Key Photovoltaic Terminology

 PV Module: 

A module is a solar panel.  Each module is made up of solar cells of crystalline-silicon.  The back is glass and the sides are aluminum.  The term ‘solar panel’ was originally used to describe a collection of modules placed on a racking system and pre-wired, which was then shipped to the job site.  The term has stuck and the words ‘module’ and ‘panel’ are now used interchangeably, however, it is more correct to say ‘module’ when talking about a single solar panel and an ‘array’ as a collection of modules.

 Standard Test Conditions (STC):

STC is defined as 1,000W/m2 @ 25C (77F) @ 1.5 Am.  This is the laboratory testing of each module, which gives an output rating of Voltage Open Current (Voc) and Current Short Circuit (Isc).  This rating system does not include any derate values, therefore, it does not reflect the actual output of the PV array.  Other rating systems, such as the CEC (California Energy Commission) and PTC (PV USA Test Conditions) provide more accurate output ratings based on more realistic conditions during the test.

 Balance of System (BOS) Components: 

BOS components refer to the hardware used to complete the installation of a solar energy system.  BOS does not refer to the modules or the inverter, but rather the smaller pieces used for construction, such as waterproof flashings for attaching the base of the rack to the roof rafters and specialized bolts.

 PV String & PV Array: 

A string is a group of modules wired together.  An array is one or more strings wired together that create an entire system.  Most residential solar systems are comprised of 2 strings of solar modules wired in series, which are then connected in parallel, which creates a high voltage and low amperage PV array.

 Temperature Coefficient: 

Solar modules put out more power when the temperature is lower.  PV systems must be designed to account for the maximum possible voltage, thus, we find the temperature coefficient on a chart that associates to the coldest temperature ever recorded at the site location (i.e. the maximum possible voltage that an array could produce while receiving a peak sun hour on a very cold day).  This number is used in equations to size PV arrays.

An Astronomical Unit (AU) is 93 million miles, or the distance between the Earth and the Sun.  The speed of light is 186,000 miles/second making solar radiation take around 8.3 minutes to reach Earth.


I-V Curve:

The Max Power Point Tracker (MPPT) continuously tracks the amperage (I) and voltage (V) intake from the system.  The MPPT balances the volts and amps to create the highest output Wattage in real time.  The I-V curve shows the volts and amps on an XY axis graph.

 Solar Irradiance:

Solar radiation power per unit area, or watts per meter squared (W/m2).

 Solar Irradiation: 

Solar radiation power per unit area per amount of time, or watt-hours per meter squared (Wh/m2).

For example:  600 W/m2 for 8 hours equals 4,800Wh/m2 or 4.8kWh/m2.

 Solar Constant:

Irradiance value for Earth @ 1,366W/m2.  This could also be understood as how powerful photons from our Sun are at a distance of 1 Astronomical Unit.

 Air Mass:

Distance of atmosphere that sunlight travels through.  Air Mass is equal to 1 when the sun is directly overhead at sea level.  Air mass is effected by the time and day of year, as well as by the latitude, longitude and elevation of a specific location.

 Peak Sun:

The solar irradiance that reaches the Earth’s surface.  This is equal to 2/3 the solar constant @ 1,000W/m2.

 Peak Sun Hours:

Amount of hours of peak sun per day that a specific location receives.  Santa Cruz CA is at a latitude of 36 degrees North and receives around 5 peak sun hours per day.  This is a key element to the design of PV systems.


Killowatt hours per meter squared per day.  (kWh/m2/day)

 Solar Declination:

Plus or minus 23.5 degrees.  This is also the tilt of the Earth and is constituted by the seasonal shift in tilt as we orbit the sun annually.  Summer solstice has a +23.5 solar declination, the equinoxes have a declination of 0, and the Winter solstice has a solar declination of -23.5 degrees.  In addition, 23.5 degrees North creates the Tropic of Cancer, whereas 23.5 degrees South creates the Tropic of Capricorn.

 Solar Time:

A time scale based upon the apparent motion of the sun through the sky.  15 degrees longitude is one solar hour.  Thus, plus or minus 15 degrees of longitude constitutes 60 minutes of solar time, or, changes a ‘time zone.’  Greenwhich, England is the prime meridian.

 Solar Altitude Angle:

The angle between the observer, the horizon and the sun.  This is between 0 and 90 degrees.  This is basic trigonometry.

 Solar Azimuth Angle:

The angle of the sun traveling horizontally from East to West.  S=0; W= -90; E=90; N= +/- 180

 Solar Window:

Area of sky between Summer and Winter solstices.  This is critical for photovoltaic array orientation and optimization.


Efficiency, in terms of solar energy, is the conversion of photons to electrons.

 Crystalline Silicon:

(CSi) Mined from quartz and silica sands.  This is the raw material for making solar modules and was used to make microprocessors for computers in California in the 1980’s and 90’s creating the name ‘Silicon Valley.’


About danblake222

*MSc "Integrative Ecosocial Design" with Gaia University *BA "Ethnography & Photography" with Minor in Spanish Linguistics from UCSC *Artist in Residence at Omega Institute *Handyman, House Painter, Writer, Photographer, Permaculture Designer, Gardener *Photography portfolio at

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