In order to facilitate comparisons between several PV installations, JRC (European Joint Research Center) introduced the following Performance Index, which were then included in the IEC EN 61724 norm.
These indicators are related to the incident energy in the collector plane, and are normalised by the Pnom = Array nominal installed power at STC, as given by the PV-module manufacturer [kWp].
Therefore they are independent of the array size, the geographic situation and the field orientation.
We define the following quantities:
- Yr [kWh/kWp] = Reference system Yield. This is the ideal array Yield according to Pnom as defined by manufacturer, without any loss. I.e. the energy produced if the system was always running at the STC efficiency. This 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 plane of array [POA], expressed in [kWh/m²] (see below).
- Ya [kWh/kWp] = Array Yield is the array daily output energy, referred to the nominal power [kWh / KWp].
- Yf [kWh/kWp] = 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 grid-connected systems, or battery inefficiencies in stand-alone.
- PR = Performance Ratio = Yf / Yr, is the global system efficiency by respect to the nominal installed power.
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 [kWh/m²])
- or as the ideal array Yield according to Pnom (expressed as [kWh / KWp]).
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], the kWh represent incident irradiance energy (light flux)
- in the latter case [kWh/kWp], the kWh mean produced electrical energy !!!