Performance Ratio (PR)
The headline efficiency number for a PV plant, independent of weather.
Performance Ratio (PR) is the ratio of a PV plant’s actual energy yield to the yield it would have produced at its nameplate efficiency under the irradiation it actually received. It normalises out the weather, so it is the single number that says how well the plant is converting available sunlight. Modern plants run PR ≈ 0.80–0.85.
Definition
PR divides measured AC energy by the theoretical energy from the plane-of-array irradiation at STC efficiency. Because it cancels the irradiance the plant received, a falling PR points to losses inside the plant — soiling, degradation, downtime, clipping, thermal losses — rather than to a cloudy month. It is the EN 61724 benchmark metric for plant health.
Formula
PR = actual energy yield ÷ (POA irradiation × nameplate DC ÷ STC irradiance)
Typical range
Well-run utility-scale plants: PR ≈ 0.80–0.85. New plants can exceed 0.85 in mild climates; hot desert sites sit lower because of temperature losses. A PR below ~0.75, or a downward trend, signals recoverable loss.
Why it matters
PR is the term in nearly every O&M contract and performance guarantee, and the metric lenders track. A 2-point PR slip on a large plant is six figures a year in lost generation. Because it strips out weather, a falling PR is the earliest honest signal that something inside the plant — not the sky — is costing yield.
How NuraVolt tracks it
NuraVolt computes PR continuously from POA irradiance and AC output, decomposes the gap to nameplate into named loss buckets (soiling, temperature, availability, clipping, degradation), and trends each — so a PR decline is attributed to a cause and a euro figure, not just flagged.
Frequently asked questions
See also
PR with the weather’s heat penalty removed, so real losses stand out.
Energy produced per unit of installed capacity — the cross-site comparator.
The share of nameplate a plant actually delivered over time.
Actual generation measured against a physics model of what the plant should have made.
A leading recoverable drain on PR.
Hardware loss that drags PR down and never recovers with rain.
See this on your own plants
NuraVolt turns your SCADA and BMS data into early fault detection, degradation-aware BESS analytics, and audit-ready reporting. A fixed-scope audit shows you what we’d find on your portfolio.