Nominal Power in Photovoltaic

Nominal Power in Photovoltaic

Power rating is the rated capacity of photovoltaic (PV) devices, such as solar cells, panels, and systems. It is determined by measuring electrical current and voltage in a circuit while varying 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 intensity of light is 1000 W / m2, with a spectrum similar to that of sunlight striking the Earth’s surface at 35 ° N latitude in the summer (air mass 1.5), the temperature of the cells is 25 ° C. Power is measured by varying the resistive load on the module between an open circuit and a closed circuit (between the maximum and minimum resistance). The highest power so measured is the “nominal” power of the module in watts.

Nominal power is essential when designing an installation to size its cables and converters correctly. If the available area is limited, the solar cell’s efficiency and with it the nominal power per area (e.g. kW / m2) is also relevant. The price per little power (for example, $ / W) is appropriate to compare modules. For the location and physical orientation of a given facility, the expected annual production (for example, kWh) per annual production assuming nominal power, that is, the capacity factor is essential. You can estimate the projected yearly production price (for example, $ / kWh) for a given facility. Finally, with a projected value of production,


Peak power is not the same as the power under actual radiation conditions. This will be about 15-20% lower due to the considerable heating of the solar cells in practice. Also, in facilities where electricity is converted to alternating currents, such as solar power plants, the total electricity generating capacity is limited by the inverter, which is generally smaller than the system’s maximum capacity. Solar for economic reasons. Since full DC power is achieved only a few hours per year, using a smaller inverter saves money on the inverter while reducing (wasting) only a tiny portion of total energy production.


Watt Peak refers to the electrical power supplied by the 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 only source of information about an amount. . However, colloquial English sometimes combines quantity power and its unit using the watt-peak of the non-SI team and the non-SI symbol WP prefixed as within the SI, for example, kilowatt-peak (kWp), megawatts- peak (MWp), etc. As such a photovoltaic installation, it can be described, for example, as having a “one-kilowatt peak” in the sense of “one-kilowatt maximum power”. Similarly, outside of the SI, maximum power is sometimes written as “P = 1 kWp” instead of “Peak = 1 kW.”

Output power in actual conditions

The performance of photovoltaic systems varies with the intensity of sunlight and other conditions. The more sun, the more power the PV module will generate. Losses, compared to performance under optimal conditions, will occur due to non-ideal alignment of the module in inclination and azimuth, higher temperature, module power mismatch (since the panels in a system are connected in series, the lowest performance modulus defines the performance of the string to which it belongs), dirt and DC to AC conversion. The power that a module generates in natural conditions can exceed the nominal capacity when the intensity of sunlight exceeds 1000 W / m2 (which corresponds approximately to noon in summer, for example,

DC to AC conversion

Most countries refer to the nominal installed capacity of photovoltaic systems and panels by counting DC power in peak watts, denoted as WP or sometimes WDC, as most manufacturers do. And organizations of the photovoltaic industry, such as SEIA, SPE or IEA;

However, in some parts of the world, the system’s rating occurs after the power output has been converted to AC. These places include Canada, Japan (as of 2012), Spain, and some parts of the United States. In most utility-scale PV power plants using CdTe technology, AC is also provided instead of DC. The main difference lies in the small percentage (around 5%, according to IEA-PVPS) of the energy lost during the DC-AC conversion. Also, some grid regulations may limit a PV system’s output to as low as 70% of its nominal DC power (Germany). The difference between the rated maximum power and the converted AC output can be as high as 30% in such cases. Due to these two different metrics,

To clarify whether the nominal power output (“watt-peak”, Wp) is, in fact, DC or already converted to AC, it is sometimes explicitly denoted as, for example, MWDC and MWAC or kWDC and WAC. The converted WAC is also often written as “MW (AC)”, “MWac”, or “MWAC”. As for Wp, these units are not SI compliant but are widely used. In California, for example, where the nominal capacity is given in MWAC, a 15 per cent loss in DC to AC conversion is assumed. This can be highly confusing not only for non-experts, as conversion efficiency has improved to almost 98 per cent, but network regulations may also change; some manufacturers may differ from the rest of the industry and countries, such as Japan, may adopt a different metric from year to year.

Output power in actual conditions

The output power of the photovoltaic system depends on the intensity of solar radiation and other circumstances. More solar radiation means a higher performance of the photovoltaic module. Losses may be due to the module’s non-directional orientation (inclination and orientation) due to high temperature, poor module performance, dirt and DC to AC conversion. It is essential to know that the module’s maximum power can easily exceed the nominal capacity in any place where the light intensity is higher than 1000 W / m 2 (approximately equivalent to noon in the Bavarian summer).

Cost per watt

Although the watt-peak is a convenient measure and is the standardized number in the photovoltaic industry on which prices, sales and growth figures are based, it is arguably not the most critical number for performance. Real. Since a solar panel’s job is to generate electrical power at minimal cost, the amount of energy generated under real-life conditions relative to its price should be the most important number to evaluate. This “cost per watt” measure is widely used in the industry.

A brand panel and a brand B panel may give precisely the same watt value in laboratory tests, but their power output is different in an actual installation. Different degradation rates can cause this difference at higher temperatures. At the same time, although brand A may be less productive than brand B, it can also cost less; therefore, it has the potential to be financially advantageous. An alternative scenario may also be actual: a more expensive panel can produce so much more power than financially outperform a cheaper board. An accurate analysis of long-term performance versus cost, both upfront and ongoing, is required to determine which panel can lead the owner to better financial results.


Words like “The photovoltaic system has a capacity of 10 kWp” or “This is a 1.2 MWp open area solar system” are colloquial. It would have to be formally correct “The photovoltaic system has a nominal power of 10 kW”, assuming the standard test conditions “, or” This is a 1.2 MW free-field solar system (nominal power under the assumption of the common test conditions)”.

The statement “Requires an area of ​​approximately 6 to 10 m² per kWp” means that for the desired system output of 1 kW under standard test conditions, an area of ​​approximately 6 to 10 m² is required.

Consequently, the notation “nominal P = 1 kW” for photovoltaic systems is preferable to the notation “P = 1 kW p” since additions to unit symbols do not conform to the standards.

Practical relevance in Germany

The irradiance of 1000 W / m² is a transitory value under natural conditions. You get more often; the more precise the air, the further you get to the equator and the higher it 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 midday hours of a cloudless day.

The measurements of the irradiation frequency in Germany measured in half a minute also show the above values. These can even reach up to 1500 W / m² due to reflection and scattering. Due to the short time availability and the fact that the inverters are usually designed for irradiance of 1000 W / m or less (economic maximum), they are rarely used. The maximum irradiance at the edge of the Earth’s atmosphere corresponds to the solar constant E 0 and is 1367 W / m².

In typical operation, solar modules or solar cells typically have an operating temperature much higher than the 25 ° C provided in the test. Therefore, they have up to 20% lower efficiency and correspondingly lower actual power output, given irradiation of 1 kW / m². In a generally rigid alignment of a fixed photovoltaic system, cells are rarely precisely aligned perpendicular to the incident light. The irradiance is reduced by the cosine of the angle of incidence.

Watt Peak’s indication is used to compare coextensive solar modules of different productions in their efficiency and dimensioning of a solar system’s other components. It cannot be used as a sole indication for a photovoltaic system’s characterisation, since for the energy performance and the system’s economy, essential parameters such as the body (open space, roof, traced) and the site, d. H. The degree of latitude is ignored and associated with the average irradiance or the prevailing climatic conditions at the site, such as temperature.

In summary, for a photovoltaic system realized, the specification of the power in peak watts does not correspond to maximum capacity or continuous power. Since the radiation conditions are often worse and the modules usually much hotter than in standard test conditions, the total energy is reached in practice only sporadically and even more rarely exceeded.


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