please refer to our solar heating element installation guide.
Conclusion: The 1 kW solar heating element is the smart retrofit solution for any heating system.
The 1 kW solar heating element is one of the simplest, cheapest and most efficient ways to generate hot water cost-effectively using photovoltaics – regardless of whether the household has a heat pump, gas heating or oil heating system.
He offers:
• Easy installation
• high efficiency
• Full compatibility with virtually all storage devices
• Maximum utilization of excess PV power
• Self-sufficient hot water preparation without an inverter
In a typical two-person household, around 75% of hot water costs can be saved over the course of a year. With just four PV modules, approximately 2,000 kWh of solar energy can be generated annually .
With typical energy prices of 10–20 ct/kWh, this results in annual savings of approximately €200 or more . As a result, the solar heating element usually pays for itself within 3 to 5 years – and subsequently provides virtually free hot water.
FAQ about the 1 kW solar heating element
How does a PV heating element work?
A PV heating element uses the direct current from photovoltaic modules directly to heat water. It does not require an inverter, as the electricity is converted into heat within the heating element without any conversion process.
Can I install the solar heating element in any hot water storage tank?
Generally, yes. The 1 kW solar heating element fits into almost all hot water and buffer storage tanks with a 1½" or 6/4" thread.
How many PV modules do I need?
For optimal operation, 3-4 PV modules with 350-600 Wp each are sufficient. The maximum connected load is 3,000 watts.
Is the 1 kW solar heating element financially worthwhile?
Yes. Many households save up to 75% on their hot water costs per year. Through reduced gas, oil, or electricity costs, the heating element usually pays for itself after 3–5 years .
Does the solar heating element also work in winter?
Yes, but with lower efficiency. In winter, the heating element assists with hot water production, but does not take over completely. The greatest savings occur during the summer months.