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Photovoltaic Panel Efficiency and Performance

This page describes the major properties of a solar panel which are used to measure solar panel efficiency and solar panel performance. The data for each property is collected in or calculated from our solar panel database. All data in the database are from manufacturers' product datasheets, but we do not guarantee the accuracy.

A photovoltaic panel or a solar panel is an interconnected assembly of solar cells and is the basic component of a photovoltaic system. Photovoltaic panel consists of transparent front side, encapsulated solar cells and backside. It is framed with an aluminum frame, occasionally with a stainless steel or with a plastic frame. The front side material (superstrate) is usually low-iron, tempered glass. Most common backside materials (substrate) are EVA (ethylene-vinyl-acetate) and PVB (polyvinyl-burial). According to the solar cell technology popular photovoltaic panels are classified as monocrystalline, polycrystalline and amorphous solar panels, and the last one is also called thin-film panels.

Photovoltaic panel electrical performance depends on environmental conditions such as the temperature, solar irradiance, angle-of-incidence, solar spectral(air mass), and the types of PV cells. Each PV panel is rated under industrial Standard Test Conditions (STC) of solar irradiance of 1,000 W/m² with zero angle of incidence, solar spectrum of 1.5 air mass and 25°C cell temperature. Electrical characteristics from manufacturers include maximum rated power, open circuit voltage, short circuit current, maximum power voltage, maximum power current, and temperature coefficients.

Maximum Rated Power Pm (Watt): The maximum power output from a PV panel at STC which is usually labeled on the panel nameplate. The actual power output can be estimated by
Preal = Pm * S / 1000 * [1 - λ(Tcell - 25)]
Tcell = Tambient + S / 800 * (TNOCT - 20)

where S - the solar radiation on the panel surface, Tambient - the ambient temperature, TNOCT - the Nominal Operating Cell Temperature, and λ - Maximum Power Temperature Coefficient.

Rated Power Tolerance δ (%): The specified range within which a panel will either overperform or underperform its rated power Pm at STC. Power tolerance can vary greatly, from as much as +10% to -10%. A 200 watt panel with ±10% rated power tolerance may produce only 180 Watts or as much as 220 watts out of the box. To ensure expected power output, look for panels with a small negative (or positive only) power tolerance.

Panel Efficiency (%): The ratio of output power to input power from the sunlight, i.e., what percentage of light energy that hits the panel gets converted into electricity. The higher the efficiency value, the more electricity generated in a given space. You must be aware, however, that the solar cell efficiency doesn’t equal the panel efficiency. The panel efficiency is usually 1 to 3% lower than the solar cell efficiency due to glass reflection, frame shadowing, higher temperatures etc.

Fill Factor (%): The ratio of actual rated maximum power Pm to the theoretical (not actually obtainable) maximum power (Isc x Voc ). This is a key parameter in evaluating the performance of solar panels. Typical commercial solar panels have a fill factor > 0.70, while grade B solar panels have a fill factor range from 0.4 to 0.7. A higher fill factor solar panel has less losses due to the series and parallel resistances within the cells themselves.

Series Fuse Rating (Amps): Current rating of a series fuse used to protect a panel from overcurrent under fault conditions. Each panel is rated to withstand a certain number of amps. Too many amps flowing through the panel(perhaps backfed amps from paralleled panels or paralleled strings of panels) could damage the panel if it’s not protected by an overcurrent device rated at specification. Backfeeding from other strings is most likely to exist if one series string of panels stops producing power due to shading or a damaged circuit. Because PV panels are current-limited, there are some cases where series fusing may not be needed. When there is only one panel or string, there is nothing that can backfeed, and no series string fuse is needed. In the case of two series strings, if one string stops producing power and the other string backfeeds through it, no fuse is needed because each panel is designed to handle the current from one string. Some PV systems even allow for three strings or more with no series fuses. This is due to 690.9 Exception B of the NEC and is possible when the series fuse specification is substantially higher than the panel’s shortcircuit current (Isc). When required, series fuses are located in either a combiner box or in some grid-connected inverters.

Connector Type: Panel output terminal or cable/connector configuration. Most panels come with "plug and play" weatherproofed connectors to reduce installation time in the field. Connectors such as Solarlok (manufactured by Tyco Electronics), and MC and MC4 (manufactured by Multi-Contact USA) are lockable connectors that require a tool for opening. Because so many PV systems installed today operate at high DC voltages, lockable connectors are being used on panels in accessible locations to prevent untrained persons from "unplugging" the paneles, per 2008 NEC Article 690.33(C). Due to this new code requirement, most PV manufacturers are updating their connectors to the locking type. Depending on how fast this change is reflected in the supply chain, connectors on a particular panel may be an older version.

Materials Warranty (Years): A limited warranty on panel materials and quality under normal application, installation, use, and service conditions. Material warranties vary from 1 to 10 years. Most manufacturers offer full replacement or free servicing of a defective panel.

Power Warranty (Years): A limited warranty for panel power output based on the minimum peak power rating (STC rating minus power tolerance percentage) of a given panel. The manufacturer guarantees that the panel will provide a certain level of power for a period of time (at least 20 years). Most warranties are structured as a percentage of minimum peak power output within two different time frames: (1) 90% over the first 10 years and (2) 80% for the next 10 years. Panel replacement value provided by most power warranties is generally prorated according to how long the panel has been in the field.

Cell Type: The type of silicon that comprises a specific cell, based on the cell manufacturing process. Each cell type has pros and cons. Monocrystalline PV cells are the most expensive and energy intensive to produce but usually yield the highest efficiencies. Though polycrystalline and ribbon silicon cells are slightly less energy intensive and less expensive to produce, these cells are slightly less efficient than monocrystalline cells. However, because both poly- and ribbon silicon panels leave fewer gaps on the panel surface (due to square or rectangular cell shapes), these panels can often offer about the same power density as monocrystalline modules. Thin-film panels, such as those made from amorphous silicon cells, are the least expensive to produce and require the least amount of energy and raw materials, but are the least efficient of the cell types. They require about twice as much space to produce the same power as mono-, poly-, or ribbon-silicon panels. Thin-film panels do have better shade tolerance and high-temperature performance but are often more expensive to install because of their lower power density.
Sanyo’s "bifacial" HIT panels are composed of a monocrystalline cell and a thin layer of amorphous silicon material. In addition to generating power from the direct rays of the sun on the panel face, this hybrid panel can produce power from reflected light on its underside, increasing overall panel efficiency.

Cells in Series: Number of individual PV cells wired in series, which determines the panel design voltage. Crystalline PV cells operate at about 0.5V. When cells are wired in series, the voltage of each cell is additive. For example, a panel that has 36 cells in series has a maximum power voltage (Vmp) of about 18V. Why 36? Historically, panels known as 12V were designed to push power into 12V batteries. But to deliver the 12V, they needed to have enough excess voltage (electrical pressure) to compensate for the voltage loss due to high temperature conditions. Panels with 36 ("12V") or 72 ("24V") cells are designed for battery-charging applications.
Panels with other numbers of cells in series are intended for use in grid-tied systems. Due to the increased availability of step-down/MPPT battery charge controllers, grid-tied panels can also be used for battery charging, as long as they stay within the voltage limitations of the charge controller.

Maximum Power Voltage Vmp: The voltage where a panel outputs the maximum power. Grid-tied inverters and MPPT charge controllers are built to track maximum power point throughout the day, and Vmp of each panel array, as well as array operating temperatures must be considered when sizing an array to a particular inverter or controller. Series string sizing software programs for grid-tied inverters allow you to input both the high and low temperatures at your installation site, and calculate the correct number of panels in series to maximize system performance.

Maximum Power Current Imp: The maximum amperage where a panel outputs the maximum power. This specification is most commonly used in calculations for PV array disconnect labeling required by NEC Article 690.53(1), as the rated maximum power-point current for the array must be listed. Maximum power current is also used in array and charge controller sizing calculations for battery-based PV systems.

Open-Circuit Voltage Voc: The maximum voltage generated by a PV panel exposed to sunlight with no load connected. All major PV system components (panels, wiring, inverters, charge controllers, etc.) are rated to handle a maximum voltage. Maximum system voltage must be calculated in the design process to ensure all components are designed to handle the highest voltage that may be present. Under certain low-light conditions (dawn/dusk), it’s possible for a PV system to operate close to open-circuit voltage. PV voltage will increase with decreasing air temperature, so Voc is used in conjunction with historic low temperature data to calculate the absolute highest maximum system voltage. Maximum system voltage must be shown on the PV array disconnect label required by NEC code.

Short-Circuit Current Isc: The maximum amperage generated by a PV panel exposed to sunlight with the output terminals shorted. The PV circuit's wire size and overcurrent protection (fuses and circuit breakers) calculations per NEC Article 690.8 are based on panel short-circuit current. The PV system disconnect(s) must list short-circuit current (per NEC 690.53).

Short-Circuit Current Temperature Coefficient α (%/°C): The change in panel short-circuit current per degree Celsius at temperatures other than 25°C. It is most commonly used to calculate maximum system current (per NEC Article 690.7) for system design and labeling purposes. For example, consider a series string of ten 8A (Isc) panels installed at a site with a record low of 15°C. Given a Isc temperature coefficient 0.04%/°C), the decrease in current will be 0.32A, making for an overall maximum system current of 7.68A.

Open-Circuit Voltage Temperature Coefficient β (%/°C): The change in panel open-circuit voltage at temperatures other than 25°C. If given, It is most commonly used to calculate maximum system voltage (per NEC Article 690.7) for system design and labeling purposes. For example, consider a series string of ten 43.6V (Voc) panels installed at a site with a record low of -10°C. Given a Voc temperature coefficient of -160mV/°C, The voltage per panel will rise 5,600mV (= 160mV x (-10°C – 25°C)), making for an overall maximum system voltage of 492V (= 10 x (5.6V + 43.6V)), which is under the 600VDC limit for PV system equipment.

Maximum Power Temperature Coefficient δ(%/°C): The change in panel output power for temperatures other than 25°C. It is used to calculate how much panel power will be lost or gained due to temperature changes. In hot climates, cell temperatures can reach an excess of 70°C (158°F). Consider a panel maximum power rating of 200W at STC, with a temperature coefficient of -0.5%/°C. At 70°C, the actual output of this panel would be approximately 155W. Panels with lower power temperature coefficients will fare better in higher-temperature conditions. Thin-film panels have relatively low temperature coefficients which reflects better high-temperature performance.

Nominal Operating Cell Temperature: The temperature of each panel at an irradiance of 800 W/m2 and an ambient air temperature of 20°C and wind speed is 1 m/s at a module tilt angle 45°C. NOCT is a very critical parameter that is required by various performance, qualification and energy rating standards/methods. It can be used with the maximum power temperature coefficient to get a better real-world estimate of power loss due to temperature increase. The cell temperature of open-rack panels , however, is governed by several external factors such as ambient temperature, irradiance level, wind speed, wind direction, and tilt-angle of the panel in an array. The difference in cell temperature and ambient temperature is dependent on sunlight’s intensity (W/m2). For example, if a particular panel has an NOCT of 40°C and a maximum power temperature coefficient of -0.5%/°C, power losses on temperature can be estimated at about 7.5%(=0.5% x (40°C – 25°C)).

Air mass has an effect on power output. Air mass is the optical path length relative to that at the zenith at sea level. So by definition, the sea-level airmass at the zenith is 1. Airmass increases as the angle between the source and the zenith increases, reaching a value of approximately 38 at the horizon. AM = (cos(z) + 0.5 x (96 - z)-1.6364)-1 x P/Psea-level (P/Psea-level = exp(-0.1184 x h)) where z is the zenith angle of the sun in degree, P is atmospheric pressure, and h is the site altitude in kilometer. Air mass has a much greater effect on the triplejunction amorphous modules than mono or polycrystalline modules.

The most informative measure of performance is panel efficiency, or what percentage of light energy that hits the panel gets converted into electricity. You must be aware, however, that the solar cell efficiency doesn’t equal the panel efficiency. The panel efficiency is usually 1 to 3% lower than the solar cell efficiency due to glass reflection, frame shadowing, higher temperatures etc. That is why some manufacturers are more happy to tell you their cell efficiencies instead of panel efficiencies.

The second performance measurement is the power tolerance which indicates the rated power range that the manufacturer can guarantee. For example, a -10% lower end tolerance means that the actual peak power could be 10% lower than the plate rated power.

The third important performance measure is the temperature coefficients which show how the panel outputs will follow the temperature changes. Three most common temperature coefficients are usually available in panel data sheets, and should be considered as important parameters in design stage of PV systems.

Solar panels must withstand heat, cold, rain and hail for many years. Many Crystalline silicon module manufacturers offer warranties that guarantee electrical production for 10 years at 90% of rated power output and 25 years at 80%

Panel certification: Panel certifications are required to get the approval for federal and state rebates in USA. Every Market region has specific sets of standards which must be met by solar panels. Most popular certification standards are

  • IEC 61215 (crystalline silicon performance), IEC 61646 (thin film performance), IEC 61730 ((crystalline modules, safety), IEC 62108(concentrating PV performance), IEC 61701 (salt resistance) ) for Europe
  • UL 1703, UL 8703 (CPV) for USA and Canada
  • CE mark (European Union, Iceland, Liechtenstein, Norway, Switzerland, Turkey)
  • TÜV or VDE certificates indicate the panels have passed the testing of IEC standards, while UL certificate implies the UL 1703 testing
  • IEC standard allows 1000 volt maximum system voltage, while UL allows 600 volt only. The maximal system voltage limits how many panels can be cascaded in one single string. For example, given panels with 40V of Voc, 25 panels can be cascaded in one series string in Europe, but only 15 panels are allowed to do so in USA and Canada.
  • Beside the common certifications, some countries and regions have extra requirements. Some USA states require PTC rating of California CEC, UK requires its MCS certification, while Australia requires panels have to meet Application Class A, or Class C of IEC 61730.

Flash Report: Most manufacturers provide flash reports of their solar panels sold, including every single panel's flash test data. During a flash test, a solar panel is exposed to a short (1 - 30 millisecond), bright (1 watt per M2) flash of xenon light source. The spectrum of the flash light is designed to be close to the spectrum of the sun. The output is collected by a testing computer and the data is compared to a pre-configurated reference solar panel which has its power output calibrated to standard solar irradiation. The results of the flash test are compared to the specifications of the pv module datasheet and are printed somewhere on the pv panel. The flash testing system is usually re-corrected by the reference panel in certain interval (usually two hours). The data in a flash report includes the pv panel barcode, Pmax, Voc, Isc, Im and Vm. Your supplier should be given these data before you hit final buying trigger or after you sign the purchase contract.

Common Solar Panel Defects: The following defects are common during solar panel quality testing:

  • Scratches on the frame and/or glass
  • Excessive or uneven glue marks on glass or frame
  • Gap between frame and glass due to poor sealing
  • Always lower output than stated in data sheet
  • Always lower fill factor than indirectly stated in data sheet
  • Inconsistant cell colors
  • Inconsistant cell alignments
  • Undurable panel label printing

Solar Panel Grading: Based on the types and degrees of above defects, solar panel grading comes to play. Grade A normally means a panel has no above defect and is covered by manufacturer's standard warranty, while you may not be able to find "Grade A" in manufacturer's documents at all. Grade B usually means the panel has some "cosmetic imparfections" or "cosmetic blemishes" of the above, but has the "same" electrical output as Grade A. Grade B is rarely covered by manufacturer's standard warranty and is usually traded underneath the market. If the nameplate of your panel has word "Grade x" or the like, you are alerted to check with the manufacturer what it means.

Please note that these pages are meant strictly for informational purposes to those seeking retailers/wholesalers/manufacturers/providers of or information about renewable energy products and/or services. Inclusion or exclusion from these pages does not imply a recommendation or lack thereof by Posharp and/or its collaborator(s) for any particular company or organization. The information are collected from the Internet or input by companies.