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In order to facilitate comparisons between several PV installations, JRC (European Joint Research Center) introduced the following Performance Index, now fixed in the IEC EN 61724 norm.
These indicators are related to the incident energy GlobInc in the collector plane, and are normalized by the Pnom = Array nominal installed power at STC, as given by the PVmodule manufacturer [kWp].
Therefore they are independent of the array size, the geographic situation and the field orientation.
In these definitions the yield energies are expressed as [kWh / KWp / day]. In other words, these quantities are numerically equal to the Equivalent operating time under a constant irradiance of 1 kW/m², that is, they can also be expressed as [Hours/day] when running at 1 kW/m², or [kWh/m²/day] (see the remark below).
We define the following quantities:
 Yr = Reference system Yield is the ideal array Yield according to Pnom as defined by manufacturer, without any loss. It can be understood as each incident kWh should ideally produce the Array Nominal Power Pnom during one hour. Yr is numerically equal to the incident energy in the array plane, expressed in [kWh/m²/day].
 Ya = Array Yield is the array daily output energy, referred to the nominal power [kWh / KWp / day].
 Yf = System Yield is the system daily useful energy, referred to the nominal power [kWh / KWp / day].
 Lc = Collection Loss = Yr  Ya, is the array losses, including thermal, wiring, module quality, mismatch and IAM losses, shading, dirt, MPP, regulation losses, as well as all other inefficiencies.
 Ls = System Loss = Ya  Yf, include inverter loss in gridconnected systems, or battery inefficiencies in standalone.
 PR = Performance Ratio = Yf / Yr, is the global system efficiency with respect to the nominal installed power and the incident energy.
For standalone systems (or every system with limited load), we also introduce:
 Lu = Unused energy, the potentially available energy at the array output, which can't be used because the system is "saturated" (full battery, or limited load in DCgrid system).
This should be determined during the simulation, and we have: Ya = Yr  Lu  Lc.
In this case Lc is the collection loss, only when the system is able to use the produced energy.
Important remark about units
There is often a unit's confusion with the quantity Yr, which may be understood
 either as the incident energy (with units [Hours at 1kWh/m² / day] or [kWh/m² / day])
 or as the ideal array Yield according to Pnom (expressed as [kWh / KWp / day]).
This numerical identity results of the STC definition: one kWh/m² of irradiance should produce one kWh/kWp of electricity.
The confusion comes from the fact that the kWh are not the same:
 in the former case [kWh/m² / day], the kWh represent incident irradiance energy (light flux)
 in the latter case [kWh / kWp / day], the kWh mean produced electrical energy !!!