If you own a solar plant, your monthly report is full of numbers: kilowatt-hours generated, peak power, availability, carbon avoided. One number tells you more than all the others about whether the plant is doing its job. It is called the performance ratio, or PR.
What performance ratio measures
Performance ratio compares the energy your plant actually delivered with the energy it could theoretically have produced given the sunlight that fell on it. Because it accounts for the weather, a cloudy month does not lower the PR. That makes it the right number for spotting problems that are within your control.
In simple terms: PR = actual energy output ÷ (irradiation on the modules × installed capacity ÷ reference irradiance). The result is expressed as a percentage.
What a good number looks like
No plant reaches 100%. Temperature, cable losses, inverter efficiency and a little soiling always take a share. In Bangladesh's hot climate, a well-designed and well-maintained rooftop plant typically runs at a PR of around 75–82%.
| PR range | What it usually means |
|---|---|
| 80% and above | Healthy plant, clean modules, no significant faults |
| 75–80% | Normal for hot months; check trend against previous years |
| 70–75% | Investigate: soiling, string faults or inverter issues likely |
| Below 70% | Act now: significant energy is being lost |
Why PR moves with the seasons
Even a perfectly maintained plant will show a lower PR in April and May than in December and January. Solar cells lose efficiency as they heat up, and module temperatures on a metal roof in the pre-monsoon months can run well above 60 °C. The dry season adds soiling on top. During the monsoon, cooler modules and natural rain cleaning often lift PR, even though total generation falls because there is less sunlight.
That is why the most useful comparison is month against the same month in previous years, or against a temperature-corrected PR, rather than one month against the next.
How to read it in a monthly report
- Compare against the design PR, the figure predicted in your yield simulation.
- Watch the trend, not a single month. A slow slide usually means soiling or degradation.
- Look for sudden drops, which point to a tripped string, a failed inverter or a communication fault.
- Check availability alongside PR. A plant that was offline for three days has a different problem from one that ran poorly all month.
Our plant monitoring service calculates PR daily and flags deviations automatically, so problems are fixed before they show up in the monthly report.
Written by
O&M Desk
Monitoring & maintenance team, Emerged Power
Field notes from the team that keeps our plants clean, cool and online.