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Puzzled by PV Performance Ratios?

[fa icon="calendar"] June 15, 2017 / by Jan-Willem Sips

Jan-Willem Sips

Blog JSi - Performance Ratio Formula.png

In most PV plant monitoring software you can choose the definition of the Performance Ratio (PR). So, a specific PV plant can have different Performance Ratios at the same time? Yes, and no…

Puzzled? Is Performance Ratio misleading? You don’t have to be puzzled, and no, Performance Ratio is not about misleading, it is about facts! Read our whitepaper – 6 key influences that determine PV performance ratios – to get a better understanding.


A new standard for performance monitoring of PV plants, is everything now clear?

Recently IEC released a new standard for performance monitoring of PV plants (IEC 61724-1: Photovoltaic system performance, monitoring; March 2017). This standard includes definitions and calculations of Performance Ratio; however, not just one method but many!

All these definitions share the same Performance Ratio basics:

final (actual) PV plant yield divided by reference (design) PV plant yield times 100 %.
The yield is AC power generated, expressed in kWh or MWh.

So, Performance Ratio is a figure that represents how well a PV plant performs compared to the designed expectation. At present, Performance Ratio figures just above 80% are quite common. Reaching a higher performance ratio is possible, for example with better monitoring and regular maintenance.

However, two influencing factors are beyond human control:

  • Solar irradiance: this varies from day to day, and from year to year; more sunlight means more output.
  • Temperature: this also varies and higher panel temperatures mean less output for the same amount of light.

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Performance Ratio by PV plant phase; from rough figures to tough maths

In each phase of a PV plant’s life, from prospecting and operations to refunding and refurbishment, the Performance Ratio plays a role.

In the pre-operational phase the Performance Ratio is a purely model-based figure, using data from prospecting and precise calculations during design. Whereas, in the operational phase, the actual yield, irradiance and (panel) temperatures are measured continuously for a correct Performance Ratio calculation.

In the more complex calculations, the actual yield is compared to a model-based yield, resulting in a specific type of Performance Ratio (Energy Performance Index or Power Performance Index).


Performance Ratio by constraints 

During the operational phase of a PV plant the Performance Ratio can be calculated with several constraints. Contractual agreements might mention the following exceptions, generally boosting the Performance Ratio:

  • Exclusion of the half hour past sunrise and before sunset each day;
  • Exclusion of periods of irradiance below a certain level;
  • Exclusion of malfunctioning parts of the PV park or grid outage, for the time they are not available;
  • Panel temperature corrections;
  • Expected or maximum ageing of the PV plant, resulting in lower output each year.


Ask the right questions

If someone uses Performance Ratio figures for a plant, just ask some questions to understand how they were derived:

  • Which phase is the plant in?
  • Which Performance Ratio definition is being used?
  • Which constraints for the calculation are being applied?

Performance ratios for any individual solar energy project come in a variety of forms, theoretical and actual, related to different phases of the project; design, installation, commissioning, contractual agreements and operational. Download our whitepaper to learn more about the influencers of Performance Ratio.

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Topics: pv, performance ratio, pv performance

Jan-Willem Sips

by Jan-Willem Sips

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