In principle, rooftop solar systems are suitable for all owners of single or multi-family houses and companies. However, the size of the rooftop solar system and thus the yield is mainly dependent on the available roof area. Four to six solar modules with a surface area of 7 to 13 square meters are required for each kilowatt peak (kWp) nominal output, depending on the module output and rooftop orientation.
Photovoltaic systems put additional weight on the roof. Therefore, a structural engineer should inspect the roof before installation. The extra load from the rooftop solar system is primarily dependent on the respective type of installation.
There are usually no other special requirements. For example, private rooftop solar systems fall under the category of "structural systems" and, therefore, typically do not require official approval as long as they are not subject to monument protection. However, to maximize the yield of the solar system, you should consider a few tips:
- South-facing rooftops are considered optimal, but west or east-facing and the corresponding intermediate levels are advantageous today. Installation on a north-facing roof can sometimes be worthwhile with compelling, high-performance solar modules.
- An angle of inclination of the solar modules of 35 to 45 degrees has proven particularly productive in Germany. In this way, the most significant possible part of solar energy can be used over the entire year.
- Avoid shading the solar modules, such as chimneys, trees, or nearby houses, as this can lead to yield losses. However, if shading cannot be avoided, special module optimizers can be purchased.
How big should a photovoltaic system be?
When planning a solar system, the first exciting thing is what output it should provide. The maximum possible electricity yield depends primarily on the location and the corresponding solar radiation, the available roof area, and the orientation/inclination of the roof.
A solar system generates an average of between 800 and 1,100 kWh of solar electricity per kilowatt peak (1,000 Wp) of installed rated output in Germany, depending on the global radiation values. Therefore, with an annual power consumption of 5,000 kilowatt-hours, it makes sense to install a system with at least 6 kWp.
However, the dimensioning of the system should not only be based on the average power consumption but also the desired share of self-consumption or self-sufficiency of the interested party. The higher this should be, the larger the solar system should be dimensioned. However, the following applies here: Although 100 percent self-consumption is feasible, it does not yet make economic sense. Instead, a 60 to 80 percent self-consumption share should be targeted when planning the system.
Tip: The larger the system, the higher the savings are usually. However, to avoid paying the EEG levy on self-consumed electricity, the system should not exceed a nominal output of 10 kW when first installed.
Advantages and disadvantages of a solar system
Having your rooftop solar system has various advantages. This is how you generate environmentally friendly solar power and thus contribute to the energy transition. The electricity generated is also comparatively inexpensive at five to twelve cents per kilowatt-hour. According to the BMWi, the average electricity price for a household in 2021 was 32.36 cents / kWh. Compared to 2019, the electricity price has increased by 1.78 cents / kWh.
On average, a self-consumption of 30 to 35 percent can be achieved with a solar system. This means that up to 35 percent of the annual electricity requirement is covered by solar power, which reduces the amount of electricity purchased from the public grid. Due to the low production costs for solar energy, electricity costs can be saved with a PV system. The higher the self-consumption, the higher the savings.
More than 60 percent of solar system operators also decide to purchase an electricity storage system. This makes it possible to store the solar power that is not directly consumed temporarily. It can also be used in the evening or at night, increasing self-consumption by up to 80 percent. This further reduces electricity costs.
Only the initial investment costs can be mentioned as a disadvantage of a photovoltaic system. Some prospects cannot or do not want to carry this in the form of a one-off payment. In this case, however, there are alternative solutions. For example, some companies now offer the option of renting solar systems. Financially, however, the financing of the solar system, which many banks support with cheap solar loans, has proven its worth.
The cost of a rooftop solar system
The prices for PV systems have fallen significantly in recent years. According to the Fraunhofer Institute, investment costs have fallen by an average of 13 percent per year since 2006 and have fallen by 90 percent since 2010. While in 2006, an average of 5,000 euros per kilowatt peak of installed rated output was paid, today this value is around 1,500 euros. Solar modules, in particular, and electricity storage devices have shown particularly positive price curves for consumers in the past. However, delivery bottlenecks, material shortages, and supply chain interruptions have led to a slight price increase in the last two years. In the long term, it is expected that the module prices will fall again if the products are further developed, and delivery bottlenecks do not put a spanner in the works.
A model calculation of the yield potential shows how much money can be saved with a solar system. This is based on the average power consumption of 5,000 kWh, an electricity price of 32 cents/kWh, and the current feed-in tariff of 6.83 cents/kWh.
Solar storage entails additional investment costs. A storage system with a capacity of 10 kWh costs around 8,000 to 10,000 euros on average. With investment costs of 19,000 euros, the system paid for itself after just under 14 years.
Battery storage is worth it.
The sample calculation shows that battery storage in connection with a PV system is financially worthwhile. However, besides the cost savings, there is another advantage: the increasing degree of self-sufficiency and thus increased independence. In particular, homeowners who are not at home during the day benefit significantly more from the electricity they generate themselves.
The electricity stored can be used, for example, to support a heat pump at night. A heat pump generates an average of four to five kilowatt-hours of heat energy from one kilowatt-hour of electricity. So if solar power is used for 10 cents/kWh on average to generate heat, one kilowatt-hour of thermal energy costs the system operator 2 to 2.5 cents. With electricity from the public grid at 32 cents / KWh, this value would be 6.4 to 8 cents.
Power storage is also helpful for owners of an electric car. In connection with a wall box or e-charging station, the self-generated solar power can be used to charge it cost-effectively and in an environmentally friendly way. System operators are thus less dependent on the public charging station infrastructure and rising electricity prices.
What funding opportunities are there?
To further promote the expansion of renewable energies in Germany, the federal and state governments offer a range of funding programs. These are intended to support the plant operator in financing the project and are granted in a low-interest loan or a non-repayable grant.
Funding from the federal states: In addition to the KfW funding, many federal states also offer funding programs for PV systems. In addition, electricity storage or e-charging stations are often subsidized with a newly built system. These are non-repayable grants that can cover a variable percentage of the investment costs in most cases.
The subsidy programs generally aim at operators whose systems do not exceed the 10 kW or 30 KW limit. However, the exact requirements and funding conditions can be found in the individual programs.
In some cases, funding from other bodies may not be combined with the corresponding funding program of the federal state. Therefore, interested parties should take this into account when applying.
Plant operators subject to sales tax must pay the tax collected through the feed-in tariff to the tax office. The sales tax is offset against the sales tax (input tax) paid for purchasing the system. The difference is then delivered to the tax office or paid out to the system operator.
Since system operators produce the electricity under tax law as entrepreneurs but use it as a private individual, the sales tax on the self-consumed solar power must also be paid to the tax office. The amount of sales tax depends on self-consumption and the price of the electricity drawn from the grid.
The tax office will reimburse plant operators who choose to pay sales tax for the VAT paid when purchasing the plant. For this purpose, the input tax paid is stated when the sales tax is first registered. After that, all necessary documents and receipts must be submitted. If the tax office has checked it, this invoice shows how much money system operators can get back.
The small business regulation reduces the accounting and tax work with the tax office. The monthly sales tax returns are no longer necessary, and the annual report is also much easier, as it remains the same every year as a so-called zero report. In addition, no sales tax has to be paid on self-consumed solar power. This can be particularly worthwhile for system operators with an exceptionally high level of self-consumption due to an electricity storage system or an e-car.
In principle, the costs for the purchase and the ongoing operation and maintenance of a PV system can be deducted from taxes. For example, loan interest, insurance contributions, or the cost of an electricity meter can be claimed as business expenses for tax purposes. In addition, the acquisition costs for the components, assembly, or potential upgrades can be amortized over the useful life of 20 years.
What else needs to be considered during installation?
In addition to the financial aspect, there are other points to consider when installing a solar system. For example, the system must first be registered with the Federal Network Agency. An online portal was set up expressly for registration. Here, the system can be written quickly and effortlessly by the system operator.
The name and address of the system operator, an email address, the exact location and the nominal output of the system in kW, and the date of commissioning must be specified. The registration should take place at the latest on the day of commissioning. If this does not happen, the system operator loses the right to the feed-in tariff.
In addition, the rooftop solar system must be registered with the responsible network operator, and an application for connection to the grid must be submitted, as the last feed the solar power into the public power grid and pays the EEG remuneration to the system operator. According to the law, the network operator has up to eight weeks to carry out a network compatibility test. Therefore, the application should always be made before installing the system.
Once the system has been installed, it is put into operation. The commissioning is documented in the commissioning report. This protocol is sent to the network operator and the Federal Network Agency certificate. Only then can the feed-in tariff for the solar power fed into the grid be paid to the operator.