Solar Toolbox – Solar Electric Terms

AC – Alternating Current, the standard form of electrical current supplied by the utility grid and by most fuel-powered generators. The polarity (and therefore the direction of current) alternates.

Active Materials – In the positive plates of batteries, the active material is lead dioxide. In the negative, it’s metallic sponge lead. When a circuit is created, these materials react with sulfuric acid during charging and discharging.

Ampere (Amp) – A unit of measurement for the electron flow or current through a circuit.

Ampere-Hour (Amp. Hr., AH) – A unit of measure for a battery’s electrical storage capacity, calculated by multiplying the current in amperes by the time in hours. (Example: A battery which delivers 5 amps for 20 hours provides 5 amps x 20 hours = 100 AH of capacity.)

Anode – The positive electrode in an electrochemical cell (battery). Also, the earth ground in a cathodic protection system. Also, the positive terminal of a diode.

Battery – A device that converts the chemical energy contained in its active materials directly into electrical energy by means of an electrochemical oxidation-reduction (redox) reaction.

Battery Capacity – The total number of ampere-hours that can be withdrawn from a fully charged battery.

Bonding – is simply a matter of taking all of the electrical and metallic masses in a facility and connecting them with conductors, bringing them to the same electrical potential. The primary reason for bonding is personnel safety, so someone touching two pieces of equipment at the same time does not receive a shock by becoming the path of equalization if they happen to be at different potentials.

Capacity Rating – The time in minutes that a new, fully-charged battery will deliver 25 amperes or 75 amperes at 80°F and maintain a terminal voltage equal to or greater than 1.75 volts per cell.

Cathode – The negative electrode in an electrochemical cell. Also, the negative terminal of a diode.

CEC – Canadian Electrical Code.

Cell – The basic current-producing unit in a battery. In consists of a set of positive plates, casing. A cell’s nominal voltage is 2 volts. (Example: A 12-volt battery has 6 cells.)

Cycle – One discharge of a battery plus one recharge.

Charge Controller – A device that controls the charging rate and/or state of charge for batteries.

Circuit – The path followed by a flow of electrons. A closed, or short, circuit is a complete path. An open circuit has a broken path.

Current – The rate at which electricity flows through a circuit, to transfer energy. Measured in Amperes, commonly called Amps.

Cutoff Voltage – The voltage levels (activation) at which the charge controller disconnects the array from the battery or the load from the battery.

Cycle – The discharge and subsequent charge of a battery.

DC – Direct Current, the type of power produced by photovoltaic panels and by storage batteries. The current flows in one direction and polarity is fixed, defined as positive (+) and negative (-). Nominal system voltage may be anywhere from 12 to 180V. See voltage, nominal.

Deep Cycle – Type of battery that can be discharged to a large fraction of capacity many times without damaging the battery.

Depth of Discharge (DOD) – The percentage of capacity actually removed from a battery compared to the total rated capacity.

Direct Current – Direct Current, the type of power produced by photovoltaic panels and by storage batteries. The current flows in one direction and polarity is fixed, defined as positive (+) and negative (-). Nominal system voltage may be anywhere from 12 to 180V.

Electrolyte – In a deep cycle battery, it is a dilute solution of sulfuric acid and water.

Equalization – An overcharge performed on flooded lead-acid batteries after they have been fully charged. This maintenance step helps eliminate stratification and sulfation.

Energy – The product of power and time, measured in Watt-Hours. 1000 Watt-Hours = 1 Kilowatt-Hour (abbreviation: KWH). Variation: the product of current and time is Ampere-Hours, also called Amp-Hours (abbreviation: AH). 1000 watt consumed for 1 hour = 1 KWH. See power.

Fixed Tilt Array – A PV array set at a fixed angle. Typical roof mount or ground rack mounted arrays are fixed. Fixed angle arrays tend to be tilted at anywhere from latitude minus 15 degrees to latitude plus 10 degrees.

Gassing – Gas by-products, primarily hydrogen, produced when charging a battery. Also, termed out-gassing.

Gel Type Battery – Lead-acid battery in which the electrolyte is composed of a silica gel matrix.

Grounding – is a matter of bringing the bonded equipment mass to the potential of the surface of the earth which it occupies. The purpose of grounding and bonding is to protect you from the danger of electric shock. Grounding and bonding achieves this by ensuring that when an electric fault occurs, the potential of the metal parts of enclosures will be maintained at a safe value with respect to earth and/or the fault will cause the system to be disconnected by a protective device.

Hydometer – A tool used to measure the specific gravity of the electrolyte solution.

Imp or Maximum Power Current – The maximum amount of current (I) output available at a solar module maximum power point (see MPP).

Insolation – The solar radiation incident on an area over time. Equivalent to energy and usually expressed in kilowatt-hours per square meter.

Inverter – An electronic device that converts DC voltage to AC voltage. In battery-based solar-electric systems, an inverter may take the 12, 24, or 48 volts DC and convert it to conventional household AC power.

Irradiance – The solar power incident on a surface. Usually expressed in kilowatts per square meter. Irradiance multiplied by time equals Insolation.

I-U Characteristics – The plot of the current versus voltage characteristics of a photovoltaic cell, module, or array. Three important points on the I-V curve are the open-circuit voltage, short-circuit current, and peak power operating point.

Isc or Short Circuit Current – The current (I) produced by an illuminated PV cell, module or array when its output terminals are shorted.

Junction Box – A PV generator junction box is an enclosure on the module where PV strings are electrically connected and where protection devices can be located, if necessary.

Low Voltage Cutoff – The voltage level at which a controller will disconnect the load from the battery.

Maximum Power Current or Imp – The maximum amount of current (I) output available at a solar module maximum power point (see MPP).

Maximum Power Voltage or Vmp – The maximum amount of voltage (V) present at a solar module maximum power point (see MPP).

MPP – Maximum Power Point; The point on the current-voltage (I-V) curve of a module under illumination, where the product of current and voltage is maximum. For a typical silicon cell, this is at about 0.45 V.

N-Type Silicon – Silicon material that has been doped with a material that has more electrons in its atomic structure than does silicon.

NOCT – Nominal Operating Cell temperature; The estimated temperature of a PV module when operating under 800 W/m2 irradiance, 20°C ambient temperature and wind speed of 1 meter per second. NOCT is used to estimate the nominal operating temperature of a module in its working environment.

Nominal Voltage – A reference voltage used to describe batteries, modules, or systems.

Ohm – A unit of measurement for electrical resistence within a circuit.

One Axis Tracking – A system capable of rotating about one axis.  Solar “Tracker” Mounting Systems hold solar panels at an optimal angle for energy production and adjust that angle throughout the day.  One Axis Trackers can adjust the solar panels along one axis only (east to west tracking typical).

Open Circuit Voltage or Voc – The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases. Or, The voltage of a battery when there is no load attached (not receiving or delivering energy). This measurement is best taken when the battery has been at rest for at least 6 hours.

Overcharge – Forcing current into a fully charged battery. The battery will be damaged if overcharged for a long period.

Peak Sun Hours – The equivalent number of hours per day when solar irradiance averages 1,000 w/m2. For example, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the irradiance for six hours been 1,000 W/m2.

Peak Watt (Wp) – The amount of power a photovoltaic module will produce at standard test conditions (normally 1000 W/m2 and 25° cell temperature).

Photovoltaic – The phenomenon of converting light to electric power. Photo = light, Volt = electricity. Abbreviation: PV. PV – The common abbreviation for photovoltaic.

Polycrystalline Silicon – A material used to make PV cells which consist of many crystals as contrasted with single crystal silicon.

Power – The rate at which work is done. It is the product of Voltage times Current, measured in Watts. 1000 Watts = 1 Kilowatt.

Power Factor – The cosine of the phase angle between the voltage and the current waveforms in an ac circuit. Used as a designator for inverter performance. A power factor of 1 indicates current and voltage are in phase and power is equal to the product of volt-amperes. (no reactive power).

PV Array – A group of PV (photovoltaic) modules (also called panels) arranged to produce the voltage and power desired.

PV Array-Direct – The use of electric power directly from a photovoltaic array, without storage batteries to store or stabilize it. Most solar water pumps work this way, utilizing a tank to store water.

PV Cell – The individual photovoltaic device. The most common PV modules are made with 33 to 36 silicon cells each producing 1/2 volt.

PV Module – An assembly of PV cells framed into a weatherproof unit. Commonly called a “PV panel”. The smallest replaceable unit in a PV array. An integral, encapsulated unit containing a number of PV cells. See PV array.

Resistance – The property of a conductor which opposes the flow of an electric current resulting in the generation of heat in the conducting material. The measure of the resistance of a given conductor is the electromotive force needed for a unit current flow. The unit of resistance is ohms.

Separator – A divider made of porous material that is placed between the positive and negative plates in a battery cell and allows current to flow through it, while preventing direct contact between the plates which would cause a short circuit.

Short Circuit Current or Isc – The current produced by an illuminated PV cell, module, or array when its output terminals are shorted.

Solar Tracker – A mounting rack for a PV array that automatically tilts to follow the daily path of the sun through the sky. A “tracking array” will produce more energy through the course of the day, than a “fixed array” (non-tracking) particularly during the long days of summer.

Specific Gravity (S.G.) – A measurement of the strength of battery electrolyte by comparing its density to that of pure water.

Standard Test Conditions – Conditions under which a module is typically tested in a laboratory: (1) Irradiance intensity of 1000 W/m2, AM1.5 solar reference spectrum, and (2) a cell (module) temperature of 25°C, plus or minus 2°C.

Stratification – A condition where the concentration of acid is greater at the bottom of the battery than at the top.

String – A number of modules or panels interconnected electrically in series to produce the operating voltage required by the load.

Sulfation – The formation of lead sulfate on the positive and negative electrodes.

Tracking Array – A PV array that follows the path of the sun. This can mean one-axis, east to west daily tracking, or two-axis tracking where the array follows the sun in azimuth and elevation.

Trickle Charge – A small charge current intended to maintain a battery in a fully charged condition.

Two Axis Tracking – A system capable of rotating independently about two axes (e.g., vertical and horizontal).

Utility Grid – Commercial electric power distribution system. Synonym: mains.

Vented Cell – A battery designed with a vent mechanism to expel gases generated during charging.

Vmp or Maximum Power Voltage – The maximum amount of voltage (V) present at a solar module maximum power point (see MPP). The voltage at which a photovoltaic module or array transfers the greatest amount of power (watts). Common Abbreviations: Vpp, Vmp.

Voc or Open Circuit Voltage – The maximum voltage produced by an illuminated photovoltaic cell, module, or array with no load connected. This value will increase as the temperature of the PV material decreases.

Volt – The unit of electromotive force that will force a current of one ampere through a resistance of one ohm.

Voltage Drop – Loss of voltage (electrical pressure) caused by the resistance in wire and electrical devices. Proper wire sizing will minimize voltage drop, particularly over long distances. Voltage drop is determined by 4 factors: wire size, current (amps), voltage, and length of wire. It is expressed as a percentage.

Voltage, Nominal – A way of naming a range of voltage to a standard. Example: A “12 Volt Nominal” system may operate in the range of 11 to 15 Volts. We call it “12 Volts” for simplicity.

Watt – The unit of electrical power. The power developed when a current of one ampere flows through a potential difference of one volt.

Watt Hour (Wh) – A unit of energy equal to one watt of power connected for one hour.

Waveform – The characteristic shape of an AC current or voltage output.