How it works
PR compares measured energy with a reference. The reference is the energy the plant would produce if its modules converted the incoming sunlight at their rated efficiency, with no other losses. It equals the irradiation in the plane of the modules, measured on site by pyranometers or reference cells in kWh/m², times the plant's DC capacity in kWp, divided by the standard test irradiance of 1 kW/m². The measured AC energy divided by this reference is the PR. The IEC 61724 series sets out the definitions.
The losses that hold PR below 100% include module temperature, soiling, shading, mismatch, cabling, inverter conversion, clipping and downtime. A healthy modern plant typically shows a PR of around 75% to 85%. Temperature is the largest weather-driven loss, so PR falls in hot months. Many contracts use a temperature-corrected PR, which adjusts the reference to the measured module temperature. That makes summer and winter figures comparable and isolates the losses the operator can control.
Why it matters
PR is how owners and contractors agree whether a solar plant is doing its job. EPC contracts often guarantee a PR at handover, and O&M contracts guarantee one each year, with damages for shortfalls. Asset managers use it to compare plants in different climates on the same terms. A falling PR at steady irradiance points to a problem worth finding, such as soiling, failed strings or inverter trouble.
A worked example
A plant has 60 MWp of modules. In June its pyranometers record 180 kWh/m² of plane-of-array irradiation. The reference energy is 60,000 kWp × 180 kWh/m² ÷ 1 kW/m² = 10,800,000 kWh, or 10,800 MWh. The meter recorded 8,640 MWh, so the PR is 8,640 ÷ 10,800 = 80%. If the O&M contract guarantees 82%, the shortfall is 2 points, or 0.02 × 10,800 = 216 MWh.
Illustrative figures.
Across markets
In India, contracts lean on PR guarantees and an annual CUF tied to the PPA. In the US, EPC handover often rests on a capacity test, for example one based on ASTM E2848, which compares measured power with a modelled value at reference conditions, alongside energy or PR guarantees. In Europe, PR guarantees are a common term in EPC and O&M contracts.