Nominal power (photovoltaic): Difference between revisions

The '''nominal power''' is the [[nameplate capacity]] of [[photovoltaic]] (PV) devices, such as [[solar cell]]s, [[solar module|panels]] and [[PV system|systems]], and is determined by measuring the [[electric current]] and [[voltage]] in a [[electric circuit|circuit]], while varying the [[Electrical resistance|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 [[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 [[Solar cell efficiency|efficiency]], the ratio of the device's electrical output to the incident energy.

However, in some places of the world, a system's rated capacity is given after the power output has been converted to [[Alternating current|AC]], being the output to the grid. 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 [[Cadmium telluride photovoltaics|CdTe]]-technology.{{Citation needed|date=January 2020}}

Some grid regulations may limit the AC output of a PV system to as little as 70% of its nominal DC peak 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.<ref name="iea-pvps-snapshot-1992-2014">

In order to clarify whether the nominal power output (''"watt-peak"'', W_p) is in fact DC or already converted into AC, it is sometimes explicitly denoted as, for example, MW_DC and MW_AC or kW_DC and kW_AC. The converted W_AC is also often written as "MW (AC)", "MWac" or "MWAC". Just as for W_p, these units are non [[SI]]-compliant but widely used. In California, for example, where the rated capacity is given in MW_AC, a downrating of 15 percent in the conversion from DC to AC is assumed.<ref name="Gipe-DC-AC-conversion">{{cite web|last1=Gipe|first1=Paul|title=Solar PV DC Conversion Factor for AC kW|url=http://www.wind-works.org/cms/index.php?id=38&tx_ttnews%5Btt_news%5D=208&cHash=bebbcb4dacd986e38d771a9bd496bdfb|websitetitle=https://www.webcitation.orgSolar PV DC Conversion Factor for AC kW|publisherlast1=Wind-worksGipe|accessdatefirst1=4Paul|date=20 SeptemberNovember 20142009|website=Wind Works|archiveurl=https://www.webcitation.org/6SLA3c7xv?url=http://www.wind-works.org/cms/index.php?id=38&tx_ttnews%5Btt_news%5D=208&cHash=bebbcb4dacd986e38d771a9bd496bdfb|archivedate=4 September 2014|dateaccessdate=2023 NovemberJanuary 2009|url-status=dead2020}}</ref> This can be extremely confusing not only to non-experts, as the conversion efficiency has been improving to nearly 98 percent,<ref>{{cite web|title=Recent Facts about Photovoltaics in Germany|url=http://www.ise.fraunhofer.de/en/publications/veroeffentlichungen-pdf-dateien-en/studien-und-konzeptpapiere/recent-facts-about-photovoltaics-in-germany.pdf|website=ise.fraunhofer.de/en|accessdate=4 September 2014|archiveurl=https://www.webcitation.org/6SLCNRPTc?url=http://www.ise.fraunhofer.de/en/publications/veroeffentlichungen-pdf-dateien-en/studien-und-konzeptpapiere/recent-facts-about-photovoltaics-in-germany.pdf|archivedate=4 September 2014|page=44|date=28 July 2014|url-status=dead}}</ref> grid regulations may change, some manufacturesmanufacturers may differ from the rest of the industry, and countries, such as Japan, may adopt a different metric from one year to the other.

== Cost- per- watt ==

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|cost per watt]]" measure is widely used in the industry.

It can happen that a panel from brand&nbsp;A and a panel of brand&nbsp;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&nbsp;A can be less productive than brand&nbsp;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-goingongoing, is required to determine which panel may lead the owner to better financial results.