ITI Solar Technician Electrical (AITT Theory) key terms

The electrical pressure that pushes charge between two points, measured in volts. One volt equals one joule of energy transferred per coulomb of charge moved through the circuit.

The rate at which electric charge flows past a point, measured in amperes. One ampere equals one coulomb of charge passing per second in a closed circuit.

The opposition a material offers to current flow, measured in ohms. It converts electrical energy into heat and depends on material, length, cross-sectional area and temperature.

The relationship stating that current through a conductor equals voltage across it divided by its resistance (I = V/R). It applies to linear resistive elements and underpins most circuit calculations.

The rate at which electrical energy is transferred or consumed, measured in watts. In DC circuits it equals voltage multiplied by current; in AC circuits the power factor also applies.

The total electrical work delivered over time, usually billed in kilowatt-hours. One kilowatt-hour equals one kilowatt of power sustained for one hour, equivalent to 3.6 megajoules.

Series wiring joins components end to end so the same current passes through all, adding voltages. Parallel wiring connects them across the same nodes, so voltage is shared and currents add.

Two rules for circuit analysis: current entering a junction equals current leaving it, and the sum of voltage changes around any closed loop is zero. They apply to any network.

Current that reverses direction periodically, typically 50 Hz in India. Its magnitude and direction follow a sine wave, and its effective heating value is given by the RMS figure.

The root-mean-square value of an alternating quantity, equal to the steady DC value that would produce the same heating effect. For a pure sine wave it is peak value divided by 1.414.

The ratio of real power in watts to apparent power in volt-amperes in an AC circuit. It equals the cosine of the phase angle between voltage and current, ranging from zero to one.

The reduction in voltage along a conductor or component caused by its resistance and current. Excessive drop in cables wastes energy and can cause equipment malfunction or nuisance tripping.

A handheld instrument combining voltmeter, ammeter and ohmmeter functions, often with continuity and diode tests. It is the basic tool for commissioning checks and fault finding on PV systems.

An instrument that measures current by surrounding a conductor with a magnetic jaw, without breaking the circuit. Many models also measure DC current, which is useful for battery and string checks.

A megohmmeter that applies a high test voltage to measure leakage through insulation, reported in megohms. It verifies cable and module insulation integrity before energising a PV installation.

A two-terminal semiconductor that conducts readily in one direction and blocks the reverse. It is used for rectification, reverse-current blocking and bypass functions in photovoltaic modules.

A diode connected across a group of solar cells, or across a module, to provide an alternate current path when cells are shaded or faulty. It limits hot-spot heating and preserves string output.

A diode placed in series with a PV string or module to prevent reverse current from batteries or parallel strings flowing back through it during darkness or shading.

A circuit of diodes that converts alternating current into pulsating direct current. Bridge rectifiers are common in equipment power supplies and in some inverter front ends.

A semiconductor switch controlled by an electrical signal, used to switch power rapidly in inverters and converters. MOSFETs suit high-frequency low-voltage work; IGBTs suit higher-power applications.

A switching technique that varies the on-time proportion of a fixed-frequency square wave to control average output voltage. It is used in charge controllers, inverters and motor drives.

The reference conditions for rating PV modules: irradiance of 1000 W/m², cell temperature of 25 °C and air mass 1.5 spectrum. Nameplate wattage is quoted at these conditions.

The instantaneous solar power density falling on a surface, measured in watts per square metre. It varies through the day and with clouds, haze and the angle of the sun.

The daily solar energy received expressed as the number of hours at 1000 W/m² that would deliver the same energy. It is used to estimate daily generation per kilowatt-peak.

The voltage measured across a PV module or string with no load connected, at its highest for a given temperature. It falls as cell temperature rises and sets the maximum system voltage.

The current that flows when module terminals are directly connected through an ammeter of near-zero resistance. It is the highest current the module can produce at a given irradiance.

The operating point on a PV device's current-voltage curve where the product of voltage and current is greatest. Trackers continuously adjust the load to stay at this point.

The ratio of a module's maximum power to the product of its open-circuit voltage and short-circuit current. A higher fill factor indicates a better-quality cell with lower series losses.

The percentage change in a PV parameter, usually power or voltage, for each degree Celsius rise in cell temperature. It is negative for power and voltage, so hot modules produce less.

A number of PV modules connected in series to build up the required DC voltage. Modules in a string carry the same current, so mismatched or shaded modules reduce the whole string's output.

A localised overheated area on a module caused by a shaded, cracked or mismatched cell dissipating power instead of generating it. Bypass diodes and good installation practice limit damage.

The study of how nearby objects such as trees, buildings and poles cast shadows on a PV array through the year. It identifies shading losses and guides array placement and string layout.

The angle at which a PV module is inclined from the horizontal. It is usually chosen near the site latitude to maximise annual yield, or steeper to favour winter generation.

A device between PV array and battery that regulates charging voltage and current to prevent overcharge and often over-discharge. PWM and MPPT types differ in how they handle array voltage.

An electronic control method that continuously adjusts the operating voltage of a PV array to extract maximum power under changing irradiance and temperature. It raises yield especially in cold or variable conditions.

The proportion of a battery's rated capacity that has been withdrawn, expressed as a percentage. Limiting DoD extends cycle life, particularly for lead-acid batteries.

The remaining stored energy in a battery expressed as a percentage of its full capacity. It is estimated from voltage, current integration or specific gravity in flooded lead-acid cells.

A device that converts direct current from a PV array or battery into alternating current suitable for appliances or the grid. Off-grid inverters may also incorporate chargers and transfer switches.

A protective function that stops a grid-connected inverter from energising a line after the utility supply fails. It protects utility workers and prevents out-of-phase reconnection hazards.

A billing arrangement in which exported solar energy is credited against imported grid energy, typically through a bidirectional meter. The consumer pays for the net difference over the billing period.

Deliberate connection of exposed metalwork and circuit points to the earth through a low-resistance path. It limits touch voltage during faults and gives protection devices a return path to operate.

A protective switch that trips when the current difference between live and neutral conductors exceeds a set threshold, indicating leakage to earth. It protects people from electric shock.

A resettable automatic switch that opens on overload or short circuit, protecting cables and equipment. Its rating and trip curve must be coordinated with cable current-carrying capacity.

Selecting conductor cross-section so it safely carries expected current while keeping voltage drop and temperature rise within limits. It considers derating for ambient temperature and grouping.

The ratio of a plant's actual energy output to the output it would produce at rated efficiency under the same irradiance. It captures losses from temperature, soiling, wiring and inverter conversion.

The electrical energy a PV system delivers over a period, usually expressed in kilowatt-hours per installed kilowatt-peak. It depends on irradiance, tilt, losses and system availability.

A record of how much power a site consumes over time, showing daily and seasonal variation. It guides battery sizing, inverter rating and decisions about grid or generator backup.

The charge a battery can store, quoted in ampere-hours at a specified discharge rate. Multiplying by nominal voltage gives watt-hours, which is more useful for system sizing.

A plot of a PV device's output current against voltage across its operating range. Its shape reveals series resistance, shunt losses and the maximum power point used for performance checks.

The condition in which a grid-connected inverter keeps supplying a local circuit after the utility supply has been disconnected. Anti-islanding protection is required to prevent it.