Reading performance ratio like an engineer, not a dashboard

What the number is actually measuring

Performance ratio compares actual energy output to the theoretical output the plant should produce given the irradiance it received. It's meant to strip out weather so you can compare a cloudy week to a clear one on equal footing. A high performance ratio doesn't mean high generation; it means the plant converted the available sunlight efficiently, whatever that sunlight happened to be.

That distinction matters because a dashboard showing a stable, healthy performance ratio during a low-generation month can quietly hide the fact that a string is underperforming, if the irradiance data feeding the calculation is off, or if the baseline it's measured against was never corrected for degradation.

The three most common misreadings

First: treating a single day's performance ratio as meaningful. Daily numbers swing with cloud cover, temperature, and even brief shading events, and a single bad day usually isn't a fault, it's noise. Look at a rolling window, not a single reading.

Second: comparing performance ratio across plants with different irradiance sensors, or worse, none at all. If one plant estimates irradiance from a nearby weather station and another has an on-site sensor, the two performance-ratio numbers aren't really comparable, even if the dashboard puts them side by side.

Third: not correcting for temperature. Panel output drops as cell temperature rises, and a performance-ratio calculation that ignores this will systematically read low on hot, clear days and mask real faults on cooler ones. A temperature-corrected performance ratio is a materially different, more useful number.

Checks worth running before you trust the dashboard

Confirm the irradiance source and whether it's an on-site sensor or a modeled estimate. Confirm whether the calculation is temperature-corrected. Check that the design baseline used for the theoretical output has been adjusted for expected annual degradation, most panels lose a fraction of a percent of output every year, and a baseline that hasn't been updated in five years will overstate the real gap.

If none of that has been documented anywhere, the performance-ratio number on the dashboard is directionally useful at best. It's worth pinning down before using it to make a cleaning or maintenance decision.

What a trustworthy setup looks like

A performance ratio you can actually act on comes from an on-site or reasonably close irradiance sensor, a temperature correction applied consistently, and a design baseline that's revisited at least annually. Once those three things are in place, a sustained drop in performance ratio is a real signal worth investigating, not something you have to second-guess first.