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AGM (Absorbed Glass Mat) battery that is maintenance free and holds acid in a fiberglass mat to prevent it from spilling even if the outer battery case is broken. Typically much more expensive and less cycle life than flooded acid batteries. Mats can be assembled in flat layers or in a spiral design (Optima brand).

Absorption Charge: Second stage of three stage battery charging in which voltage is held for a set time period at the bulk voltage setting, while amperage is gradually reduced as the batteries come up to charge. (See Bulk Charge, Float Charge)

AC: (See Alternating Current)

AIC: Amps Interrupt Capacity: DC fuses should be rated with a sufficient AIC to interrupt the highest current possible in the circuit. See Arc for what happens when the breaker or fuse isn't rated high enough.

Air Mass: The air mass is an indication of the length of the path solar radiation travels through the atmosphere. An air mass of 1.0 means the sun is directly overhead and the radiation travels through one atmosphere (thickness). High Altitude locations have less Air Mass and so higher irradiation levels.

Alternating Current (AC): An electric current that alternates periodically. Household electricity is of this type.

Ambient Temperature: The temperature of the surrounding area.

Amorphous Silicon: Thin film PV silicon having no crystalline structure. Manufactured by depositing layers of doped silicon on a substrate. Has inferior performance to, and shorter life span than, Multi- and Mono-crystalline silicon.

Amorphous Panel: PV module made with amorphous silicon cells.

Amperage (I): The measure of electrical flow. The amount of "current" (the number of amps) drawn by an electrical device. Rate that electrons pass a point in the circuit per second

Ampere (Amp, A): Unit of electrical current.

Amp-Hour (Ah): A measure of electrical energy equal to one amp of current for one hour. Used to measure the energy stored in a battery.

Angle of Incidence: The angle that sunlight striking a solar module makes with a line perpendicular to its surface.

Anode: The earth ground in a cathodic protection system.

Array: (See PV Array)

Array Current: The electrical current produced by a PV array when it is exposed to sunlight.

Array Operating Voltage: The voltage produced by a PV array when exposed to sunlight and connected to a load.

Arc:electricity conducting through the air itself. Used in welding metals, but also caused by loose connections or conductors too close to each other and/ or other materials. 12 volt DC can arc a 1/4", while 100,000 volts (Lightning) can arc thousands of feet. Within the arc, air turns into a plasma that has much lower resistance than normal.
Drawing an Arc, starts by making full contact between electrical connectors, and while current is flowing slowly pull the connectors apart. The arc can burn these contact points badly if sustained for more than a split second.

AWG: (American Wire Gauge), standard for wire sizing in the United States. Formerly known as Brown & Sharp. Wires are sized from #40 (smallest) to #0000 (4/0) largest. See wire & connectors guide for comparison table.

Azimuth: Horizontal angle measured clockwise from true north; 180º is true south.

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Balance of System (BOS): the additional components besides the batteries, and Solar modules, that are necessary to safely complete a Power system. (I.e. fuses, disconnects, meters, junction boxes, charge controllers, etc.)

Base Load: The average amount of electric power that a utility must supply to the grid at any given time.

Battery: A device that stores electrical energy. Usually, through a electrochemical reaction, it produces electricity. A rechargeable battery can reverse the chemical reaction to store electrical energy as chemical energy.

Battery Bank: A group of batteries wired together in series and/or parallel to form a unified electrical energy storage unit.

Battery Capacity: The total number of ampere-hours that can be withdrawn from a fully charged battery. (See Amp-Hour & Rated Battery Capacity)

Battery Cell: The smallest unit or section of a battery that can store electrical energy and is capable of furnishing a current to an external load. For lead-acid batteries the voltage of a cell (fully charged) is about 2.2 volts dc. ( for example, a 12 volt battery is actually made of 6 separate cells) NiCad and NiMh cells are about 1.2 volts per cell.

Battery Cycle Life: The number of times a battery can be discharged and recharged before failing. Battery manufacturers specify Cycle Life as a function of discharge rate and temperature.

Battery Self-Discharge: Loss of energy by a battery that is not under load.

Battery State of Charge (SOC): The percentage of total battery capacity available at that moment, or 100 percent minus the depth of discharge. (See Depth of Discharge)

BTS: Battery Temperature Sensor.

Blocking Diode: A diode used to prevent undesired current flow. In a PV array the diode is used to prevent current flow towards a failed module or from the battery to the PV array during periods of darkness or low current production.

British Thermal Unit (Btu): The quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.

Bulk Charge: The first stage of three stage charging in which full available current is sent to the batteries. In this stage, voltage gradually rises to the bulk voltage setting, when Absorption charging begins. (See Absorption Charge, Float Charge)

Bypass Diode: A diode connected in parallel with a PV module to provide an alternate current path in case of module shading or failure.

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Capacitance (C): The storage of energy in an electrostatic field, which creates an opposition to a change in voltage. A capacitor has two metal plates, separated by an air space or other insulating material, across which an electrical charge can build up but not flow. Capacitance in a circuit creates a phase difference between the voltage and the current in that circuit, in which the voltage lags or peaks after the amperage. The unit of capacitance is the farad (F).

Capacity: The total number of ampere-hours that can be withdrawn from a fully charged battery at a specified discharge rate and temperature. (See Battery Capacity)

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

CEC: California Energy Commission, in charge of California's Grid tie Incentive programs that rebate up to $5 per installed watt of PV power.

Cell: each separate part of a larger whole. Batteries are made of separatebattery cells. Each cell has its own independent chemical action. Solar Modules are made of numerous separatePV cells.

Charge: The process of adding electrical energy to a battery.

Charge Controller: A device that controls the charging rate from a solar array or other charging source, and prevents the batteries from being over charged. It also prevents current from back flowing back into the panels at night.

Charge Rate (C): The rate at which a battery is recharged. Normally expressed as a ratio of battery capacity to charging current. For instance, C/10 indicates the current that would recharge a battery in 10 hours (not factoring in loss for efficiency) and would indicate a 1 amp charge for a 10 Ah battery, a 10 amp charge for a 100Ah battery and a 22 amp charge for a 220Ah battery.

Cloud Enhancement: (See Edge of Cloud Effect)

Compact Fluorescent (CF): Fluorescent light bulb that can replace a regular light bulb in most fixtures or lamps. Typically uses one quarter the energy that a standard incandescent bulb uses for the same amount of light.

Concentrator: a PV module that uses a lens or mirrors to magnify the amount of sunlight striking a PV cell. Requires direct radiation--diffuse radiation is unusable by concentrators. (See Diffuse Radiation; Direct Radiation)

Conductor: A material through which electricity can flow, usually made of metal, and usually a wire. Some types of metal conduct electricity better than others. Aluminum is a better conductor than steel, copper is better than aluminum, and silver is better than copper.

Conversion Efficiency: The ratio of the electrical energy produced by a PV cell to the total solar energy striking the cell. Usually around 10%, but as high as 18%.

Converter: A device that converts DC electricity from one voltage to another (up or down).

Coulombic Efficiency: In battery charging, the ratio between amount of charge (Ah) output to amount of charge (Ah) input. Efficiency drops at high charge rates, as with generator charging. 85-90% is typical when charging from PV. This is factored in during the design process at

Current ( I ): The flow of electricity in a conductor, measured in Amperes (Amps).

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DC: (See Direct Current)

Daisy Chaining: Directly interconnecting PV modules in parallel to boost current without separate fusing. While permissible to some degree, this method of wiring solar modules can exceed the module terminals' current rating and lead to meltdown of the terminals especially in the last module that carries the most current.

Declination: The angle in degrees difference between magnetic north, as read on a compass, and true north. Declination is different on different parts of the globe. The declination for one's locale can be determined in many ways. Contact the nearest office of the Forest Service or BLM. Many compasses come with declination maps.

Deep Cycle: Type of battery that can be discharged close to its full capacity many times without damaging the battery.

Depth of Discharge (DOD): The percent of the rated battery capacity that has been withdrawn. (See Battery State of Discharge)

Diffuse (Indirect) Radiation: Light received from the sun after reflection and scattering by the atmosphere (clouds) and ground. (See Concentrator; Direct Radiation)

Diode: Electronic component that allows current flow in one direction only; like a check valve for electricity. (See Blocking Diode; Bypass Diode)

Direct Current (DC): Electric current flowing in only one direction.

Direct Radiation: Light received by direct solar rays, unobstructed by clouds, haze, etc. (See Diffuse Radiation)

Discharge Rate: How fast current is withdrawn from a battery. It is usually expressed in hours . For instance, a C/5 discharge rate is a current that would discharge the battery in 5 hours. Batteries have less capacity at higher discharge rates, so their storage capacity is rated at 5 hr., 20 hr, and 100 hr discharge rates. (See Puekert's Number)

Distributed Generation: A system of powering an electric grid, not from a few large central power plants, but from many smaller power systems distributed over a wide area.

Dual Axis Tracking: Tracking Array that follows the sun by both adjusting both up and down tilt and side to side motion. The second adjustment may actually pivot to track azimuth, or simply tilt the modules in both axis.

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EMF: 1) Electromagnetic flux/Electromagnetic field: the magnetic "force field" generated around a conductor through which a varying current flows (as is the case with a wire in an AC circuit). Other conductors in close proximity and parallel to the first conductor will have an electric charge induced in them. This is the principle (inductance) behind the transformer, in which two coils of wire are placed next to each other, the EMF of one coil inducing a voltage in the second coil. (See Inductance; Transformer)
2) Electromotive force: voltage.

Edge of Cloud Effect: The increase in solar intensity caused by reflected irradiance from nearby clouds.

Efficiency: The ratio of input power (or energy) to output power (or energy) expressed as a percentage.

Electricity: The flow of electrons in a circuit. A circuit must be a circle (a loop) of a conductor for electrons to flow. These electrons are part of the conductor material, they don't leave the circuit, and they aren't used up. They flow in one direction in a DC circuit, and they flow back and forth in an AC circuit.

Electrolyte: The medium that provides the ion transport mechanism between the positive and negative electrodes of a battery.

Electrical Grid: The electrical utility distribution network that connects "on grid" homes to power plants.

Energy: The capacity or potential for doing work (e.g. electricity stored in batteries). (See Power)

Energy Density: The ratio of the energy available from a battery to its volume (Wh/m³) or weight (Wh/kg).

Equalization (EQ): The process of mixing the electrolyte in batteries (accomplished by the bubbling of the electrolyte) by periodically overcharging the batteries for a short time.

Equinox: Time of year when day and night are equal, and the sun is directly over the equator. Occurs twice a year at the Spring equinox (beginning of spring, around March 21) and the Fall equinox (beginning of autumn, around Sept. 21). These are the ideal times of year when the angle of an adjustable array should be changed from Winter tilt to Summer or back.

Fixed Tilt Array: A PV array set in at a fixed angle with respect to horizontal.

Flat-Plate Array: A PV array that consists of non-concentrating PV modules.

Float Charge: A charge current to a battery that is equal to or slightly greater than the battery's self discharge rate. The final stage of a three stage charger in which voltage is held at the float voltage setting. (See Bulk Charge, Absorption Charge)

Flooded Acid Battery: Most common lead acid battery, uses liquid electrolyte in each cell. Usually open celled with removable caps to add water periodically. Some sealed maintenance free versions of flooded acid batteries are used in cars, but are not suitable for solar power.

Frequency: The number of repetitions per unit time of a complete waveform, expressed in Hertz (Hz) (cycles per second). AC electricity in America oscillates 60 times a second, a frequency of 60 Hz.

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