The nominal power is the nameplate capacity of photovoltaic (PV) devices, such as solar cells, panels and systems, and is determined by measuring the electric current and voltage in a circuit, while varying the resistance under precisely defined conditions. These Standard Test Conditions (STC) are 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 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 watts. 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, the ratio of the device’s electrical output to the incident energy.
The nominal power is important for designing an installation in order to correctly dimension its cabling and converters. If the available area is limited the solar cell efficiency and with it the nominal power per area (e.g. kW/m2) is also relevant. For comparing modules, the price per nominal power (e.g. $/W) is relevant. For a given installation’s physical orientation and location the expected annual production (e.g. kWh) per annual production assuming nominal power i.e. the capacity factor is important. With a projected capacity factor the price per projected annual production (e.g. $/kWh) can be estimated for a given installation. Finally, with a projected value of the production, the amortization of the cost of an installation can be estimated.
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, 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 (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.
Watt Peak refers to the electrical power delivered by solar modules under standard test conditions (STC) with the following parameters:
Cell temperature = 25 ° C
Irradiance = 1000 W / m²
Sunlight spectrum according to AM = 1.5.
The International Bureau of Weights and Measures, which maintains the SI-standard, states that the physical unit and its symbol should not be used to provide specific information about a given physical quantity 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.
Power output in real conditions
The output of photovoltaic systems varies with the intensity of sunshine and other conditions. The more sun, the more power the PV module 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), 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.
Conversion from DC to AC
Most countries refer to installed nominal nameplate capacity 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 SEIA, SPE or the IEA-PVPS.
However, in some places of the world, a system’s rated capacity is given after the power output has been converted to AC. These places include Canada, Japan (since 2012), Spain, and some parts of the United States. AC instead of DC is also given for most utility-scale PV power plants using CdTe-technology. The major difference lies in the small percentage (about 5%, according to the IEA-PVPS) of energy lost during the DC-AC conversion. In addition, some grid regulations may limit the output of a PV system to as little as 70% of its nominal DC power (Germany). In such cases, the difference between nominal peak-power and converted AC output can therefore amount to as much as 30%. 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, for example, 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 loss of 15 percent in the conversion from DC to AC is assumed. This can be extremely confusing not only to non-experts, as the conversion efficiency has been improving to nearly 98 percent, grid regulations may change, some manufactures may differ from the rest of the industry, and countries, such as Japan, may adopt a different metric from one year to the other.
Output power in real conditions
The output power of the photovoltaic system depends on the intensity of the solar radiation and other circumstances. More solar radiation means higher photovoltaic module performance. Losses may be due to the non-directional orientation of the module (tilt and / or orientation) by high temperature, poor module performance, dirt, and DC conversion to AC. It is important to know that the maximum power of the module can easily exceed the rated power anywhere where the light intensity is higher than 1000 W / m 2 (roughly equivalent to noon in summer Bavaria).
Although watt-peak is a convenient measure, and is the standardized number in the photovoltaic 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 on-going, is required to determine which panel may lead the owner to better financial results.
Words such as “The photovoltaic system has a capacity of 10 kWp ” or “This is a 1.2 MWp open area solar system” are colloquially. It would have to be formally correct “The photovoltaic system has a rated power of 10 kW” assuming the standard test conditions “or” This is a 1.2 MW free-field solar system (rated power under the assumption of the standard test conditions) “.
The statement ” Requires an area of approximately 6 to 10 m² per kW p ” means that for a desired system output of 1 kW under standard test conditions an area of approximately 6 to 10 m² is required.
Correspondingly, the notation “P nominal = 1 kW” for photovoltaic systems is preferable to the notation “P = 1 kW p “, since an addition of additions to unit symbols is not conforming to standards.
Practical relevance in Germany
The irradiance of 1000 W / m² is a transient value under real conditions. It is reached the more often the clearer the air is the closer one gets to the equator and the higher one is above sea level. It also depends on how close the sun is to the highest point. It is usually reached in Germany only in the noon hours of an unclouded day.
Measurements of the frequency of irradiation in Germany measured on a half-minute basis also show values above. These can also reach up to 1500 W / m² due to reflection and scattering. Due to the temporary short availability and the fact that inverters are usually designed for an irradiance of 1000 W / m and below (economic maximum), they are rarely used. The maximum of the irradiance at the edge of the earth’s atmosphere corresponds to the solar constant E 0 and is 1367 W / m².
In normal operation, solar modules or solar cells usually have a much higher operating temperature than the 25 ° C. provided in the test and therefore have up to 20% lower efficiency and a correspondingly lower actual power output, given an irradiation of 1 kW / m². A generally rigid alignment of a fixed photovoltaic system, the cells are rarely aligned precisely perpendicular to the incident light, whereby the irradiance is reduced by the cosine of the angle of incidence.
The indication in Watt Peak is used for the comparison of coextensive solar modules from different production in their efficiency and the dimensioning of the different components of a solar system. It can not be used as a sole indication for the characterization of a photovoltaic system, as it for the energy yield and for the economy of the system essential parameters such as the body (open space, roof, tracked) and the site, d. H. the degree of latitude and associated therewith the average irradiance, or the prevailing at the site climatic conditions such as the temperature are ignored.
In summary, for a photovoltaic system actually realized, the specification of the power in watts peak does not correspond to a maximum power or continuous power. Since the radiation conditions are often worse and the modules usually much warmer than under standard test conditions, the peak power is achieved in practice only sporadically and even more rarely exceeded.
Source from Wikipedia