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
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
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
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
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
panels have relatively low temperature coefficients which reflects better
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
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)).
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.