performance ratio: how to compare solar plant performance correctly

The amount of electricity generated does not, on its own, show how well a solar plant is performing. During sunny periods, generation can increase even if additional losses have developed in the system. Conversely, lower generation during cloudy periods does not necessarily indicate a technical problem. Performance ratio, or PR, is a dimensionless performance metric that compares actual generation with reference generation based on the irradiation received and the installed nominal panel capacity.

PR is neither a measure of panel efficiency nor an indicator of return on investment. It describes the overall system performance within the selected measurement boundaries, incorporating temperature, conversion, wiring and other losses. For owners and investors, it is a useful comparison tool provided that the report clearly states the calculation methodology.

PR is calculated as PR = (E_AC / P_STC) / (H_POA / G_STC). E_AC is the AC electrical energy measured over the selected period in kWh, P_STC is the total nominal DC capacity of the panels in kWp under standard test conditions, H_POA is the irradiation received in the plane of the panels over the same period in kWh/m², and G_STC is the reference irradiance of 1 kW/m². The result can also be expressed as a percentage.

It must be stated whether energy is measured at the inverter outputs or at the plant's grid connection point. In the latter case, the result may also reflect transformer losses and auxiliary consumption, depending on the metering arrangement. The nominal DC capacity of the panels must not be replaced with the AC capacity of the inverters in the calculation without justification.

Plants with panels facing different directions require representative irradiation data for each significant panel group and a consistent aggregation method. For bifacial modules, the way in which the rear-side irradiation contribution is accounted for must be described separately, as different methodologies can produce different PR results.

A dirty, shaded or incorrectly installed irradiance sensor can produce misleading results. If the sensor records less irradiation than is actually received, the calculated PR may appear unjustifiably high. Sensor positioning, cleanliness, calibration and data availability are therefore just as important as electricity meter readings.

Energy and irradiation data must cover the same time intervals, with aligned timestamps. Missing records must not automatically be treated as zero values. The report must state any data gaps, the principles used to replace missing data and any periods excluded from the calculation.

Monthly or annual PR is calculated from the total energy and irradiation for the relevant period, rather than as a simple arithmetic mean of daily PR values. Otherwise, days with low and high irradiation receive equal weighting, which can distort the comparison.

Conventional PR is not entirely independent of weather conditions. Higher module temperatures reduce their power output, so results for summer and cooler periods can differ even in a technically sound system. Snow, shading, inverter clipping and generation curtailment imposed by the grid operator also affect the result.

When comparing plants, their design, DC-to-AC capacity ratio, module technology and measurement boundaries must be taken into account. There is no single universal PR threshold that reliably indicates good or poor performance across all sites. A more appropriate reference is the specific plant's design performance or a comparable historical period.

If a contract uses temperature-corrected PR or excludes periods of grid curtailment from the calculation, the rules must be defined in advance. The report should present the actual result and the adjusted metric separately, so that the assessment of technical performance does not obscure the actual amount of lost generation.

A fall in PR is a reason to investigate, not a diagnosis of a specific defect. First, check the data, weather conditions and recorded operating restrictions. Then compare inverter and string performance to establish whether the deviation affects the entire plant or only part of it.

If the deviation is related to shading from vegetation, grass cutting at the solar plant is required. Where electrical connection problems are involved, a qualified specialist must inspect the cables and connectors. Microcracks in panels, however, cannot be confirmed from PR or thermographic data alone; their assessment may require electroluminescence testing and additional electrical measurements.

Following remedial action, assess performance under comparable conditions and retain records of the work carried out. pvcare.eu can assist with monitoring, maintenance in accordance with LVS EN IEC 62446-2:2020, robotic panel cleaning using filtered water, grass cutting and thermal drone inspections with a report available on dronepv.eu, as well as arrange regular work under an O&M contract.