nameplate capacity
Nameplate capacity, also known as the rated capacity, nominal capacity, installed capacity, maximum effect or gross capacity,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 ...
(PV) devices, such as
solar cell
A solar cell, also known as a photovoltaic cell (PV cell), is an electronic device that converts the energy of light directly into electricity by means of the photovoltaic effect.
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 is exp ...
. It is determined by measuring the
electric current
An electric current is a flow of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is defined as the net rate of flow of electric charge through a surface. The moving particles are called charge c ...
and
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
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.
Nominal power is also called peak power because the test conditions at which it is determined are similar to the maximum irradiation from the sun. Thus this quantity approximates the theoretical maximum production of the panel on a clear sunny day with the panel oriented perpendicular to the sun.
The nominal power is generally not reached under actual radiation conditions. In practice, actual conditions will allow for approximately 15-20% lower generation 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 op ...
, 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 WDC or Watt-peak (Wp).
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 the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...
hitting the Earth's surface at latitude 35°N in the summer (
airmass
In astronomy, air mass or airmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from below Earth's atmosphere . It is formulated as the integral of air density along the light ray.
As i ...
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 (physics), 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 quantification (science), quantify the rate of Work ...
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 making mistakes or wasting materials, energy, efforts, money, and time while performing a task. In a more general sense, it is the ability to do things well, successfully, and without waste.
...
, the ratio of the device's electrical output to the incident energy.
Units
In the context of domestic PV installations, the
kilowatt
The watt (symbol: W) is the unit of Power (physics), 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 quantification (science), quantify the rate of Work ...
(symbol kW) is the most common unit for nominal power, for example Ppeak = 1 kW.
Colloquial English sometimes conflates the quantity power and its unit by using the non-standard label watt-peak (symbol Wp), possibly prefixed as in kilowatt-peak (kWp), megawatt-peak (MWp), etc. For example, a photovoltaic installation may be described as having "one kilowatt-peak of power" ("P = 1 kWp").
However, in the
International System of Units
The International System of Units, internationally known by the abbreviation SI (from French ), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official s ...
(SI), the
physical unit
A unit of measurement, or unit of measure, 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 ...
(and its symbol) should not be used to provide specific information about the conditions assumed for measuring a given
physical quantity
A physical quantity (or simply quantity) is a property of a material or system that can be Quantification (science), quantified by measurement. A physical quantity can be expressed as a ''value'', which is the algebraic multiplication of a ''nu ...
.
Conversion from DC to AC
Solar power needs to be converted from
direct current
Direct current (DC) is one-directional electric current, flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor (material), conductor such as a wire, but can also flow throug ...
(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 day, 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 some hours each day 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 The balance of system (BOS) encompasses all components of a photovoltaic system other than the photovoltaic panels.
This includes wiring, switches, a mounting system, one or many solar inverters, a battery bank and battery charger.
Other optio ...
(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, maximum effect or gross capacity,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 American 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 i ...
International Energy Agency
The International Energy Agency (IEA) is a Paris-based autonomous intergovernmental organization, established in 1974, that provides policy recommendations, analysis and data on the global energy sector. The 31 member countries and 13 associatio ...
( 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.
Power output in real conditions
The output of
photovoltaic system
A photovoltaic system, also called a 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 abso ...
s varies with the intensity of sunshine and other conditions. The more sun, the more power the
PV module
A solar panel is a device that converts sunlight into electricity by using photovoltaic (PV) cells. PV cells are made of materials that produce excited electrons when exposed to light. These electrons flow through a circuit and produce direct ...
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.