Solar Glossary
A reference guide to the key terms used in off-grid solar system design.
AC (Alternating Current)
Electrical current that periodically reverses direction. Mains power in New Zealand and Australia is 230V AC at 50Hz. Household appliances run on AC power.
AGM (Absorbent Glass Mat)
A type of sealed lead-acid battery where the electrolyte is absorbed into fibreglass mats between the plates. Maintenance-free and spill-proof, making them popular for marine and caravan use.
Ah (Amp-hours)
A unit of battery capacity. A 100Ah battery can theoretically deliver 1 amp for 100 hours, or 10 amps for 10 hours.
Array
Two or more solar panels connected together, either in series, parallel, or a combination of both.
BMS (Battery Management System)
An electronic system that monitors and protects a lithium battery pack. It manages cell balancing, over-charge protection, over-discharge protection, and temperature monitoring.
Bus Bar
A solid metal strip or bar used as a common connection point for multiple wires, typically for positive or negative distribution in a battery or fuse board.
Charge Controller
A device that regulates the voltage and current from solar panels to the battery, preventing overcharging and damage. See also MPPT and PWM.
C-Rate
The rate at which a battery is charged or discharged relative to its capacity. A 1C rate for a 100Ah battery means 100A. A 0.2C rate means 20A.
DC (Direct Current)
Electrical current that flows in one direction only. Solar panels, batteries, and 12V/24V appliances all operate on DC power.
DC-DC Charger
A device that charges a secondary (house/leisure) battery from a vehicle's alternator while the engine is running. Modern smart alternators require DC-DC chargers rather than simple VSRs.
Deep Cycle Battery
A battery designed to be regularly discharged to a significant portion of its capacity (typically 50-80%) and then recharged, unlike a starting battery which delivers short bursts of high current.
DoD (Depth of Discharge)
The percentage of a battery's total capacity that has been used. A 100Ah battery discharged to 70Ah remaining has a 30% DoD. Deeper discharge reduces battery cycle life.
Equalisation Charge
A controlled overcharge applied to flooded lead-acid batteries to equalise the voltage across all cells and prevent stratification of the electrolyte.
Float Charge
A low-level maintenance charge applied to a battery once it is fully charged, to keep it at 100% without overcharging.
Fuse
A safety device containing a thin wire that melts and breaks the circuit if too much current flows, protecting wiring and equipment from damage or fire.
Gel Battery
A type of sealed lead-acid battery where the electrolyte is mixed with silica to form a gel. More resistant to vibration than flooded batteries but sensitive to overcharging.
Ground Fault
An unintentional electrical path between a current-carrying conductor and a grounded surface, which can cause shock or fire hazards.
Imp (Current at Maximum Power)
The current output of a solar panel when operating at its maximum power point. Used in system design calculations.
Inverter
A device that converts DC power (from batteries) to AC power (for household appliances). See also Pure Sine Wave and Modified Sine Wave.
Inverter/Charger
A combined unit that functions as both an inverter (DC to AC) and a battery charger (AC to DC), such as the Victron MultiPlus.
IP Rating (Ingress Protection)
A two-digit code indicating the degree of protection against solids (first digit, 0-6) and liquids (second digit, 0-9). IP67 means dust-tight and protected against temporary immersion.
Isc (Short Circuit Current)
The maximum current a solar panel can produce when its output terminals are connected directly together. Used for sizing fuses and charge controllers.
LiFePO4 (Lithium Iron Phosphate)
A lithium battery chemistry popular in off-grid solar systems. Offers longer cycle life, lighter weight, deeper DoD, and flat discharge curves compared to lead-acid batteries.
Load
Any device or appliance that consumes electrical power in the system.
MCB (Miniature Circuit Breaker)
A resettable safety device that automatically breaks the circuit when current exceeds a rated value. Unlike fuses, MCBs can be reset after they trip.
Modified Sine Wave
A type of inverter output that approximates a sine wave using a stepped square wave. Cheaper than pure sine wave but may cause problems with sensitive electronics, motors, and some chargers.
Monocrystalline
A type of solar cell made from a single crystal of silicon. More efficient than polycrystalline cells, producing more power per unit area.
MPPT (Maximum Power Point Tracking)
An advanced type of charge controller that optimises the power output from solar panels by continuously adjusting the electrical operating point. More efficient than PWM controllers, especially when panel voltage is significantly higher than battery voltage.
Nominal Voltage
The approximate voltage used to classify a battery or solar panel (e.g. 12V, 24V). Actual operating voltages differ — a "12V" solar panel typically has a Vmp of 18-20V.
Off-Grid
A power system not connected to the mains electricity grid, relying on solar panels, batteries, and possibly generators for all electrical needs.
Open Circuit Voltage (Voc)
The maximum voltage a solar panel produces when not connected to any load. Important for sizing charge controllers, as Voc increases in cold temperatures.
Parallel Wiring
Connecting solar panels (or batteries) positive-to-positive and negative-to-negative. This increases current (amps) while keeping voltage the same.
Pmax (Maximum Power)
The maximum power output of a solar panel under Standard Test Conditions (STC), measured in watts. This is the panel's "rated" wattage.
Polycrystalline
A type of solar cell made from multiple silicon crystals. Slightly less efficient than monocrystalline but historically cheaper to manufacture.
Pure Sine Wave
A type of inverter output that produces a smooth, continuous sine wave identical to mains power. Required for sensitive electronics, variable-speed motors, and medical equipment.
PWM (Pulse Width Modulation)
A basic type of charge controller that connects the solar panel directly to the battery and regulates charging by rapidly switching on and off. Less efficient than MPPT, especially when panel voltage is much higher than battery voltage.
Series Wiring
Connecting solar panels (or batteries) positive-to-negative in a chain. This increases voltage while keeping current the same.
Shunt
A precision low-resistance resistor installed in the battery negative cable. By measuring the tiny voltage drop across the shunt, a battery monitor can calculate current flow in and out of the battery.
STC (Standard Test Conditions)
The laboratory conditions under which solar panel specifications are measured: 1000 W/m² irradiance, 25°C cell temperature, and AM1.5 air mass. Real-world output is typically lower.
String
A group of solar panels wired in series forming a single electrical circuit.
System Voltage
The nominal DC voltage of an off-grid electrical system, typically 12V, 24V, or 48V. Higher voltages allow smaller cable sizes and less power loss over distance.
Temperature Coefficient
The rate at which a solar panel's voltage or power output changes with temperature. Voc increases as temperature drops below STC (25°C), which matters for controller sizing in cold climates.
Vmp (Voltage at Maximum Power)
The voltage output of a solar panel when operating at its maximum power point. This is the voltage the panel operates at under optimal load.
Voc (Open Circuit Voltage)
See Open Circuit Voltage.
VSR (Voltage Sensitive Relay)
A simple relay that connects the starting and house batteries when the starting battery voltage rises (engine running). Not suitable for vehicles with smart alternators.
Watt (W)
A unit of power. Watts = Volts × Amps. A 100W solar panel at 12V produces approximately 8.3 amps.
Wh (Watt-hours)
A unit of energy. Watt-hours = Watts × Hours. A 100W appliance running for 3 hours uses 300Wh of energy. Used to calculate daily power consumption and battery sizing.