SOL•THOR in a German townhouse
Self-sufficient water heating through a school project
Hot water from an off-grid photovoltaic solution instead of an oil-fired heating system – a cost-effective alternative.
Facts about the project
- Planner/Installer/Owner
- Location
- Photovoltaic output and orientation
- Number of modules and type
- Heat storage size
- Building type
- Year of construction and size
- my-PV product
- Heating element (my-PV, other providers, etc.)
- System control
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A terraced house in Bavaria – more precisely, near Nuremberg – has been using solar energy for self-sufficient hot water production since early 2025. What exactly does that mean, and how did the system come to life?
The starting point: a 50-year-old terraced house
The home, owned and lived in by Stefan L., is 98 m² in size and equipped with a 150-liter tank integrated into an oil-fired heating system. Until recently, this setup ensured hot water supply throughout the year. However, with rising operating costs – triggered by the energy market disruptions that began in late 2021 – the homeowner began looking for an environmentally friendly alternative for domestic hot water generation.
The solution emerged from a collaboration with the Rudolf Diesel Technical College in Nuremberg.
Personal customer opinion and resumee
“Our personal conclusion: The system from my-PV impresses with its well-developed technology and allows for detailed monitoring of energy data. Installation is straightforward, and operation via the touchscreen is intuitive. Additionally, the system is flexible and suitable for both indoor and outdoor use. Overall, we are very satisfied with the solution and the benefits it brings,” says Stefan L., owner of a townhouse near Nuremberg.
A project team for self-sufficient water heating
“We are a project team of aspiring mechanical engineering technicians from the Rudolf Diesel Technical College in Nuremberg, and we’re developing our final thesis in the field of renewable energy,” explains homeowner Stefan L. “We discovered my-PV through the school’s annual Technician Exchange, where companies and student teams present their projects.”
One project in particular stood out: “Suntrapper”, a team supported by my-PV the previous year, which aimed to provide hot water using PV electricity. “This sustainable approach really inspired us and motivated us to implement a similar idea of our own,” Stefan L. continues.
my-PV was impressed by the initiative and provided the students with a device to bring their project to life. “We would like to take this opportunity to sincerely thank my-PV for the collaboration and support!”
What is the advantage of self-sufficient water heating?
“Our system is designed to heat domestic hot water using a photovoltaic installation,” explains Stefan L. “Since the existing hot water tank doesn’t have an opening for a screw-in heating element, and there’s no space in the basement for an additional external water storage tank, we developed an alternative solution.”
The team implemented a bypass system with an external pressure vessel, allowing the water to be heated through circulation. The heating element is installed inside this pressure vessel, and once the water is heated, it is pumped back into the main hot water tank.
“Our setup runs completely off-grid as a stand-alone system – we intentionally decided not to connect it to the power grid,” says the project owner.
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Why not a grid-connected photovoltaic system?
A grid-connected photovoltaic system – as it's referred to in technical jargon – with an inverter, grid connection approval, and all the related components, is not required in this case. Instead, domestic hot water is heated in a storage tank or via the bypass system using an autonomous thermal solution: the SOL•THOR by my-PV, in combination with a my-PV screw-in heating element.
The SOL•THOR is directly connected to the photovoltaic modules – in this way, similar to a solar thermal system, only heat is generated from solar energy. Why generating heat from photovoltaic energy offers many advantages compared to traditional solar thermal systems is summarized here.
The SOL•THOR can be connected to 1 to 10 photovoltaic modules. It's important to observe the input voltage range for proper operation. On our website, we offer an Excel configuration tool to help size the system correctly.
The SOL•THOR allows for direct use of the DC electricity from photovoltaic modules to power a standard heating element (ideally the my-PV screw-in heating element), without the need for a grid connection or inverter. This setup can be used for both domestic hot water and heating water applications, and both are approved.
Were there any obstacles during installation?
During the implementation – the project in Nuremberg was one of the first to receive a SOL•THOR unit – there were a few minor challenges.
"One issue was the unclear information regarding the wiring of the solar modules: while the first page of the datasheet recommends installation with 1 to 10 solar modules, it only later mentions the maximum input voltage of 230 V," explains Stefan L., describing the challenge.
This detail has since been revised – thanks in part to feedback from the project team at the Rudolf Diesel Technical School – to ensure a smoother and more straightforward commissioning process.
How high is the domestic hot water demand in the home?
The 98 m² household is home to two people, with an average hot water consumption of around 60 liters per day. In summer, the demand is slightly higher due to more frequent showers – an ideal use case, as this is also when photovoltaic systems generate the most energy.
“With our new hot water supply powered by photovoltaics, we now have the opportunity to optimize our consumption even further. Since the hot water can now be produced with little to no financial cost, we’re planning to connect our dishwasher and washing machine directly to the hot water system,” says Stefan L., optimistic about further improvements. “This will help reduce grid electricity use even more, increasing the overall efficiency and cost-effectiveness of the system.”
The optional hot water backup function offered by the SOL•THOR was not activated in this project.
“We don't use additional hot water heating via the general power grid, as our system is mainly intended as a support solution. Since we don’t have a PV system feeding electricity into the home’s grid, using grid electricity wouldn’t make economic sense for us – especially since our oil burner still serves as the primary heat source,” explains the Bavarian homeowner.
When is autonomous heat generation worthwhile?
“In our view, expanding the system with photovoltaic heat offers significant advantages. Especially on sunny days, we can save heating oil while also reducing our ecological footprint. To heat the 150-liter tank by 50 °C, we theoretically need around 9 kWh of energy. With an estimated solar output of 11 to 15 kWh per day, the system is more than adequately sized for the summer and should also provide sufficient energy during the transitional months in spring and autumn,” the homeowner explains confidently.
How to carry out a theoretical calculation of the heat demand in a domestic hot water or buffer storage tank is explained in this article.
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SOL•THOR
in use
The direct current power manager utilizes direct current from photovoltaic modules directly for heat generation. Solar power is efficiently converted into heat by transferring the direct current from the photovoltaic modules exclusively and with minimal loss to a connected heating element.
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