PV Assessment Reports

A PV system converts sunlight directly into electrical energy using solar cells. The solar cells consist of semiconductor materials (usually silicon), in which charge carriers are released by the photovoltaic effect when exposed to light. The generated direct current is converted by an inverter into grid-compatible alternating current and can be fed into the public grid or consumed directly.

The three main types are monocrystalline, polycrystalline, and thin-film modules. Monocrystalline modules have the highest efficiency (20–24%) and require less area but are more expensive to manufacture. Polycrystalline modules achieve 15–20% efficiency and are slightly cheaper. Thin-film modules have the lowest efficiency (6–13%) but are flexible and perform better in diffuse light and high temperatures.

kWp (Kilowatt Peak) denotes the nominal power of a PV module under standardized test conditions (STC: 1,000 W/m² irradiance, 25°C cell temperature, AM 1.5). In practice, this output is rarely achieved as real-world conditions deviate from STC. The actual yield depends on location, orientation, shading, temperature, and system losses.

In Germany, the specific annual yield ranges from approximately 850 to 1,100 kWh/kWp depending on location and system quality. Southern Germany tends to achieve higher values than the north. A well-planned and maintained system in southern Germany can readily achieve 1,000–1,100 kWh/kWp per year.

The technical lifespan of a PV system is at least 25–30 years; many systems remain operational even after 30+ years. Module manufacturers typically provide a performance warranty of 25–30 years, guaranteeing that modules will still deliver at least 80–87% of their nominal power. Inverters have a shorter lifespan of approximately 10–15 years and typically need to be replaced once during the system's lifetime.

A south-facing orientation with a tilt of approximately 30–35° is optimal in Germany. Deviations toward southeast or southwest result in only minor yield losses (approximately 5%). East-west orientations with a shallow tilt are even advantageous for self-consumption optimization, as generation is distributed more evenly throughout the day. Pure north-facing orientations are not economically viable.

The system size depends on the available roof area, electricity consumption, desired self-consumption ratio, and budget. Since the amendment of the EEG and the removal of the 70% curtailment rule for new systems, it is recommended to utilize the available roof area as fully as possible. For residential homes, systems between 5 and 15 kWp are typical; for commercial properties and multi-family buildings, significantly larger.

Shading from trees, neighboring buildings, chimneys, antennas, or other structures must be carefully analyzed. Even partial shading of individual cells can significantly reduce the output of an entire string in conventionally wired modules. Modern module optimization (half-cell technology, module-level power optimizers, micro-inverters) can substantially reduce shading losses. A professional shading analysis considers the sun's path throughout the entire year.

A battery storage system is economically viable when the difference between electricity purchase costs and feed-in tariff is large enough and self-consumption can be significantly increased as a result. At current electricity prices and storage costs, a battery can be cost-effective if it increases self-consumption from a typical 30% to 60–80%. The economic viability depends heavily on the individual load profile, storage system size, and acquisition costs.

The roof must be able to support the additional weight of the PV system (approximately 10–15 kg/m² for rooftop-mounted systems, up to 25 kg/m² for flat-roof systems with ballasting) as well as wind and snow loads. For older buildings or borderline structural conditions, a structural analysis by a structural engineer is required. Particular attention must be paid to penetrated roof membranes and the fastening of the mounting substructure.