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The solar powered house

Photovoltaic heat for hot water and heating is becoming more and more popular – due to rising energy prices, the solar-powered house is the future!

The time of energy transition has arrived, with the current climatic, geopolitical and economical developments. We at my-PV have been working for years to establish the solar-powered house as a vision for the future. The signs have never been as good as they are today. But what is a solar-powered house? This article aims to answer this and many other related questions.

Power-to-Heat: What is a solar-powered house?

The solar-powered house refers to a residential building that covers its energy requirements as much as possible based on photovoltaic’s power. However, this also means that space heating and hot water are also covered by ecologically sustainable PV-based electricity. This model is commonly referred to as “Power-to-Heat”:

The heat is always generated using photovoltaics when there is surplus power. This excess generated power is not fed into the grid but rather used in-house for space heating or hot water.


Heating with electricity – an image problem

For a long time, heating with electricity was considered to be uneconomical and harmful to the climate. This is not the case today! With the energy transition and the expansion of renewable energies, electric heating is becoming efficient, economical and environment friendly. However, certain requirements must be met.

Electric direct heating systems used to be badly portrayed mainly from the time of night storage heaters. Dirty coal and nuclear power were burned at night because the large power plants could not be shut down. Electricity was only converted 1:1 into heat without adding environmental energy to the system. With the use of photovoltaic electricity for space heating, this balance changes.

A hot water boiler and electric underfloor heating are efficient "storage mediums" for heat generated by photovoltaics. The electricity that is not consumed immediately, is stored or diverted in the form of warm water, in the underfloor heating or with classic battery storage (ESS).  PV systems do not have to be regulated, the solar power is used in its entirety. A "backup" for times when there is not enough photovoltaic power comes from the public grid.

Altogether, this technology is already that mature that it even outdoes heat pumps. Our company building is the perfect reference model for power-to-heat technology – including negative operating costs!

More information about heating with photovoltaics can be found here.


Are all types of houses worth considering electric heating?

In principle: No. There are certain basic requirements: A solar-powered (not to confuse with classic solar thermal) direct heating for space heating only makes sense in buildings with a low heating requirement. By this, we mean properties should not have an energy index of more than 50 kWh/m²a.

For a single-family home that has been built using the latest thermal technology or has been thermally renovated professionally, a classic, water-based heating system is not necessary during installation in terms of performance and material use and is completely oversized.

The reason lies in the hydraulic heat distribution, which is complex and heat loss. Electric heaters, on the other hand, generate the heat directly where it is needed and the required heating load of three to six kW can easily be achieved with solar-based electricity.

For buildings with a specific heating requirement of less than 50 kWh per square meter per year, there are better and, above all, simpler options. This saves a large amount of operating cost for the building services and instead uses part of the budget to fill the available roof area as completely as possible with photovoltaic modules.

It is therefore important to know: A purely solar-powered primary heating system only makes sense in residential buildings that have been built or thermally renovated according to state-of-the-art technology and therefore have a specific heating requirement of 50 kWh/m²a or less!


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Figure 1: Conventional water-based heating systems are usually oversized for the low heating requirements (dark grey) of today's single-family homes.


Solutions for old buildings

For houses with a higher energy index, the general recommendation is:

  • First carry out a thermal renovation of the shell (window replacement, facade and ceiling insulation) and

  • then select a suitable heating system.

One thing is clear to us: One of the greatest potentials with regard to energy transition and climate protection is in the thermal renovation of buildings. We are looking forward to many exciting projects in this sector in the future.


Hot water in the solar-powered house

In the solar-powered house of the future, not only will the room be heated using photovoltaic-based power, but also the hot water(DHW). Hot water from solar energy is usually only known from solar thermal systems. However, these are unnecessarily complicated and high-maintenance, which hardly work efficiently, especially during the transitional period and in winter. But there is a different and simpler way:

Precisely modulated immersion heaters use excess photovoltaic power from your system to heat hot water storage tanks. The immersion heater, which is installed in a hot water or buffer storage tank, heats the water electrically during the day. This way, the electricity produced for this purpose can be used to the maximum extent for one's household. At the same time, energy losses are kept low because there is no thermal fluid circuit in this system.

You can read more about hot water with photovoltaics here.


How large does the photovoltaic system of a solar-powered house have to be?

The size of the required photovoltaic system depends on the one hand on the power consumption and on the other hand on the available roof area, but also on parameters such as profitability, self-sufficiency or climate protection. Experience has shown that the average size of a photovoltaic system in a family home is between 7 and 15 kWp.

In a single-family home of this type, the annual energy requirement for room heating is around 4,000 kWh. It therefore corresponds to about the same amount of energy that is required in such a property for electricity, hot water generation and electromobility. In order to be able to feed the energy supply for all four sectors to a significant extent from your own photovoltaic system, the system output is typically 10 kWp or more.

With modern modules, this only requires an area of 50 square meters (assuming module output 330 Wp, 1.6 m² module area). This amount of space is usually available on the roofs of single-family houses!


How does the heat get into the solar-powered house? 

Usually via electric heating mats in the floor, wall or ceiling. These have several advantages over water-based underfloor heating:

  • They are significantly cheaper

  • most of these products can be laid on top of existing screeds

  • in the case of renovations, only the floor covering has to be redone and not the entire structure has to be torn down and replaced

As with water-based underfloor heating, electric heating mats also offer the advantage that the mass of the floor can be thermally activated. This can thus be used as a day storage for photovoltaic surpluses, whereby fluctuations in generation are well compensated. In a house with 120 square meters of floor space, the screed can still store 25 kWh of energy with a temperature increase of just 4 degrees (e.g. from 22°C to 26°C).

If you like it particularly cozy and warm for a short time, for example in the bathroom, you can add infrared panels selectively and temporarily. For the general heat output, however, a storage mass such as the floor, wall or ceiling is preferable.

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Why is PV heat more ecological than a heat pump?

A heat pump can “only” generate heat. It does not contribute to the electrical consumers in the household. In contrast, PV supplies normal electrical household consumers with priority over heat generation and thus makes a major contribution to reducing the amount of electricity purchased from the grid. When using power-to-heat, the degree of self-sufficiency increases!

In low-energy houses, solar-electric building technology is on par with an air heat pump in terms of energy efficiency. The comparison between a heat pump system and linearly (!) output-controlled PV heat generation is shown below using an exemplary project of a single-family home.

General conditions

  • Location: Upper Austria

  • PV power: 10 kWp

  • Living space: 120 m², heating demand approx. 30 kWh/m²a

  • Household electricity consumption: 4,000 kWh/a

  • No battery storage, no electromobility.

  • Hot water requirement: 200 l/d (4 people)

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Figure 3: Energy balance air heat pump without photovoltaics

As shown in Figure 3, the heat pump uses green energy, which is captured by the evaporation of a refrigerant from the surrounding air, to provide the heat. Its own drive power is always fed from the mains. The sum of these two amounts of energy represents the heat output that can be used for hot water preparation and building heating. In any case, the regular electricity demand for electrical household consumers must also be covered from the public grid. In total, an annual amount of energy of almost 7,000 kWh, which is in this case drawn from the grid in order to supply household consumers and provide the heat.

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Figure 4: Energy balance with photovoltaic heat generation

A purely solar-electric building version is now being considered for the same building. In this example, a photovoltaic system with 10 kWp is taken into account in the calculation. Photovoltaics now supplies electrical household consumers with priority over heat generation. In addition to the lower grid purchase of only 4,800 kWh in the PV variant, there is an additional feed-in yield of approx. 3,900 kWh.

The annual performance factor of solar-electric building technology is 9.59. As with heat pumps, this key figure describes the heat output / grid purchase factor without household electricity consumers.


How can a purely electric building technology have an annual performance factor?

An electric heat generator does not have an annual performance factor on its own, just like the water circuit of an underfloor heating system. Only the overall system enables such a factor. The system components required for the heat pump are: evaporator, compressor, condenser and throttle valve. Environmental energy is absorbed via the evaporator. With air heat pumps, the heat source is the ambient air. It is heated by the sun.

In the solar-electric variant, the photovoltaic system replaces the evaporator. The original source of environmental energy is therefore also the sun. The energy source is now electricity instead of a refrigerant. Thin cables not only replace complex pipe systems, but also technology that cannot do without regular maintenance.

With regard to the solar electric concept, photovoltaics has the same function as the evaporator is in the case of heat pumps. In the case of PV, the sun is used as the original energy source to bring environmental energy into the system.

The big advantage is that electricity instead of heat is now available for power distribution. "Cables instead of pipes" make the system uncomplicated and more cost-effective. This also has a significant impact on maintenance costs.


Why not combine heat pumps and photovoltaics?

For various reasons: On the one hand, the electrical drive power of heat pumps is generally not linearly variable, which is a basic requirement for the combination with photovoltaics. With purely electrical heat sources in combination with suitable power controllers, however, the heat generation is “PV-ready”.

Another aspect is the cost. Two systems (PV and conventional heating combined) are of course correspondingly more expensive and often a financial hurdle for the broad mass of builders and homeowners. Moreover, there are the monetary expenses for the maintenance of the heat pump.

Third point: the volume. Building technology that works without moving parts is not only low-maintenance but also low-noise.


How high are the costs for acquisition and operation?

Many people are not even aware of how cheap photovoltaics have become in the meantime. Nowadays, a 395 W module can be purchased for as little as 200 euros. This fact creates the economic basis for the concept "cables instead of pipes".

In contrast to heat pumps, investment and operating costs can be directly influenced in many areas with solar-electric building technology through the dimensioning of the photovoltaic system. With increasing PV power, construction costs increase. On the other hand, this reduces operating costs, since a larger proportion of the energy required now comes from in-house generation.

The customer is therefore free to decide where he wants to save on the output of the PV system. Of course, projects that are initially equipped with smaller PV systems can later increase their degree of self-sufficiency by adding more modules and thereby increase the annual performance factor.


Conclusion on the solar-powered house

The house of the future is purely electric, mostly solar electric! "Cables instead of pipes" not only simplify the installation and operation of future building services. With the appropriate dimensioning of the photovoltaic system, the concept is also more ecological than a water-based heating system with a heat pump and in terms of costs even cheaper!


Would you like to calculate your personal building project?

We offer you a simple tool for the individual calculation of your energy system with our "Power Coach". Electricity, hot water, heating and, if necessary, e-cars are included in the analysis. The basis is a purely electrical energy system with photovoltaics as the central energy source. The best thing to do is to try it out yourself!


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