Performance Ratio
The Performance Ratio is the ratio of the energy effectively produced (used), with respect to the energy which would be produced if the system was continuously working at its nominal STC efficiency. The PR is defined in the norm IEC EN 61724.
The energy potentially produced at STC conditions is indeed equal to GlobInc * PnomPV, where PnomPV is the STC installed power (manufacturer's nameplate value). This equivalence is explained by the fact that at STC (1000 W/m², 25°C) each kWh/m² of incident irradiation will produce 1 kWh of electricity.
Therefore for a grid-connected system:
PR = actual energy / (irradiance * DC size)
Reference: https://jira.manamonitoring.com/browse/MANA-1531
Note, similar to Predicted Power, we have here instantaneous PR (kW%) and aggregated PR (kWh%)
PR is calculated when/if irradiance > 150w/m2, or the "Minimum Irradiance Threshold" set for the site.
The values are averaged to get hourly/daily values.
It is possible to review how PR was calculated by exporting "Combined System Readings" for the system that has irradiance data (usually the site aggregator):
Once you export it you can see exactly how the PR values are calculated.
Some notes:
- PR is calculated based on momentary Irradiance and momentary Power for instances in time when irradiance > 150 W/m2, averaged out for the day.
Example
Looking at Maemae Elementary, we see that PR is calculated as 266% for 9/1/2024:
To validate this number, we can export combined readings for this system and calculate manually ourselves:
In Excel, set a column to be calculated as Solar Power * 1000 / (Irradiance * 123.12).
123.12 is the DC size of the system:
The formula in Excel:
It is then easy to calculate the average PR value for the given day:
Settings that Affect PR
Parameters that affect PR calculation are: site DC size and the site irradiance threshold.
The Site Irradiance Threshold can be set on the site form:
If PR seems incorrect, then either these parameters are incorrect, or we are getting incorrect power or irradiance readings. If power is unavailable then the system uses average power, which comes from energy readouts so those may also be a culprit.
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