What is solar power? Donauer glossary

Technical terms from A to Z




The absorber serves as the direct receiver of the solar radiation. It is blackened or coated with a selective coating. On its surface, the solar radiation is converted into heat and transferred to a heat transfer medium (water/antifreeze mixture).


Specifies the portion of the solar radiation absorbed by the roof that is converted into heat.

Alternating current (AC)

Electricity whose polarity constantly changes. In the German power supply grid, the alternating current has a frequency of 50 Hz (hertz), i.e. it assumes the positive or negative value of a (ideally) sinusoidal half-wave 50 times in one second. Alternating voltage or current is generated by rotating generators or inverters.

Amorphous silicon

(Abb. a-Si) Atoms in amorphous material form an irregular structure (amorph: Greek for formless). Due to the high level of absorption capability of the a-Si, a solar cell made of this material requires layer thicknesses of only a few µm. Which is why the term thin-film technology is also used in this context.

Anti reflection layer

A transparent layer on the surface of solar cells for reducing reflection losses. This special coating allows more light to penetrate into the cells, since the silicon surface would otherwise reflect back 30% of the sunlight.

Aperture area

Glass surface of a collector through which the solar radiation can enter. The aperture area is the reference unit for the collector efficiency according to DIN 4757 or in EN 12975.

Aligning a PV system

To ensure optimum alignment, the module surfaces should face south and be tilted 30°. Deviations in the south east or south west direction or roof pitches between 20° and 60° only result in marginal yield reductions with grid-connected PV systems.

Azimuth angle

The azimuth angle specifies how many degrees the surface of the thermal collector or photovoltaic module deviates from the exact south-facing direction.



Emergency power system for ensuring the power supply. During a power outage, the system switches to a self-sustaining power supply in a few milliseconds and your devices are powered by the energy stored in the battery.


Batteries (accumulators) are used to operate electrical equipment independently of the grid. There is a large numbers of energy storage technical: electrochemical (lead, nickel cadmium, lithium batteries), electrostatic (supercondensers), electromechanical (flywheels and compressed air), thermochemical (hydrogen fuel cell).


Eine PV-Anlage erhöht in der Regel nicht das Risiko eines Blitzeinschlags. Aus Sicherheitsgründen und um Schädigungen vorzubeugen, werden PV-Anlagen jedoch gemäß den Blitzschutznormen und den Vorgaben des Verbandes der Sachversicherer (VdS) errichtet.

Bypass diode

If a solar cell is partially or completely shaded, less current flows through it.
The series connection in a module causes the shaded solar cell to heat up and can destroy it. Bypass diodes provide protection of shaded cells. Depending on the number of cells, a solar module has between one and three bypass diodes.


CIS (CIGS) thin-film modules

Due to their high absorption in thin-film solar cells, copper-indium-diselenide compound semiconductors are used.

CdTE thin-film modules

Due to their high absorption in thin-film solar cells, Cadmium telluride compound semiconductors are used.

Charge controller

The charge controller in grid-independent systems serves as the battery manager. It controls the flow of electricity between solar modules, consumers and the battery and protects against overcharging and deep discharging.

Climate chamber

A climate chamber is a testing instrument or part of a measuring arrangement and is used to generate and maintain a defined climate which generally includes temperature and humidity for the duration of the measurement or test.


An elementary component of a collector system that converts the solar radiation into heat as efficiently as possible. The heat is collected and transported by a heat transfer medium and transferred to a storage tank by a heat exchanger. A flat collector consists of a well-insulated housing that contains an absorber. The housing is covered with a sheet of solar glass.

Collector circuit (solar circuit)

Circuit between the collector and a collector system storage tank that consists of collector, piping, heat exchanger, pump and safety equipment.

Collector circuit yield

The thermal energy that is transferred from the collector circuit (solar circuit) to the solar storage tank. In most cases, a specific collector circuit yield is specified. An annual collector circuit yield of more than 400 kWh/(m2 *a) is a good result for a flat collector system; an amount greater than 500 kWh/( m2 *a) is an excellent result for a large-scale system.


The strength of the charge quantity flowing through a conductor (e.g. in the form of electrons flowing through a copper cable) is referred to as electrical current. The unit of measure for current is ampere (abb. A)




Abbreviation for direct current. This abbreviation refers to the direct current that is stored in solar cells or modules and accumulators.

Diffuse radiation

The solar radiation that falls on the Earth’s surface is divided into direct and diffuse radiation. Diffuse radiation is all the radiation that does not coincide with the geometrical straight-line path from the sun to the observation point, but is rather reflected or scattered, for example, through the atmospheric components. Clouds or smog also create diffuse radiation.

Direct current (DC)

Flow of electricity without a shift in direction, such as the current generated by batteries or photovoltaic modules.

Direct radiation

Solar radiation that reaches the Earth’s surface on a direct path from the sun. The diffuse radiation is added to the direct solar radiation to determine the global radiation.



An E-bike is a bicycle that is powered by an electric motor. A decision is made whether the electric motor completely powers the bicycle on its own or if the rider provides additional assistance by pedaling. In German-speaking countries, the E-bike therefore also has various names. The electric motor of a "Pedelec" supports the rider with a drive as soon as he pedals. A controller controls this drive up to a speed of 25 km/h in the Pedelec and up to 35 km/h in the light moped class. The electric motor is powered by a battery.


The efficiency specifies the relationship of two measurements on a system - output power to input power. Efficiency is a current value and depends on the operating conditions of the system at the observed point of time. The efficiency of a solar cell or module is defined as the relationship between the power irradiated and the emitted electrical power. Since the efficiency is surface-dependent, make sure to observe the data that was used for calculating the surface, e.g. the entire module surface or only the active cell surface within the module.

The efficiency of a collector is defined as the ratio of the heat output of the collector (output) compared to the irradiation level in the collector plane (input). The collector efficiency is only one of several factors that influence the efficiency of a solar power system.

Electroluminescence measurement

Electroluminescence is a form of luminescence in which a solid state is excited to emit electromagnetic radiation, for example, in the form of light by applying an electrical field or electrical current.

Electromobility / E-mobility

In general terms, electromobility refers to the use of electrically-powered vehicles in freight and passenger transportation. Pure electric vehicles do not require any fuel. Instead, they drive solely using electricity drawn from the integrated battery. They enable locally emission-free driving, i.e. they do not emit any harmful substances and greenhouse gases.

Energy payback time

The time a solar power system requires to generate the electricity that was required to manufacture it. PV solar power systems currently available on the market pay themselves off between 2 and 4 years depending on the technology and quality. Their estimated service life ranges between 25 and 30 years. According to a study conducted by the Technical University of Berlin, PV systems based on amorphous silicon have an energy payback period ranging between 17 and 41 months.


Facade installation

A PV system that is mounted on a building facade or permanently integrated into the facade.

Flat roof installation

In order to achieve an optimum energy yield, the PV modules on a flat roof must be mounted with a tilt of approximately 30° using a supporting structure.


A flasher is a technical measuring facility (simulation room), which subjects the module to a temporary flash under STC conditions. Here, the standard test condition (STC) defines radiation on the module level of 1000 W/m² at a module temperature of 25 °C. Voltage and current values are determined and multiplied to determine the performance.

This basic data is listed in the data sheets of the respective manufacturer. Due to manufacturing tolerances, we recommend requesting the flash list of the supplied modules to check the actual performance of the batch.


Als Freilandaufstellung bezeichnet man eine PV-Anlage, die auf einer Freifläche, wie zum Beispiel einem Feld, aufgestellt ist.

Feed-in tariff

Tariff for feeding electricity produced from renewable energy sources into the general power supply grid.



A generator or solar generator refers to all of the interconnected PV modules as a whole.

Global radiation

Sum of the direct radiation and diffuse radiation that falls on a horizontal surface. The Earth’s atmosphere decreases the irradiance of the extra-terrestrial solar radiation (solar constant) by absorption, reflection and scattering, so that the irradiation level on the Earth’s surface is reduced at our latitudes to 1,000 W/m2 (summer, clear sky, noon). Depending on metrological conditions and astronomical laws (that, among other things, determine the course of the seasons), the solar energy supply fluctuates. The average annual sum of global radiation on a horizontal receiving surface is, for example, approximately 1,000 kWh/(m² *a) in the region of Hanover/Germany. That corresponds to approximately the energy content of 100 liters of heating oil or 100 m³ of gas.

German Renewable Energy Sources Act (EEG)

Act for promoting the generation of electricity from renewable energy sources. It primarily serves to protect the climate and is part of a whole series of legal measures devised to reduce the dependency on fossil fuels, such as crude oil, natural gas and coal.

Grid connection

(Grid-parallel operation) Connection of a photovoltaic system to the grid of the electricity supplier by an inverter for completely or partially feeding the photovoltaic generated electricity into the grid. Grid-connected photovoltaic systems do not require energy storage. This function is virtually performed by the power supply grid.

Ground-mounted installation

Ground-mounted installation refers to a PV system that is installed on an open surface, for example, a field.



The use of two or more different power sources (PV, diesel generator, wind turbine, public grid) in energy supply systems.

Heat transfer medium

Fluid in the collector circuit of a solar energy system that transports the heat from the collector to the storage tank. The heat transfer medium is usually a mixture of water and glycol to ensure frost protection.



This refers to a block made of semiconductor material, such as silicon. Ingots can be monocrystalline or polycrystalline.


Converts the direct current produced by the modules into grid-compliant alternating current. Using a MPP controller, the inverter takes the power from the photovoltaic generator at the maximum power point of the I-U characteristic curve.


Kilowatt peak

(Abb. kWp) Measurement for the performance of a PV system under standard test conditions (1 kWp = 1,000 Wp).

Kilowatt hour

(Abb. kWh) Unit of the energy (1 kWh = 1,000 Wh).



Lightning protection

A PV system generally does not increase the risk of a lightning strike. For safety reasons and to prevent damages, PV systems are, however, constructed according to lightning protection standards and the specifications of the German association of property insurers (VdS).

Light moped

For historical reasons, a moped (motor-driven bicycle) has pedals so the rider can continue driving with an empty tank or as the case may be today, with a dead battery. The moped can also be driven solely using the accelerator throttle. In this case, the light moped can reach a maximum speed of 20 km/h. This allows the rider to reach his destination without breaking a sweat. When the rider pedals the moped, the torque sensor is activated and he can reach a speed of up to 35 km/h. This speed requires an operating license and an insurance indicator. E-light-moped riders require an inspection document. The minimum age is 15. Bikeways may be used with a "moped" label. Wearing of a helmet is not compulsory.

Low light performance

Module behavior during reduced radiation (e.g. 200 W/m²).


Maximum Power Point

(Abb. MPP) The point of maximum power. The maximum power output can be derived from this operating point on the I-U characteristic curve of a solar cell or module. MMP tracking allows this point to be detected and set during any operating status.


The electrical wiring of several solar cells that are weather-resistant, protected against environmental influences and both electrically insulated and encapsulated. A module forms the basic unit of a photovoltaic system.

Monocrystalline silicon

Name for the silicon that has only one crystal.

Multicrystalline silicon

Polycrystalline silicon that consists of several crystals.

Module string

Series connection of several modules.


According to the legal definition, a moped is a two-wheeled vehicle with an electric drive and a maximum speed of 45 km/h that falls under the class of low-powered motorcycle and may have a maximum continuous rated output of 4,000 watts. The European operator license class M (EU operator license) is required, which can be obtained when the rider is 16. The operator license classes A1, A, B and T include class M. An insurance indicator is required; the rider must wear a helmet and be at least 16 years old.


On-roof installation

For on-roof installations, the solar modules are installed (parallel to or mounted) above the roof covering using special mounting systems.

Open circuit voltage

(Abb. UL or U) The electrical voltage of a solar cell or module when both poles are not interconnected, in other words, no current flows between the two poles.

Operating consumption

Electricity generated by a photovoltaic system that is consumed within the separate household grid at the same time; it is also called self-consumption or direct consumption.


Peak power

Peak power of a solar module. It is labeled with a lower case "p" behind the energy output kilowatt: kwp. The peak power indicates the performance that a solar module generates under standard test conditions. It is commonly referred to as "nominal value" or "rated output".


Pedelec is the name for an electric bicycle, which is simultaneously powered by an electric motor and a rider's muscle power. Pedelec stands for pedal electric cycle. According to legal regulations, it is considered a "bicycle". Which is why wearing a helmet is not compulsory. There is no speed limit and no prescribed minimum age. You do not require an operating license and may also drive on bikeways. The electric motor assists the rider up to a maximum speed of 25 km/h. The rider must apply pure muscle power to reach higher speeds.


(Abb. PV) is the technology that uses solar cells to convert solar energy (photons) into electric energy.

Photovoltaic generator

The individual modules are first connected in a series (in a row) to form strings and these strings are then connected parallel to the PV generator to achieve sufficiently high voltages and currents, for example, to feed into the public grid using inverters.

Photovoltaic principle

Describes the formation of an electrical current in a semiconductor when charge carriers are excited (internal photon effect) during the irradiance of light (photons). If the charge carriers are extracted, electrical energy is gained in the form of electrical current.

PID Effekt: Potential induced Degradation

LID Effekt: Light induced Degradation
Aging; also referred to as degradation. Degradation of the solar cell efficiency coupled with the duration of the light. This effect, also referred to as the Staebler Wronski effect, has not yet been completely scientifically proven. Generally, only the amorphous solar cells are affected by aging. Laboratory experiments have demonstrated that the efficiency stabilizes after approximately 1,000 operating hours.

Polycrystalline silicon

Consists of small-scale connected crystals that range in size from several millimeters to several centimeters. The block casting method is a standard manufacturing process used for polycrystalline silicon.

Primary energy

Energy that can be extracted from a natural source like crude oil, coal, natural gas, hydropower, solar radiation, etc. Primary energy sources can to some extent be used directly by the end user. In most cases, primary energy is, however, first converted into secondary energy.

Primary circuit (collector circuit, solar circuit)

As seen from the heat source (collector), it is the first closed loop. It is the circuit in which the energy is generated.

PV Cycle

The organization PV Cycle is developing a voluntary return and recycling system for old modules across Europe. The initiative, which is headquartered in Brussels, is supported by the European Commission. The organization's initiative will set up collection points throughout Germany over the next few years. If the next collection point is farther than 50 km away from residential areas or workshops, the organization PV Cycle offers a free collection service for 30 modules or more. PV Cycle members are voluntarily absorbing the costs for developing the entire system.
Donauer Solartechnik has set up a collection point at its headquarters in Gilching.


The measurement unit of power is watt (W) or kilowatt (kW). 1 kW = 1,000 W = 1,000 J/s


Reflection losses

Radiation that is reflected by the surface of a collector or module or solar cell and thus can no longer contribute to the generation of heat or electricity.

Rated output

Maximum possible power output of a solar cell or module. The rated output is defined as the peak output at the maximum power point under standard test conditions (STC) and is indicated in watt peak (abb. Wp).

Roof pitch

Roof slope is the angle of a roof with the horizontal surface. Roof slopes between 20° and 60° are suitable to ensure the optimized yield integration of a PV system. Corresponding mounting systems can bring the modules into a suitable position, even with an unfavorable roof slope.

Roof-integrated installation

In roof-integrated installations, the modules are integrated into the roof covering. In this case, the modules, combined with a corresponding mounting system, perform all the functions of the roof covering, such as protection against rain or snow.



The shade on the roof cast by trees, buildings or antennas is the enemy of every PV system, since the solar cells are connected in a series and each solar cell that is in the shade interrupts the smooth energy flow and thus adversely affects the performance of the system.

Secondary energy

Results from the conversion of primary energy. For example, coke or briquettes from coal and gasoline, diesel fuel or heating oil from crude oil.

Secondary circuit

This refers to the second closed loop as seen from the heat source (collector). It is the circuit that transports the energy to the consumer device.

Selective coating

Special coating on the absorber of a collector that absorbs nearly all of the (short-wave) solar radiation and simultaneously reduces the radiation losses that increase with the rising temperature by radiating (emission) the (long-wave) thermal emission.


Chemical element that makes four bonds with neighboring atoms and in doing so, can form hard and brittle crystals with a stable diamond structure. After oxygen, silicon is the second most common element in the Earth’s crust, however only occurring as silicon dioxide SiO2 (quartz, sand). Silicon is the semiconductor that plays the most important role in the electronic and photovoltaic industry to date. The raw material silicon dioxide can be processed into monocrystalline, polycrystalline or amorphous silicon.

Smart Metering

An "intelligent" meter (also called smart meter) is a meter that enables the power supply company to remotely read out the recorded meter readings using the integrated additional functions or supplementary modules. It is available for gas, water, electricity and (district) heating. The primary purpose of intelligent meters is to acquire variable compensation for services rendered depending on the overall demand and network utilization. Power supply companies aspire to use this for enhancing utilization of the grid and the existing power station infrastructure as well as to avoid investments for expanding or at least resetting the peak load. The operating consumption of photovoltaic systems can also be controlled using smart metering.


SOC or state of charge refers to the battery's current state of charge. This is indicated in percent.

Solar energy

In the narrower sense, the energy that travels from the Sun to the Earth in the form of photons.

Short-circuit current

(Abb. IK or I) The current provided by a solar cell or module when both clamps are connected without any additional resistance (short-circuit).

Solar bike port

The solar bike port was especially developed by Donauer Solartechnik for charging E-bikes. Two solar modules on the roof of the bike port generate the energy required for charging the e-bike. The bike is not connected to the power grid. It operates completely independently. The e-bikes can be charged around the clock, even without any sun. The only thing e-bike riders have to do is connect the battery and press the button to start charging the bike. The sun does the rest. This means: no polluting energy sources and especially no need to lay underground cables.

Solar thermal energy

Using solar energy for the direct generation of heat. The corresponding energy converter is known as a collector.

Solar yield

Specifies the utilizable solar heat, in other words, the remaining heat that can be used from the storage tank after deducting all thermal losses of the collector system.

Spectral sensitivity

The spectral sensitivity of a solar cell describes the solar cell's capability of generating electrical energy from light of a specific wavelength.

Standard test conditions (STC)

Standardized conditions for determining the rated output (measurement of the I-U curve characteristics) of PV modules: irradiation level 1,000 W/m2 with perpendicular light incidence; radiation spectrum corresponds to AM 1.5; cell temperature of 25°C.


Name for several solar cells connected in a series within one module.

Stand-alone system

This system is powered by a photovoltaic system that operates independently of the electrical grid. The electricity generated using solar modules is stored in batteries. In particular, in remote areas (weekend house) or at locations where a grid connection does not make sense due to the costs for installing new power lines (lighting in parks)

Switchover device

It is used in emergency power systems (backup systems). It automatically disconnects the photovoltaic system from the grid in case of an emergency and switches over to the self-sufficient power supply in only a few milliseconds. This is how schools and hospitals maintain an uninterrupted power supply, for example.


Temperature difference controller

Controls the heat transport from the collector to the storage tank by switching the collector pump on and off. The control signal is the temperature difference at the collector outlet and in the lower part of the storage tank.

Temperature coefficient

Specifies how much the open circuit voltage or power output and thus the efficiency of a solar cell or a module decreases for each degree Celsius as the cell temperature increases. Since the above sizes of crystalline solar cells have comparably high negative temperature coefficients, modules, particularly those made of crystalline silicon cells, should be well-ventilated.

Textured surface

Surface structure; also called texture. Specific roughening of solar cell surface using mechanical or chemical procedures. It improves the coupling of incident sunlight. With crystalline solar cells, for example, this is done by the controlled manufacturing of reversed pyramid structures.

Thermal imaging camera

A thermal imaging camera (also referred to as a thermography camera) is an imaging device that is similar to a conventional camera, but receives infrared radiation. The radiation used by infrared cameras lies in the wavelength range between 0.7 and 1000 µm. Thermal imaging cameras, however, use the spectral range between 3.5 and 14 µm (mid-infrared, in short MIR) due the typical emission wavelengths close to the ambient temperature (Wien's displacement law). This range is also suitable for measuring and rendering images or temperatures in the ambient temperature range, if the emissivity is known.

Thin-film technology

Name for the manufacturing technology of thin-film solar cells that are deposited directly onto a low-cost substrate (glass, metal foil, plastic foil). The advantages of thin-film technology lie within the material and energy savings during the industrial manufacturing process, the simple doping ability and the ability to produce large-scale solar cells with integrated series interconnection.

Tilt angle

Angle between a tilted receiving surface and the horizontal surface. Depending on the latitude of the installation location, there are different ideal tilt angles for a solar energy system.


Using a tracking system, the module surface of a PV system is rotated and follows the position of the sun during the course of the day. The system energy yield can be increased by approximately 20% - 40%. depending on the type of tracking (single-axis or dual-axis) and location.



(Abb. V) Unit of measure for electrical current.


(Abk. V) Potential difference between two points, e.g. both poles of a battery. The voltage (U) is the cause of the electrical current (I); both measurements are linked together by the resistance (R) of a conductor according to the ohm´s law (U = R * I). The unit of measure of the electrical current is volt (abb. V).



Name for a thin slice of semiconductor material (e.g. silicon). Wafers are used as the basic material for manufacturing computer chips and crystalline solar cells. The crystalline slices are general sawed from semiconductor blocks (ingots) and are between 0.2 and 0.3 millimeters thick.

Watt peak

(Abb. Wp) Measurement for the performance (rated output) of solar cells and modules. For the purpose of comparison, module prices are usually listed in euros/Wp.