kWp (kilowatt-peak) kWh (kilowatt-hour) Specific yield (kWh/kWp) PVGIS Performance ratio (PR) Self-consumption ratio Net metering (netting) String MPPT Voc and Vmp DC/AC ratio Bypass diode Shading loss Degradation Payback period Grid-tied, off-grid and hybrid systems Days of autonomy and depth of discharge (DoD) Bill of materials (BOM) Row spacing and GCR Pre-suitability check (mounting)
kWp (kilowatt-peak)
The nominal power of a PV array under standard test conditions (1,000 W/m² irradiance, 25 °C cell temperature). System size is quoted in kWp; actual output on site varies with weather and temperature.
kWh (kilowatt-hour)
A unit of energy: 1 kW produced or consumed for one hour. Bills and production estimates are compared in kWh; kWp measures power, kWh measures energy.
Specific yield (kWh/kWp)
Annual energy produced per 1 kWp installed. It reflects the site's irradiation, orientation, tilt and losses together, and makes systems of different sizes comparable. Learn more
PVGIS
The European Commission Joint Research Centre's free solar tool. It uses satellite-based irradiation data to estimate monthly irradiation and PV output for a location. Learn more
Performance ratio (PR)
The ratio of a system's actual output to the energy it could theoretically produce at the same irradiation. It summarises temperature, cable, inverter, soiling and shading losses in a single figure.
Self-consumption ratio
The share of the energy produced that is used on site at the moment it is produced. It is usually high for businesses that operate during the day and determines how much of the saving comes straight off the bill. Learn more
Net metering (netting)
Offsetting energy exported to the grid against energy imported over a set period. The period (e.g. hourly or monthly) and how surplus is valued are set by regulation and can change; confirm current rules with the distribution company. Learn more
String
A group of modules connected in series. Voltages add up in series while current stays the same; the string voltage must stay within the inverter's input limits. Learn more
MPPT
Maximum Power Point Tracker: the circuit in an inverter or charge controller that operates the connected modules at the voltage giving maximum power. Modules with different orientation or tilt go to separate MPPT inputs. Learn more
Voc and Vmp
Voc is the module's open-circuit voltage and Vmp its voltage at the maximum power point. Voltage rises as temperature falls, so the cold-weather string Voc is checked against the inverter's maximum DC input voltage. Learn more
DC/AC ratio
The ratio of the array's DC power to the inverter's nominal AC power. A ratio slightly above 1 is common; too high a ratio causes inverter clipping in sunny hours. The right value depends on location, orientation and the inverter manufacturer's limits.
Bypass diode
Diodes connected in parallel with cell groups inside a module. When a cell group is shaded, current flows around it, limiting hot spots and string losses. Learn more
Shading loss
Energy lost to shade from surrounding obstacles (chimneys, trees, neighbouring buildings) or from rows shading each other. Because modules are in series, one shaded module can affect the whole string, so it should be assessed per module. Learn more
Degradation
The gradual decline of module output over the years. The manufacturer states the annual rate in the datasheet and warranty; it is included in long-term production and payback calculations.
Payback period
The time it takes for the annual savings and income from a system to cover its investment cost. Electricity price increases, degradation, maintenance costs and financing terms all change the result. Learn more
Grid-tied, off-grid and hybrid systems
A grid-tied (on-grid) system works alongside the grid. An off-grid system is independent of the grid and covers night-time demand from batteries. A hybrid system is grid-connected and also uses batteries. Learn more
Days of autonomy and depth of discharge (DoD)
Days of autonomy is how many days the batteries can supply the loads without sun. Depth of discharge is how much of the battery capacity is used; it is usually higher for lithium than for gel batteries and directly affects battery life. Learn more
Bill of materials (BOM)
The list of quantities and lengths of items such as rails, hooks, clamps, profiles, screws and cables for a project. It is the common basis for the proposal cost, warehouse issue and installation. Learn more
Row spacing and GCR
The distance between module rows in ground-mount and flat-roof systems, set so rows do not shade each other when the winter sun is low. GCR (ground coverage ratio) is the ratio of module area to ground area. Learn more
Pre-suitability check (mounting)
A preliminary assessment of the mounting structure for buckling, bending, deflection and shear. It guides the design early on; it does not replace an approved structural project or a licensed engineer's calculation. Learn more