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PV Lexicon


Ampere-Hours (Ah)

Ampere-Hour (Ah) is the capacity of a battery to deliver the same current over a set period of time. For example, one battery has a 112 Ah capacity over a 100 hour discharge period but only an 88 Ah capacity if discharged equally over 8 hours.

Batteries, Sealed AGM

These batteries use glass mat separator spacked tightly between the flat plates to hold the electrolyte. AGM cells should always be installed flat, never vertically, to minimize stratification and the drying out of the cells prematurely. AGM batteries perform better than GEL batteries in cold weather conditions, as they lose less capacity in low temperatures. AGM batteries excel in high current, high power, standby and shallow cycle applications.

Sealed Gel

Instead of the liquid electrolyte typically used in open cells, the electrolyte is mixed with silica to produce a gel. Sealed Gel batteries offer the added benefit of non-maintenance, non-spillable or leakable, superior deep cycle life, minimal gassing and stratification. GEL batteries provide 2-3 times more cycles as compared to AGM, depending on DOD and ambient temperatures. Due to the physical properties of the gelled electrolyte, a GEL battery’s capacity declines faster than AGM when the temperature drops below 0ºC.

Batteries  Cycles

One period of discharge and recharge is called one cycle. Battery performance may be measured by the expected number of cycles it may deliver at varying depths of discharge, usually in good quality batteries from 1200 to 5000 cycles.


A term for the design and integration of photovoltaic (PV) technology into the building envelope, typically replacing conventional building materials. This integration may be in vertical facades, replacing view glass, spandrel glass, or other facade material; into semitransparent skylight systems; into roofing systems, replacing traditional roofing materials; into shading "eyebrows" over windows; or other building envelope systems.


Cell Types

The type of silicon that comprises a specific cell, based on the cell manufacturing process. Monocrystalline PV cells are the most expensive and energy intensive to produce but usually yield the highest efficiencies. Though polycrystalline and ribbon silicon cells are slightly less energy intensive and less expensive to produce, these cells are slightly less efficient than monocrystalline cells. However, because both poly- and ribbon silicon panels leave fewer gaps on the panel surface (due to square or rectangular cell shapes), these panels can often offer about the same power density as monocrystalline modules. Thin-film panels, such as those made from amorphous silicon cells, are the least expensive to produce and require the least amount of energy and raw materials, but are the least efficient of the cell types. They require about twice as much space to produce the same power as mono-, poly-, or ribbon-silicon panels. Thin-film panels do have better shade tolerance and high-temperature performance but are often more expensive to install because of their lower power density.

HIT panels are composed of a monocrystalline cell and a thin layer of amorphous silicon material. In addition to generating power from the direct rays of the sun on the panel face, this hybrid panel can produce power from reflected light on its underside, increasing overall panel efficiency.

Charge Controller

A component of a PV island (Off-Grid) system that controls the flow of current to and from the battery to protect it from over-charge and deep-discharge. The charge controller may also indicate the system operational status. They are to main types of charge controllers: PWM and MPPT ( for details see below: PWM and MPPT charge controller).


Chemical symbol for carbon dioxide, a combination of carbon and oxygen. Carbon dioxide is generated e.g. when burning carbon containing substances, and thus also in many production processes, so that when determining the ecological footprint of a product it is also necessary to indicate its CO2 emissions. Too much CO2 promotes the greenhouse effect on Earth. CO2 absorbs part of the sun’s heat (infrared radiation), while shorter wave radiation, i.e. the greater part of sunlight, can pass through it. This property makes carbon dioxide into what is referred to as a “greenhouse gas”. With a solar power system, within just a few years more CO2 is saved than is necessary for the production of PV modules.


Connector Types

PV module output terminal or cable connector comes with "plug and play" weather-proofed connectors. The mainly used connectors standard is the MC4, from Multi-Contact. Other popular types of connectors are MC3 (older one) and Sunclix from Phoenix Contact.

Cost-Effectiveness (performance ratio)

PV systems are known for their good returns as compared eg. to financial products. This applies above all when such systems are operated in countries with existing feed-in compensation schemes that guarantee a purchase price for generated solar electricity. However, such calculations of returns often neglect the risks that could arise in connection with a PV system with a 25 year useful life. Risks include weather related fluctuations and the performance of the PV modules themselves. High performance PV modules are known for their high degree of efficiency and their elevated reliability, which contribute to minimising the risks and guaranteeing higher returns.

DoD - Depth of Discharge | SoC - State of Charge 

DoD is the ratio of amp hours (Ah) removed from a battery versus its full capacity. For example 25Ah are removed from a 100Ah battery, thus it's depth of discharge is 25% (DoD) and the battery is at a 75% state of charge (SoC).

Efficiency (PV technology, cell, module, inverter)

The efficiency η (pronounced ‘eta’) of a solar cell or a module is defined as the ratio between the power output and the power input (the absorbed light). In other words, it indicates how well the solar cell and PV module converts light into electricity. The higher the efficiency value, the more electricity generated in a given space. The solar cell efficiency doesn’t equal the PV module efficiency. The module efficiency is usually 1 to 3% lower than the solar cell efficiency due to glass reflection, frame shadowing, higher temperatures etc. 

The efficiency of commercially available photovoltaic modules is of between 6 and 9% for thin film modules, around 14% polycrystalline and up to 20% for monocrystalline modules. The efficiency of grid connected inverter is up to 98% and the inverter losses (~2%) must also be taken into the equation.


Electrical Parameters (PV Moduels/Panels)

Maximum Power Voltage (Vmp)

The voltage where a panel outputs the maximum power. Grid-tied inverters and MPPT charge controllers are built to track maximum power point throughout the day, and Vmp of each panel array, as well as array operating temperatures must be considered when sizing an array to a particular inverter or controller. Series string sizing software programs for grid-tied inverters allow you to input both the high and low temperatures at your installation site, and calculate the correct number of panels in series to maximize system performance.

Maximum Power Current (Imp)

The maximum amperage where a panel outputs the maximum power. This specification is most commonly used in calculations for PV array disconnect labeling, as the rated maximum power-point current for the array must be listed. Maximum power current is also used in array and charge controller sizing calculations for battery-based PV systems.

Open-Circuit Voltage (Voc)

The maximum voltage generated by a PV panel exposed to sunlight with no load connected. All major PV system components (panels, wiring, inverters, charge controllers, etc.) are rated to handle a maximum voltage. Maximum system voltage must be calculated in the design process to ensure all components are designed to handle the highest voltage that may be present. Under certain low-light conditions (dawn/dusk), it’s possible for a PV system to operate close to open-circuit voltage. PV voltage will increase with decreasing air temperature, so Voc is used in conjunction with historic low temperature data to calculate the absolute highest maximum system voltage.

Short-Circuit Current (Isc)

The maximum amperage generated by a PV panel exposed to sunlight with the output terminals shorted. The PV circuit's wire size and overcurrent protection (fuses and circuit breakers) calculations are based on panel short-circuit current. The PV system disconnect(s) must list short-circuit current.

Short-Circuit Current Temperature Coefficient α (%/°C)

The change in panel short-circuit current per degree Celsius at temperatures other than 25°C. It is most commonly used to calculate maximum system current for system design and labeling purposes. For example, consider a series string of ten 8A (Isc) panels installed at a site with a record low of 15°C. Given a Isc temperature coefficient 0.04%/°C, the decrease in current will be 0.32A, making for an overall maximum system current of 7.68A.

Open-Circuit Voltage Temperature Coefficient β (%/°C)

The change in panel open-circuit voltage at temperatures other than 25°C. If given, It is most commonly used to calculate maximum system voltage for system design and labeling purposes. For example, consider a series string of ten 43.6V (Voc) panels installed at a site with a record low of -10°C. Given a Voc temperature coefficient of -160mV/°C, The voltage per panel will rise 5,600mV (= 160mV x (-10°C – 25°C)), making for an overall maximum system voltage of 492V (= 10 x (5.6V + 43.6V)), which is under the 600VDC limit for PV system equipment.

Maximum Power Temperature Coefficient δ(%/°C)

The change in panel output power for temperatures other than 25°C. It is used to calculate how much panel power will be lost or gained due to temperature changes. In hot climates, cell temperatures can reach an excess of 70°C. Consider a panel maximum power rating of 200W at STC, with a temperature coefficient of -0.5%/°C. At 70°C, the actual output of this panel would be approximately 155W. Panels with lower power temperature coefficients will fare better in higher-temperature conditions. Thin-film panels have relatively low temperature coefficients which reflects better high-temperature performance.


Fill Factor (%)

The ratio of actual rated maximum power Pm to the theoretical (not actually obtainable) maximum power (Isc x Voc ). This is a key parameter in evaluating the performance of PV solar modules. Typical commercial modules have a fill factor > 0.70, while grade B solar panels have a fill factor range from 0.4 to 0.7. A higher fill factor solar panel has less losses due to the series and parallel resistances within the cells themselves


The feed-in tariff system has been introduced in the most EU member countries at present and it is widely known as the key financial incentive to encourage the use of renewable energy resources such as wind power and solar photovoltaics. Under a feed-in tariff, eligible renewable electricity generators, which can include homeowners, business owners, farmers, as well as private investors, are paid a cost-based price for the kWh renewable electricity they produce. This enables a diversity of technologies, providing investors a reasonable return on their investments.

Feed-in tariffs can be used to accelerate the pace at which renewable energy technologies become cost-competitive with electricity provided from the grid. The rapid deployment of renewable energy under feed-in tariffs seen in countries like Germany, Denmark and Spain has undoubtedly contributed to reducing technology costs. For instance PV solar technology costs have decreased dramatically since the 1970s, as the technologies have become more widespread, manufacturing processes have improved, innovations have been incorporated, and gains have been harnessed from economies of scale.

 See an international overview of the feed in tariffs.

Flash List (Report)

Most manufacturers provide flash reports of their solar panels sold, including every single panel's flash test data. During a flash test, a solar panel is exposed to a short (1 - 30 millisecond), bright (1 watt per M2) flash of xenon light source. The spectrum of the flash light is designed to be close to the spectrum of the STC. The output is collected by a testing computer and the data is compared to a pre-configurated reference solar panel which has its power output calibrated to standard solar irradiation. The results of the flash test are compared to the specifications of the pv module datasheet and are printed somewhere on the pv panel. The flash testing system is usually re-corrected by the reference panel in certain interval (usually two hours). The data in a flash report includes the pv panel barcode, Pmax, Voc, Isc, Im and Vm.

Grid Connected System (PV On-Grid)

Grid connected photovoltaic systems are connected to the power utility grid via an inverter in order to feed the electricity generated into the public power grid. Compensation is determined according to the pertinent FIT system. On-Grid photovoltaic systems do not require energy storage devices.

Inclination (PV generator)

The sun shines all day long, but solar modules installed on a fixed plane (e.g. on a roof) keep the same orientation all year long. It is possible to calculate the ideal angle of inclination to maximise annual electricity production. In principle, in Europe roofs with an angle of inclination of between 45° and 15° are considered to be appropriate, with hardly any difference in the energy production yield. Roofs with an angle of inclination of less than 15° usually suffer from dirt that can no longer easily be washed away by rain. In addition, snow also stays longer, leading to increased pressure on the modules. 

Roofs with a high angle of inclination have better output in the winter, whereas roofs with a small angle of inclination perform better during the summer. In addition, roofs with a high angle of inclination usually have better air circulation on the back side of the modules, which can result in a slight advantage over the course of a year. 

Solar parks, on the other hand, use a different approach: it is possible to mount modules on structures that move to match the sun’s trajectory; this is sometimes called a “sun tracking system”. The energy yield of such systems can be more than 30% higher than that of fixed modules. However, such systems are usually not suitable for rooftop installation.


A device that converts direct current electricity to alternating current either for stand-alone systems or to supply power to an electricity grid. Photovoltaic modules generate DC. An inverter transforms DC into AC. This makes it possible for the electrical energy produced with solar energy to be used by end consumers with 230 volts of alternating voltage or to feed it into the local grid. Central inverters are used in large photovoltaic systems, and string inverters in small photovoltaic systems.

Island system (PV Off-Grid)

Island systems, also known as PV Off-Grid systems are designate e.g. a building that is not connected to the public grid and which is energy self-sufficient due to the solar electricity it generates. This requires accumulator batteries usually in 12/24/48V to store the generated power.


Abbreviation of Kilowatt hour. One kWh is equivalent to 1.000 Watt per hour.


kW = kiloWatt, p = peak, i.e. the maximum possible output of a PV solar system under standard testing conditions (STC). It is measured in Kilowatt and stated as kWp (KiloWatt-Peak).



Mismatching define the loss of power, mainly in series connected PV modules. The reason can be the different performance of the individual modules, within the same models. By connecting several modules in series, through all modules should flow the same current. But the worst performing module, within the series, will impair the power of all other connected PV modules. Are there strong variations in performance, the powerful modules can not implement her superiority. Therefore it is important to connect modules with low power distribution in a separate. Quality manufacturer sorts out modules with mismatching performance. Sorting can be also made individually by installer by the current (A) listed in the modules flash list. Other mismatch losses can result from different operating conditions such as different module inclination, orientation, partial shade or different models, within the same string.


Modules (PV solar panels)

A photovoltaic module is the essential component of the PV solar power system. A solar module consists of several solar cells connected in series (cells string). Generally, these cells are enclosed in an airtight, weather-resistant housing to protect them against the elements. Solar cells are basically constructed of two layers, where one layer is negatively charged and the other is positively charged. When sunlight strikes these layers, a voltage is created between them. A direct current is produced from this voltage inside the solar cell. To make use of this current, a photovoltaic system has what is called an inverter. The inverter converts the generated direct current into alternating current so that it can be used in the household or fed into the public grid.

There are many different sizes and designs of solar modules nowadays. So that they can be integrated more effectively into existing architectures, solar modules are offered in special designs and different colours.

MPPT charge controller

MPPTs are smart DC to DC converters that optimize the match between the PV solar generator and the battery bank. While gains of 50% in solar module output are possible the typical wattage gain using an MPPT is 10 to 30% vs. an PWM charge controller. For example, an 75W PV module is rated at 4.4A and 17V that is 4.4 times 17 = 74.8 watts. But this 75 watts does not equal 75 watts of charging capacity since your battery is only charging near 13.5 V. The output of a solar module is characterized by a performance curve of voltage versus current, known as its I-V curve. For crystalline modules, the current remains fairly constant as the voltage changes relative to the voltage of battery it is charging. A battery charging at 13 V is only using 57.2 watts of power not the full 75 watts a loss of about 24%. In an extreme case, such as a fully discharged battery at 10.5 volts, you would get nearly 7 amps at 10.5 volts from the MPPT into the battery! MPPT's are most effective under these conditions: Cloudy or hazy days - when the extra power is needed the most. Cold weather - solar module output increases in cold temperatures during the winter when sun hours are low and you need the most power. Low battery charge - the lower the state of charge in your battery, the more current a MPPT puts into them. An MPPT charge controller are dedicated for higher voltage PV generators, around 150Wp and more, below this size it may be more cost effective to use an PWM charge controller.


Abbreviation for Megawatt. The output of solar systems is measured in Megawatt (MW) 1MW=1,000,000 Watt.

Nominal output

The nominal output is the highest possible yield of a solar cell or a solar module. It is measured under Standard Test Conditions (see also: STC). Nominal output is stated in Wp abbreviation for Watt Peak.


Performance Ratio

The system performance ratio (PR) is the final energy yield, divided by the reference yield. PR can be expressed either as a decimal fraction or as a percentage. The reference yield is based on the location irradiance and represents the theoretically available energy per year per kWp installed. Typical PR values are 60-80% but higher values are also possible.

Photovoltaics (PV)

Photo (Greek) = photos: light; Volt = unit of electric tension (tension through light). The generation of electric power from solar energy is called “photovoltaics”. Positive and negative charge carriers are released in the solar cell under the influence of heat and light. The direct current thus generated can drive motors or charge accumulator (storage) batteries. As Europe primarily uses alternating current, the direct current is converted into alternating current by means of an inverter.

PWM - Pulse Width Modulation

These controllers use power transistors at very high frequencies to pulse the charge current on and off in order to maintain a constant battery voltage. These controllers offer excellent charging characteristics for pv systems of up to 40A, however, due to their high switching frequency may cause noise on some telecommunications equipment.

The relays used within these controllers offer some limitations which affect performance and useful life. To extend life, relay's on/off range will be widened to reduce their cycling though this results in less efficient battery charging. Where currents are high, these relays may have a bypass circuit to temporarily handle the current during the cycle. Solid state switching devices will cycle almost endlessly without damage to them, however, the resultant voltage drop will lead to some heat generation.


Rated Power Tolerance δ (%)

The specified range within which a PV module will either overperform or underperform its rated power Pm at STC. Power tolerance can vary from +10% to -10%. A 200 watt PV module with ±10% rated power tolerance may produce only 180 watts or 220 watts. To ensure expected power output, look for PV modules with a small negative (or positive only) power tolerance.

Silicon (Si)

A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide). Chemical element capable to bond with four neighbouring atoms. Silicon is the semiconductor which has been the most important for the semiconductor industry and photovoltaics so far. The raw material silicon oxide (sand) can be processed into monocrystalline, multicrystalline or amorphous silicon.

STC - Standard Test Conditions

Standard Test Conditions are the conditions set by standards for the measurement of the maximum nominal output of a solar module. The irradiation power assumed is of 1.000 W/m2 with vertical light incidence, the radiation or sunlight spectrum is of AM (Air Mass) 1.5 and the cell temperature is assumed to be 25°C.

Sunshine hours

Sunshine hours designates the number of hours per year in which sunlight falls onto the Earth’s surface. Depending on the location in Europe, an average number of annual sunshine hours can be assumed. What is essential, however, is that the PV system should not be in the shade of trees, buildings or other items.

Tracked solar system

A tracked solar system is based on a special fastening and assembly system (also referred to as “trackers”) that enables the solar modules to be mechanically oriented towards the current position of the sun at all times. This enables a distinct increase in yield. Single axis systems rotate from east to west, while dual axis tracking systems also take the sun’s angle of elevation into account. The energy yield of such systems can be more than 30% higher than that of fixed PV systems.


In photovoltaics, a wafer is a sawn semiconductor chip (usually of silicon). A wafer acts as carrier material for the manufacture of solar cells. Chips are usually sawn from semiconductor blocks, and are 0.2 to 0.3 millimetres thick. Technological progress means that wafers are becoming ever thinner, ever faster.


Warranty for Materials (Years)

A limited warranty on modules materials and quality under normal application, installation, use, and service conditions. Material warranties vary from 1 to 20 years.

Warranty for Power (Years)

A limited warranty for module power output based on the minimum peak power rating (STC rating minus power tolerance percentage) of a given module. The manufacturer guarantees that the module will provide a certain level of power for a period of time (at least 20 years). Most warranties are structured as a percentage of minimum peak power output within two different time frames: (1) 90% over the first 10 years and (2) 80% for the next 10 years. module replacement value provided by most power warranties is generally prorated according to how long the module has been in the field.

Wp (Pm)

Watt-Peak abbreviation: Wp, indicates the output capacity of solar cells or modules, the maximum electrical output generated. The maximum rated power Pm (W) is the maximum power output from a PV panel at STC which is usually labeled on the panel nameplate. The actual power output can be estimated by

Preal = Pm * S / 1000 * [1 - λ(Tcell - 25)]

Tcell = Tambient + S / 800 * (TNOCT - 20) 

where S - the solar radiation on the panel surface, Tambient - the ambient temperature, TNOCT - the Nominal Operating Cell Temperature, and λ - Maximum Power Temperature Coefficient.



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