PV terms for operational assessment
The glossary explains existing plant documentation from A to Z. For companies, the interplay of generation, self-consumption, and remaining purchased electricity is particularly important. Technical planning and individual tax issues require a plant-specific review.
A
Depreciation Depreciation spreads eligible acquisition costs over the asset's useful life for tax purposes. Whether depreciation, investment deduction, or special depreciation is applicable to your PV system depends, among other things, on its business classification, the applicable regulations, and any potential tax exemptions. Outdated flat-rate percentages are not a suitable basis for planning. The specific tax treatment is a matter for tax advice.
AC Abbreviation for "Alternating Current" (alternating current)
Anti-reflective coating A transparent layer on the surface of solar cells to reduce reflection losses. The anti-reflective coating allows more light to penetrate the cell, thus increasing energy absorption and efficiency. The anti-reflective coating also gives the solar cell its color.
On-roof photovoltaic mounting In on-roof mounting, the solar modules are attached above the roof covering using suitable mounting systems. The roof retains its protective function. Structural analysis, wind and snow loads, and the roof's condition are all part of the planning.
PV Module Orientation
A south-facing orientation can enable a high annual yield. East-west facing surfaces distribute generation differently throughout the day. Which orientation suits the company also depends on when electricity is needed. Evaluate roof pitch, shading, available area, and load profile together; the nominal power of the modules does not change due to their orientation.
Azimuth Angle Indicates the deviation of the PV modules from south towards east or west. For a true south orientation, the azimuth angle is 0°, for a true west orientation it is +90°, and for a true east orientation it is -90°.
B
Building Permit The requirements depend on state building law, location, and type of structure. Many structures on buildings are exempt from permitting; however, other public law regulations still apply. Historic preservation and local ordinances may require additional reviews. Consult the responsible building authority or historic preservation authority before planning. Building projects exempt from permitting in Baden-Württemberg.
Lightning protection Lightning protection, surge protection and equipotential bonding are distinct protective measures that must be coordinated. The required design depends on the building, any existing lightning protection system and the specific PV installation. Planning and installation belong in the hands of qualified specialists. VDE technical information.
Domestic hot-water heat pump A domestic hot-water heat pump heats drinking water using ambient heat and electrical energy. It can be combined with or integrated into a storage tank. PV electricity can cover part of its electricity demand if generation and operation coincide. Suitability and economic viability depend on the heat source, hot-water demand, installation space and operating method; solar thermal is a different technical solution.
C
CIS cells CIS cells are thin-film cells manufactured using the elements copper, indium, gallium, sulfur, or selenium. A typical semiconductor compound used in CIS cells is copper indium diselenide (CuInSe2).
CO2 (carbon dioxide) is produced during the combustion of fossil fuels and plant biomass and is partly responsible for the warming of the Earth's atmosphere as a greenhouse gas.
CO₂ reduction In operation, PV modules generate electricity without combustion. Manufacturing, transport and disposal do cause emissions. The climate impact therefore depends on the full life cycle, electricity yield and the electricity being displaced. A fixed annual CO₂ saving per kWp does not apply to every installation and every year.
D
Roof Pitch The roof pitch is the angle of the roof to the horizontal. Together with orientation and shading, it influences the energy yield over time. For flat roofs, the pitch and layout are planned using a suitable substructure; statics, wind load, and usable roof area are also taken into account.
Historic Preservation For listed buildings or protected ensembles, a permit under historic preservation law may be required. Design and location must be coordinated with the responsible authority. Historic Preservation Permit in Baden-Württemberg.
DC Abbreviation for "Direct Current", English term for direct current.
Thin-film solar cell In thin-film solar cells, a thin photoactive semiconductor layer is applied to a substrate material. Examples of semiconductor materials used include amorphous silicon, cadmium telluride, or copper indium gallium selenide. Properties and efficiency vary depending on the material and product; a single percentage does not describe the entire technology.
E
EEG The Renewable Energy Sources Act (EEG) regulates, among other things, the connection, purchase, and promotion of electricity from renewable energy sources. The currently applicable version and any transitional rules are decisive for each installation.
Feed-in tariff The feed-in tariff is a possible form of support for renewable electricity fed into the grid. Whether and to what extent an entitlement exists depends, among other things, on the system, commissioning, power, and marketing method. The current legal regulations and published support rates are decisive, not a historical benchmark.
Self-consumption Self-consumption refers to solar power used on-site. It must coincide with generation or storage discharge. The metering concept depends on the system and supply situation as well as the requirements of the grid and metering point operator. A blanket number of required meters based solely on a 10 kWp threshold is insufficient. Self-consumption, storage and residual electricity in the company.
Feed-in meter A feed-in meter records the energy fed into the public grid in kWh. This amount is less than the total PV generation when used for self-consumption. A bidirectional meter can record grid import and feed-in in separate registers. Which measurement is required depends on the system and metering concept. Example of measuring import and surplus feed-in with SMA.
Einstein Albert Einstein (1879–1955) received the 1921 Nobel Prize in Physics, particularly for the law of the photoelectric effect; the prize was awarded in 1922. His 1905 paper described light as quanta of energy. In the external photoelectric effect, electrons can be released from a material by light. Solar cells, on the other hand, utilize the internal photovoltaic effect in semiconductors. Einstein's contribution is one of the foundations of quantum theory, but it is not a description of the structure of a modern solar cell. Nobel Prize in Physics 1921.
Energy Payback Energy payback describes the operating period after which a plant has offset the energy used in its manufacture and subsequent life cycle steps through its output. It is distinct from financial payback. Production conditions, technology, and location influence the result; old, general timeframes are not applicable to today's plants. Current assessment at Fraunhofer ISE.
ENS The abbreviation stands for a network monitoring device with associated switching elements. The term appears particularly in older plant documentation. The protective function is intended to prevent impermissible feed-in or islanding in the event of a network fault. For current planning, the specific protection concept and the applicable connection rules are decisive. VDE FNN on network and plant protection.
Renewable energies Renewable energies utilize sources that renew themselves on human timescales or are continuously available. These include solar, wind, hydropower, geothermal energy, and sustainably used biomass. Renewable does not mean unlimited availability everywhere and at all times: weather, available land, and technical infrastructure limit the respective use.
Renewable Energy Sources Act (EEG) The EEG further develops the previous legal regulations on electricity feed-in. It has been amended numerous times since its introduction in 2000. The remuneration period and support conditions must be checked against the regulations applicable to the specific installation; citing an older version of the law does not replace this check.
EVU EVU stands for energy supply company. Depending on its role, a company generates energy, trades it, supplies customers, or operates a network. The supplier and the local network operator are not necessarily the same company. Therefore, for your electricity contract, a distinction must be made between energy supply and network operation.
F
Facade Integration In building-integrated photovoltaics, modules can take over functions of the facade. For office and industrial buildings, in addition to electricity yield, aspects such as fire protection, building physics, structural engineering, design, and maintenance must be considered. Planning should be carried out by appropriately qualified specialist planners.
Flat Roof Mounting Modules can be arranged on flat roofs using elevated systems, for example facing south or east-west. Tilt, row spacing, and panel layout are determined by yield, shading, structural integrity, wind load, and maintenance access. A fixed angle specification is not a universally applicable design rule.
Flasher Data When testing a PV module with a light pulse, its electrical properties are measured under defined conditions. Flasher data includes, for example, power, current, and voltage. It serves for quality control and documentation. The specific test documentation to be provided should be explicitly agreed upon in the offer.
Funding Programs Funding programs can support investments or financing. Conditions, availability of funds, and application deadlines vary; in some cases, the application must be submitted before the project begins. Check the information provided by the respective funding agency and calculate the system with a transparent presentation of all funding assumptions.
Fossil Fuels Fossil fuels include, for example, petroleum, natural gas, lignite, and hard coal. They were formed over millions of years from organic deposits of all kinds, chemically altered by high pressure from layers that are sometimes kilometers thick, and have a high energy content. Their combustion releases large quantities of CO2. Fossil fuels are finite.
Ground-mounted installation Ground-mounted systems are installed on suitable plots of land, independent of any rooftop. There are fixed-orientation and tracking systems. Building regulations, permits, nature conservation, land use, and grid connection must be reviewed on a site-specific basis.
G
Generator The term generator or solar generator refers to the entirety of interconnected PV modules.
Generator Connection Box The electrical strings of the PV generator are connected in the generator connection box. Depending on the system design, it contains further connection and protection components. The design and protective measures depend on the specific system.
Direct current Direct current maintains its direction of flow; PV modules supply direct current. An inverter converts it for use in the alternating current grid. The grid frequency of 50 Hz means 50 complete oscillations per second, not just 50 polarity changes.
Global radiation is the sum of direct and diffuse solar radiation on a reference surface. Annual totals are often given in kWh/m² on a horizontal surface. They vary regionally and between individual years. For planning purposes, site-specific data, orientation, and tilt are important. Radiation data from the German Meteorological Service.
H
Hot Spot Shading or defects can lead to locally increased heating in a PV module. Bypass diodes can limit certain effects of shaded cell groups; they do not prevent every fault or damage. Any abnormalities should be investigated by a specialist company.
I
In-Roof Mounting With in-roof mounting, PV modules are integrated into the roof membrane and take over part of their protective function there. Existing roof surfaces are adapted for this purpose, or new buildings are planned accordingly. Watertightness, fire protection, structural integrity, and ventilation must be compatible with the system. Roofing and electrical work should be coordinated jointly by the responsible qualified specialist companies.
Off-grid system An off-grid system is a self-sufficient PV system that is not connected to a public grid.
K
Nuclear energy uses the energy released in nuclear reactions to generate electricity. Operation, fuel cycle, safety requirements, and radioactive waste are separate technical and societal issues. It is not a form of photovoltaics.
KfW KfW is a promotional bank. Whether a program is suitable for a specific energy investment depends on the current program conditions. KfW programs and requirements.
Kilowatt peak (kWp) refers to the nominal power output of a PV module or solar generator under standard test conditions. It is a comparative measure of system power, not an annual energy yield and not a guaranteed output in everyday use. The actual yield in kWh is influenced by factors such as irradiance, temperature, orientation, and shading.
L
Power Power is the energy converted per unit of time.
Performance Guarantee A performance guarantee is a contractual commitment by the manufacturer to specific power values of its modules over a defined period. The scope, measurement conditions, and procedures in the event of a warranty claim are detailed in the respective warranty terms and conditions; it is not a guarantee of the electricity yield at the site.
LG announced its exit from the solar module business in 2022. The name may therefore still appear on existing installations. For existing modules, the respective product and warranty documentation is crucial. Manufacturer's announcement regarding the exit.
M
Maximum Power Point "MPP" Operating point of the PV generator at which it delivers the highest possible power under the given conditions. The position of the MPP is not constant, but depends on the irradiance, the temperature and the type of solar cells.
N
Tracking Tracking mechanically moves the module surface towards the sun throughout the day. Single- or dual-axis systems are possible. The increased yield depends on the location and system and must be weighed against the additional investment, space requirements, energy consumption, and maintenance.
Nominal Power The nominal power of a module is specified under defined standard test conditions. The actual power output in operation depends on factors such as irradiance, temperature, and shading, and may differ from the nominal power.
Grid feed-in Grid feed-in is the supply of electrical energy to the public grid. In a PV system, the inverter converts the generated direct current into grid-compatible alternating current. If a portion is consumed or stored on-site, this portion is not simultaneously available for grid feed-in.
Grid-connected system A grid-connected PV system is connected to the public electricity grid. The grid operator and electricity supplier can be different companies. Depending on the design, the system can enable both self-consumption and feed-in.
O
OC Open Circuit refers to an open electrical circuit. Without a connected load, no external current flows; however, the open-circuit voltage Uoc may still be present at the terminals.
P
Peak power see rated power
Performance Ratio A location-independent metric, the performance ratio, is used as a measure of system quality. It characterizes the utilization of the PV system under consideration compared to a theoretically loss-free PV system.
Photovoltaics The term photovoltaics is composed of photon (Greek for light) and the physical unit for electrical voltage, volt [V], named after the physicist Alessandro Volta. When light energy (photons) strikes the crystal lattice of a doped semiconductor (usually silicon), charge carriers are released, which create an electrical voltage between the attached conductor ends as long as the light flow across the module continues.
Primary energy Primary energy is the originally offered energy in the form of petroleum, coal, natural gas or solar radiation.
PV Abbreviation for "photovoltaics", term for the conversion of light energy into electrical energy.
PV system Term for the entirety of components required for the generation and utilization of solar energy.
PV generator Technical device for converting light energy into electrical energy, usually solar modules.
PV module see solar module
Pyranometer A pyranometer is a measuring instrument for determining the incoming global solar radiation (see Global radiation).
R
REC REC is a module brand that appears in older system documentation and product comparisons. For a specific system, the model, datasheet, and warranty conditions are what matter. A historical mention does not imply any current partnership or certification with OPTUM.
S
Secondary energy Converted or further processed primary energy, e.g. heating oil or gasoline, which is obtained by refining petroleum.
Silicon Is the second most abundant element in the Earth's crust after oxygen. It is extracted from quartz sand through chemical treatment, purified, and used in the semiconductor industry as well as for the production of mono-, polycrystalline, and amorphous solar cells.
SMA SMA is a manufacturer of inverters and other components for energy systems. Features and functions should be checked based on the specific model. Manufacturer information .
Simulation program Photovoltaic systems are frequently designed using simulation programs today. These computer programs provide information about the size of the system, the design of the individual components, the achievable yields, and the economic viability.
Solar Latin "relating to the sun"
SolarEdge is a manufacturer of components for PV systems. Power optimizers can operate individual modules electrically independently at the appropriate operating point. Whether this is practical at a given location depends on the system design and losses; an economic advantage does not automatically follow.
Solar module A solar module combines several solar cells into a protected electrical unit. It converts sunlight into electrical energy. Several interconnected modules form the PV generator; therefore, the module and the entire generator are distinct units.
Solar power Direct current generated directly in solar cells
Solar thermal energy Solar thermal energy uses solar collectors to provide heat, for example for hot water or to support heating. Unlike photovoltaics, it does not generate electricity. Whether a device can be supplied with preheated water depends on the manufacturer's specifications. Yield and usable fraction of heat depend on the system, storage, and actual demand.
Solar cell A solar cell converts light into electrical energy through the photovoltaic effect. Absorbed light creates mobile charge carriers in the semiconductor material; the cell structure allows these carriers to separate, generating an electrical voltage. When connected to a circuit, direct current can flow. Structure and materials vary: in addition to crystalline silicon cells, there are, for example, thin-film and multi-junction cells. Describing them as always consisting of exactly two layers does not encompass these different technologies.
Solar cell, amorphous In amorphous silicon, the atoms are not arranged in a regular crystal lattice. Thin semiconductor layers enable various applications. The specific efficiency depends on the product and test conditions.
Solar cell, monocrystalline Monocrystalline silicon cells are made from single-crystal material. Their efficiency depends on the cell structure and manufacturing process.
Solar cell, polycrystalline / multicrystalline
Multicrystalline silicon consists of several crystal regions with grain boundaries. This technology is present in many existing installations. For comparisons, actual data sheets are more suitable than historical, generalized efficiency ranges.
Solar energy Solar radiation is the energy source for photovoltaics and solar thermal energy. Photovoltaics uses it to generate electricity, while solar thermal energy generates usable heat. Availability and yield vary depending on the time of day, season, and weather.
Storage, electricity storage
Electricity storage postpones the use of energy to a later period. Usable capacity in kWh, power in kW, efficiency, aging, and operating strategy must be considered separately. A specific self-consumption rate is not guaranteed across the board. Evaluate storage and remaining grid imports together.
Taxes The tax treatment depends on the system and the operator. Certain systems may be exempt from income tax and have a zero percent VAT rate; however, this does not mean that all commercial PV systems are generally tax-exempt. The old blanket statement that all income is taxable and that input tax is always refunded is incorrect. Consult a tax advisor to review your specific situation. Information from the Federal Ministry of Finance.
String Several PV modules connected in series form a string, which is connected to the inverter individually or together with other strings.
T
Temperature coefficient The temperature coefficient describes how a specific electrical quantity changes with temperature. Datasheets may list separate values for power, voltage, and current. When making comparisons, the reference quantity, unit, and reference temperature must be considered.
V
Shading Shade from buildings, trees, or other obstacles can reduce electricity yield. The extent to which a shaded area affects the entire system depends on cell interconnection, module design, bypass diodes, and inverter concept. Therefore, planning should consider the pattern and duration of shading; a single shaded module does not necessarily disable the entire system.
Insurance Insurance coverage for PV systems can cover various types of damage and, if applicable, lost revenue. The specific contract, exclusions, deductibles, and obligations are crucial. Check with your insurer whether your building or system insurance covers the existing installation and what additional coverage is needed.
W
Wafer Wafers are thin semiconductor discs, made from materials such as silicon. They form the starting material for many crystalline solar cells. Thickness and manufacturing processes vary depending on the technology and product generation.
Maintenance Regular monitoring and professional inspection help to identify malfunctions and deviations in yield. The scope and intervals depend on the system, manufacturer's documentation, and applicable requirements. Electrical work must be carried out by a specialist company. Classify maintenance and documentation.
Inverter The inverter converts the direct current (DC) produced by the solar generator into alternating current (AC). A crucial function of the inverter is grid monitoring, which ensures that the PV system is switched off within seconds as soon as the public grid fails or is shut down for maintenance. In a grid-connected PV system, this is also called a grid-connected inverter; in an off-grid system, it is called an off-grid inverter. There are now also inverters that maintain the power supply to the house in an off-grid system during a power outage. See www.sma.de
Module-integrated inverters A module inverter converts the direct current of one or a few modules into alternating current. This allows for precise allocation, but does not eliminate all losses.
Multi-string inverters
Multi-string inverters can operate multiple strings via separate MPP inputs. The appropriate design depends on voltages, currents, orientation, and the permissible combinations according to the datasheet.
Inverter, String String inverters convert the direct current of connected module strings. Depending on the device, several strings can be assigned to one or more MPP trackers. Not every individual string is automatically controlled independently.
Alternating Current (AC) refers to electric current whose strength and polarity repeat themselves after a constant period, based on the alternating voltage. In the German AC grid, this is 50 oscillations per second, or 50 Hz.
Efficiency Efficiency relates the usable output power to the input power. Cell, module, and system efficiency describe different levels. Laboratory records, datasheet values, and real annual yield are not equivalent. Compare products under the same test conditions. Technical assessment at Fraunhofer ISE.
Z
Central Inverter A central inverter processes the power of larger generator sets. It may be suitable for certain system designs; a fixed minimum size limit does not generally apply. Redundancy, maintenance, and the consequences of a failure must also be considered during the planning phase.
