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The nominal power is the
nameplate capacity Nameplate capacity, also known as the rated capacity, nominal capacity, installed capacity, or maximum effect, is the intended full-load sustained output of a facility such as a power station,
of
photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially u ...
(PV) devices, such as
solar cell A solar cell, or photovoltaic cell, is an electronic device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.
s,
modules Broadly speaking, modularity is the degree to which a system's components may be separated and recombined, often with the benefit of flexibility and variety in use. The concept of modularity is used primarily to reduce complexity by breaking a sy ...
and
systems A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment, is described by its boundaries, structure and purpose and express ...
, and is determined by measuring the
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving pa ...
and
voltage Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to mo ...
in a circuit, while varying the resistance under precisely defined conditions. The nominal power is important for designing an installation in order to correctly dimension its cabling and converters.Die Verwirrung um das Watt-Peak
The confusion around watt-peak, 14 August 2009.
The peak power is not the same as the power under actual radiation conditions. In practice, this will be approximately 15-20% lower due to the considerable heating of the solar cells. Moreover, in installations where electricity is converted to AC, such as solar power plants, the actual total electricity generation capacity is limited by the
inverter A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opp ...
, which is usually sized at a lower peak capacity than the solar system for economic reasons. Since the peak DC power is reached only for a few hours each year, using a smaller inverter allows to save money on the inverter while
clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...
(wasting) only a very small portion of the total energy production. The capacity of the power plant after DC-AC conversion is usually reported in WAC as opposed to Wp or WDC.


Standard test conditions

The nominal power of PV devices is measured under ''standard test conditions'' (STC), specified in standards such as IEC 61215, IEC 61646 and UL 1703. Specifically, the light intensity is 1000 W/m2, with a spectrum similar to
sunlight Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is scattered and filtered through Earth's atmosphere, and is obvious as daylight when th ...
hitting the earth's surface at latitude 35°N in the summer ( airmass 1.5), the temperature of the cells being 25 °C. The power is measured while varying the resistive load on the module between an open and closed circuit (between maximum and minimum resistance). The highest power thus measured is the 'nominal' power of the module in
watt The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Watt ...
s. This nominal power divided by the light power that falls on a given area of a photovoltaic device (area × 1000 W/m2) defines its
efficiency Efficiency is the often measurable ability to avoid wasting materials, energy, efforts, money, and time in doing something or in producing a desired result. In a more general sense, it is the ability to do things well, successfully, and without ...
, the ratio of the device's electrical output to the incident energy.


Watt-peak

The
International Bureau of Weights and Measures The International Bureau of Weights and Measures (french: Bureau international des poids et mesures, BIPM) is an intergovernmental organisation, through which its 59 member-states act together on measurement standards in four areas: chemistry, ...
, which maintains the SI-standard, states that the
physical unit A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multip ...
and its symbol should not be used to provide specific information about a given
physical quantity A physical quantity is a physical property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ' Numerical value ' and a ' Unit '. For examp ...
and that neither should be the sole source of information on a quantity. Nonetheless, colloquial English sometimes conflates the quantity power and its unit by using the non-SI unit watt-peak and the non-SI symbol Wp prefixed as within the SI, e.g. kilowatt-peak (kWp), megawatt-peak (MWp), etc. As such a photovoltaic installation may for example be described as having "one kilowatt-peak" in the meaning "one kilowatt of peak power". Similarly outside the SI, the peak power is sometimes written as "P = 1 kWp" as opposed to "Ppeak = 1 kW". In the context of domestic PV installations, the kilowatt (kW) is the most common unit for peak power, sometimes stated as kWp.


Conversion from DC to AC

Solar power need to be converted from
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
(DC, as it is generated from the panel) to alternate current (AC) to be injected into the power grid. Since solar panels generate peak power only for few hours each year, and DC to AC converters are expensive, the converters are usually sized to be smaller than the peak DC power of the panels. This means that for few hours each year the peaks are " clipped" and the extra energy is lost. This has very little impact on the total energy generated throughout a year, but saves considerable amount of balance of system (BOS) costs. Due to under-sizing of converters, AC ratings of solar plants are generally significantly lower than DC ratings, as much as 30%. This in turn increases the calculated yearly capacity factor of the plant. The downrating of peak power and the related clipping is different from the losses incurred in the conversion from DC to AC, which happen at any power level and are usually relatively small. Most countries refer to installed nominal
nameplate capacity Nameplate capacity, also known as the rated capacity, nominal capacity, installed capacity, or maximum effect, is the intended full-load sustained output of a facility such as a power station,
of PV systems and panels by counting DC power in watt-peak, denoted as Wp, or sometimes WDC, as do most manufacturers and organizations of the photovoltaic industry, such as
Solar Energy Industries Association The Solar Energy Industries Association (SEIA), established in 1974, is the national non-profit trade association of the solar-energy industry in the United States. In 2019, the group reported at least 1,000 member companies. SEIA is a 501(c ...
(SEIA), the European Photovoltaic Industry Association (EPIA) or the
International Energy Agency The International Energy Agency (IEA) is a Paris-based autonomous intergovernmental organisation, established in 1974, that provides policy recommendations, analysis and data on the entire global energy sector, with a recent focus on curbing carb ...
( IEA-PVPS). Some grid regulations may limit the AC output of a PV system to as little as 70% of its nominal DC peak power (Germany). Because of these two different metrics, international organizations need to reconvert official domestic figures from the above-mentioned countries back to the raw DC output in order to report coherent global PV-deployment in watt-peak. In order to clarify whether the nominal power output (watt-peak, Wp) is in fact DC or already converted into AC, it is sometimes explicitly denoted as MWDC and MWAC or kWDC and kWAC. The converted WAC is also often written as "MW (AC)", "MWac" or "MWAC". Just as for Wp, these units are non SI-compliant but widely used. In California, for example, where the rated capacity is given in MWAC, a downrating of 15 percent in the conversion from DC to AC is assumed.


Cost per watt

Although watt-peak is a convenient measure, and is the standardized number in the
photovoltaic Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially u ...
industry on which prices, sales and growth numbers are based, it is arguably not the most important number for actual performance. Since a solar panel's job is to generate electric power at minimal cost, the amount of power that it generates under real-life conditions in relation to its cost should be the most important number to evaluate. This cost per watt measure is widely used in the industry. It can happen that a panel from brand A and a panel of brand B give exactly the same watt-peak in laboratory test, but their power output is different in a real installation. This difference can be caused by different degradation rates at higher temperatures. At the same time, though brand A can be less productive than brand B it may as well cost less, thus it has a potential of becoming financially advantageous. An alternative scenario can also be true: a more expensive panel may produce so much more power that it will outperform a cheaper panel financially. An accurate analysis of long-term performance versus cost, both initial and ongoing, is required to determine which panel may lead the owner to better financial results.


Power output in real conditions

The output of
photovoltaic system A photovoltaic system, also PV system or solar power system, is an electric power system designed to supply usable solar power by means of photovoltaics. It consists of an arrangement of several components, including solar panels to absorb and c ...
s varies with the intensity of sunshine and other conditions. The more sun, the more power the
PV module A solar cell panel, solar electric panel, photo-voltaic (PV) module, PV panel or solar panel is an assembly of photovoltaic solar cells mounted in a (usually rectangular) frame, and a neatly organised collection of PV panels is called a photo ...
will generate. Losses, compared to performance in optimal conditions, will occur due to non-ideal alignment of the module in tilt and/or azimuth, higher temperature, module power mismatch (since panels in a system are connected in series the lowest performing module defines performance of the string it belongs to), aging factor, soiling and DC to AC conversion. The power a module generates in real conditions can exceed the nominal power when the intensity of sunlight exceeds 1000 W/m2 (which corresponds roughly to midday in summer in, for example, Germany), or when sun irradiation close to 1000 W/m2 happens at lower temperatures. Nominal power rating of solar PV plants is not comparable with the name plate MCR rating of conventional power plants as there is wide difference between its DC and AC ratings (30% to 40%). The net power that can be fed by a coal fired / nuclear power plant is around 90% of its name plate MCR after deducting internal consumption. Similarly for CCGT, around 97% of site rating and for GTPP or Hydro electric plant above 99% of site rating. The installation cost/MW generally given for Solar PV based on DC capacity is skewed when compared to net MCR of other power generation sources (including
wind power Wind power or wind energy is mostly the use of wind turbines to generate electricity. Wind power is a popular, sustainable, renewable energy source that has a much smaller impact on the environment than burning fossil fuels. Historically, w ...
and
solar thermal Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in industry, and in the residential and commercial sectors. Solar thermal collectors are classified by the United S ...
). The actual installation cost of solar PV is around 50% more to compare with net MCR of other sources for giving equal input (MW) to the AC power grid.


References

{{Photovoltaics Renewable energy economy Units of power Photovoltaics