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Energy Yield and Performance Ratio of Photovoltaic Systems

For investors and operators alike, there are two fundamental questions:

  1. How much electricity does the system generate?
  2. How well does the system perform?

Energy of incoming light

The yearly sum of global irradiation, h, that hits the module is specific to the location and should be obtained from databases, measurements, or - in the first instance - from an irradiance map. It is measured in [kWh/m2]. In Berlin, for instance, it is h ~ 1,200 kWh/m2.

Target Yield

The target yield is the theoretical annual energy production (on the DC side of the module), only taking into account eh energy of the incoming light and the module's nominal efficiency.

Performance Ratio

The Performance Ratio is the ratio between actual yield (i.e. annual production of electricity delivered at AC) and the target yield:
Performance Ratio of a PV System
The performance ratio, often called "Quality Factor", is independent from the irradiation and therefore useful to compare systems. It takes into account all pre-conversion losses, inverter losses, thermal losses and conduction losses. It is useful to measure the performance ratio throughout the operation of the system, as a deterioation could help pinpoint causes of yield losses.

Energy Output per Area

The energy, E, delivered by a system with area A can be estimated from:

Energy Yield

The pre-conversion efficiency reflects the losses incurred before the beam hits the actual semiconductor material, caused by shading, dirt, snow and reflection off the glass. The system efficiency reflects electrical losses caused by wiring, inverter and transformer.The module itself is defined by a nominal efficiency and relative efficiency as shown on previous page.

Energy per Rated Power

Sometimes, the energy yield is expressed in terms of the peak power of the module, which is independent from the area of the module. It is (with H0= 1,000 W/m2):

Energy Yield for peak power

This is a very useful ratio, since the energy yield E is a measure of the earnings potential while the peak power reflects the cost of the system. Note that the peak power in the above formula is the module's peak power, not the system's installed capacity, which is Psys = Pmoduleηsys.

Energy Losses

Sometimes it is more intuitive to think in terms of energy losses that occur at every step of the way rather than component efficiencies. Both concepts are the same, as losses = 1 - efficiency, both expressed in percentage terms.

Starting with the intensity of the incoming light (i.e. the energy that is actually available to the system), there are three major blocks of energy losses:

  1. Pre-photovoltaic losses: Attenuation of the incoming light though shading, dirt, snow and reflection before it hits the photovoltaic material. In concentrating pv systems, it also includes losses from concentration devices.
  2. Module and thermal losses: Reflecting the efficiency and temperature dependance of the solar module
  3. System losses: Reflecting losses in the electrical components including wiring, inverters and transformers.

Energy Flow through PV System

Pre-Module Losses Tolerance of rated power Consider that the module does not deliver the power as stated in the data sheet. Manufacturers provide a tolerance, often up to 5%.
Shadows Shadows may be caused by trees, chimneys etc. Depending on the stringing of the cells, partial shading may have a significant effect.
Dirt Losses due to dirt up to 4% in temperate regions with some frequent rain. Up to 25% in arid regions with only seasonal rain and dust.
Snow Dependant on location and maintenance effort.
Reflection Reflection losses increase with the angle of incidence. Also, this effect is less pronounced in locations with a large proportion of diffuse light, i.e. clouds.
Module Losses Conversion The nominal efficiency is given by the manufacturer for standard conditions.
Thermal losses With increasing temperatures, conversion losses increase. These losses depend on irradiance (i.e. location), mounting method (glass, thermal properties of materials), and wind speeds. A very rough estimate is ~8%

System Losses

~ 14%

Wiring Any cables have some resistance and therefore more losses.
MPP Ability of the MPP tracker to consistently find the maximum power point.
Inverter Inverter efficiency
Mis-sized inverter If the inverter is undersized, power is clipped for high intensity light. If it is oversized, the inverter's efficiency will be too low for low intensity light.
Transformer Transformer losses in case electricity has to be connected to a high-voltage grid.
Operation & Maintenance Downtime Downtime for maintenance is usually very low for photovoltaic systems.
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