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Photovoltaik vs. Solarthermie

Photovoltaics vs. solar thermal energy

Photovoltaic water heating has many advantages. Perhaps the biggest advantage is the ability to feed excess electrical energy into your own home network via a micro PV inverter. This can further reduce electricity costs.

Operating principles and history:

Hot water can be heated using the sun either via a solar thermal system or a photovoltaic system. In the solar thermal variant, the water to be heated is pumped into a solar collector and heated there directly by the sun. This process has been used primarily in the last 50 years. Photovoltaic hot water heating generates electricity using photovoltaic modules. This is routed in cables to the hot water tank. The electricity is converted into heat in the storage using a heating element. Since the prices for photovoltaic modules have fallen by around 90% in recent years and there are many advantages to photovoltaic hot water heating, this variant is establishing itself on the market. Below we would like to compare both types of production and present the main advantages of fothermo products.

Excess feed into the socket:

No energy is lost. As soon as the photovoltaic boiler is fully heated, excess energy can easily be fed into the power grid via a micro PV inverter. This not only saves costs for hot water preparation, but also additional electricity costs in the household for pumps, a refrigerator or computers, for example. No energy is lost. The additional feed-in of excess electricity roughly doubles the user's savings and thus halves the payback period. The payback period of a photovoltaic standalone boiler with surplus feed-in is approximately only half of the payback period of a classic solar thermal system.

40% lower investment costs – cables instead of pipes:

The investment costs for photovoltaic hot water heating are around 40% lower than for a solar thermal system. The main reason for this is the lower installation costs. Flexible and thin cables are laid much faster and easier than insulated and water-carrying pipes. Practice shows that the photovoltaic standalone boilers (200 liters and 300 liters) are installed around 50% faster than comparable solar thermal systems. Another reason for the low investment costs is the lower acquisition costs of the photovoltaic system, due to the significantly lower complexity of the individual components.

No overheating and no freezing:

The photovoltaic boiler switches off automatically when the set maximum temperature is reached. A solar thermal system can overheat and be destroyed if there is too much sunlight. The same applies to freezing of water pipes in winter. There are solutions for both problems with solar thermal systems, but these involve additional investment costs or regular maintenance. While there is a risk of overheating with solar thermal systems, fothermo products can use the excess energy and simply feed it into your own power grid via a micro PV inverter. This also reduces annual electricity costs.

Heating starts from 7 W power:

A large part of the annual hot water requirement can be covered by photovoltaics. The photovoltaic standalone boilers cover around 60% - 75% of a family's hot water needs. The advantage of photovoltaic technology is that it produces electricity or heat even in low light conditions and at low power. Whereas the solar thermal system only pumps hot water into the storage tank as soon as the water in the collectors on the roof is hotter than the water in the storage tank in the basement. Thanks to the innovative and patented MPP tracker, the fothermo products start the heating process from a photovoltaic output of 7 W. This power is produced by the photovoltaic modules just a few moments after sunrise. This is a big advantage, especially in the transition months of autumn and spring, when you want to generate as much heat as possible with the limited energy available from the sun.


Since photovoltaic products from fothermo do not have any mechanical components installed, they are practically maintenance-free. Solar thermal systems, on the other hand, have movable components installed such as pumps, valves or expansion vessels.

Easy one-man assembly:

The weight of a photovoltaic module is about 20 kg and can be carried and assembled by one person. The weight of a solar thermal collector is around 40 kg. This ensures significantly more efficient assembly and thus reduces acquisition costs.