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The absorber is the most important part of a solar collector. It absorbs the incident sunlight via a carrier fluid (as a rule, water plus antifreeze), as a result of which the liquid is heated and circulated between the collector and the storage unit. The use of black or selectively coated absorbers results in a particularly high level of efficiency.

Antireflection layer

The antireflection layer on the top of the solar cell prevents as few of the radiated light particles as possible being reflected back again.

Accumulator (storage battery)

Electrical energy is stored in an accumulator. It is used where electrical energy has been generated and needs to be stored for subsequent use.


The rows of solar cells in a module or panel are bridged with bypass diodes in order to maintain their function when the module is in the shade. If a solar cell is in the shade for a long time, it more or less becomes "blocked up", while the other cells continue trying to push electricity through it. This can result in excess voltage and overheating, which can cause irreparable damage to the module. The bypass diode bridges the cell in the shade and thus protects it from damage. It means that the relevant cells do not need to be taken out of operation, which would, in turn, result in a loss of power.

Direct/diffuse radiation

Direct radiation (i.e. radiation that throws a shadow) impacts a surface without suffering any scatter caused by components in the Earth's atmosphere. Indirect or diffuse radiation (low intensity) results from scattering due to mist, fog, clouds or such like.


If a solar panel has been certified to IEC 61215 (crystalline) or IEC 71646 (thin film), it is an indication of the panel's quality regarding mechanical stability and compliance of the electrical parameters.

Charge controller

This is the interface between a source of direct current and an accumulator or storage battery. With storage batteries, the charge controller regulates the charging and discharging process.
Direct current (DC)
Current that flows in one direction only, although it may have appreciable pulsations in its magnitude.

Direct/diffuse radiation

Direct radiation (i.e. radiation that throws a shadow) impacts a surface without suffering any scatter caused by components in the Earth's atmosphere. Indirect or diffuse radiation (low intensity) results from scattering due to mist, fog, clouds or such like.

Efficiency level

The efficiency level or degree of conversion indicates how much of the incident solar energy is converted into usable energy. PV installations nowadays achieve an efficiency of between 11 and 17 %, while thermal solar units can convert 25 – 40 % of the solar radiation.

Energy amortization time

The energy amortization time is basically the relationship between the amount of energy used for production and the amount of energy generated. It indicates how long it takes until e.g. a solar module or solar collector has supplied the energy needed for its production in the form of electricity or heat.

Global radiation

Global radiation is the total solar energy impacting the Earth. It is made up of direct solar radiation and diffuse sunlight. The ratio of one type of energy to the other depends on the angle of incidence of the sun. The German weather service in Hamburg provides comprehensive data and statistics on the regional radiation intensities.


An inverter converts direct current generated by the solar modules into alternating current that can be fed into the public grid. Inverters can be installed inside or outside buildings depending on the type of energy production. Since they generate heat and their performance falls when the ambient temperature is high, they should be installed in a cool room or on a shady side of the building. The choice of the right inverter has a major influence on the efficiency/output of a PV unit.

Abbreviation of kilowatt-hour. 1 kWh = an output of 1,000 Watts over a period of one hour.


Solar modules can only be produced with a manufacturing tolerance of +/- 5 %. Matching means correctly pre-sorting the modules according to their true output. Through identification of the serial number and the output, an effective arrangement within the string can be achieved, thereby raising the overall yield of the unit.


The switching of effective and less effective solar modules in one string, whereby the module with the lowest output in a row determines the electricity yield. .


Solar modules können nur mit einer Fertigungstoleranz von +/- 5 % produziert werden. Matching bedeutet das richtige Vorsortieren der Module nach tatsächlicher Leistung. Über die Identifikation der Seriennummer und Leistungszuordnung kann dann eine genaue Aufteilung innerhalb der Strangverkabelung erfolgen, wodurch der Gesamtertrag der Anlage gesteigert wird.

No-load voltage

The voltage of a solar cell when it is not in a loaded state, i.e. when it is not being irradiated. It is generally around 25 % higher than the nominal voltage and gives an indication of the upper voltage limit. Under ideal circumstances, the voltage range of the irradiated cell is between the nominal voltage and the no-load voltage.

Nominal output (peak performance)

The nominal (or rated) output is the nominal voltage (voltage at max. output in volts) multiplied by the nominal current (current intensity at max. output in amperes). It is dependent on a number of factors such as the intensity of the solar radiation and the operating temperature. The nominal output is achieved at a cell temperature of 25 °C, a radiation intensity of 1,000 W/m² and a given spectral distribution. Such conditions occur fairly rarely in a year, so that the real output over the average for the year is around 15 % lower than the nominal output. The nominal or rated output is also frequently called the peak performance – measured as Wp (Watt peak).

Output tolerance

This indicates the maximum deviation from the nominal output. The module manufacturers indicate this with 3.5 or 10 %. See also Matching.


Generation of electricity from solar power. 

Reimbursement for electricity fed into the grid

In Germany, the Renewable Energy Sources Act (EEG), which came into force on January 1, 2004, stipulated that electricity generated by solar power units has to be reimbursed by the grid operators and the local power supply companies at a rate of at least 45.7 cents/kWh. The actual rate varies according to the type and size of the system and drops annually by 5 %. The rate valid in the year of start-up remains, however, for a period of 20 years. The exact reimbursement rates can be found here.

Renewable Energy Sources Act (EEG)

The Renewable Energy Sources Act came into force in Germany on April 1, 2000 and was amended in mid-2004. It regulates the obligation of the energy supply companies to purchase electricity from renewable energy sources and to pay fixed rewards. Following the "Stromeinspeisegesetz" (Electricity Grid Feed Act) and the "1,000 and 100,000 roofs program", Germany thereby created the economic basis for the rapid development of the production and installation of PV units in Germany. Many other countries in Europe have since taken over large sections of the EEG.

Solar cell
In solar cells, which are usually made of silicon, positive and negative charge carriers are released when light or heat is introduced (photo effect) thereby releasing direct current that can drive a motor or charge up a battery. If solar energy is also to be used for operating electrical applications with an alternating voltage of 230 V or fed in to the public grid, an inverter is needed. There are various kinds of solar cells, which differ not least in their efficiency and production cost.

Solar collector

A collector converts, by means of an absorber, solar radiation into heat that can be used for heating, for heating up process water or for thermal ventilation (solar thermal technology). Solar collectors can convert up to 80 % of the radiant energy into heat.

Solar energy

The energy potential of the sun is virtually inexhaustible – it will last at least for the next 4 billion years, according to experts' estimates. The amount of solar energy reaching the Earth is somewhere between ten and fifteen thousand times the daily consumption. Using photovoltaics or solar thermal technology, solar energy can be used actively and converted into electricity or heat.

Solar generator

The solar generator is all the solar modules used for a PV unit.

Solar module

A solar module or solar panel is formed by connecting up individual solar cells, including the necessary components such as the module frame, glass panels etc

Storage unit

The medium for bridging the time difference between the capture of the solar radiation and the energy consumption. PV units store the generated electricity (if it is not fed into the grid) in accumulators (batteries), while thermal solar units store the generated heat in hot water storage units.

Temperature coefficient

The temperature coefficient indicates by how many percent the output of the module (i.e. electricity yield) decreases with rising (cell) temperatures. Solar panels produce their optimum performance at cold temperatures, a clear sky and a lot of sunshine.

Wp / KWp

The output of a solar module under standard test conditions. The unit is Wp (Watt peak) or KWp (Kilowatt peak).
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