Most owners look at a monthly total, which is the least informative view available. The same data, plotted four ways, answers four different questions.
Operational performance depends on both equipment and repeatable human routines; for a separate workplace-management perspective, see how employees can tell if they are being monitored.
Reviewed August 9, 2026.
The four views
The daily curve. Production against time of day, one day at a time. The view that finds problems earliest, because a healthy day has a smooth shape and any deviation from it is visible immediately.
For additional context on photovoltaic operation, maintenance and sector developments, consult UK Government.
The monthly total. What most people look at. Useful for billing and nearly useless for diagnosis, because it cannot separate weather from performance.
The annual performance ratio plot. Twelve points a year, several years on one chart. The view that finds gradual decline, which nothing else does.
And the string comparison. Strings against each other, on the same day. The view that localises a fault once one of the others has revealed it.
What the daily curve shows
A smooth arc on a clear day, a ragged one on a cloudy day, and both are normal.
A notch at the same time each day is shading — and if it appeared this year and not last, something grew or was built.
A flat top is clipping, where the array output exceeds what the inverter passes. Sometimes a design choice, sometimes a sign the array was oversized against the inverter.
A late start or early finish relative to sunrise and sunset is horizon shading or an inverter start threshold.
And an asymmetric curve — strong morning, weak afternoon or the reverse — points at orientation, shading or a partial fault on one side.
What the annual PR plot shows
A flat line at design level across years: healthy.
A gentle slope matching expected degradation: normal.
A steeper slope: a question with a date on it.
A step down: a discrete event, and the month it happened is on the chart.
And a sawtooth that recovers each time somebody cleans: soiling, and the cleaning is working.
The habit
Look at one daily curve a month, chosen from a clear day. Thirty seconds, and it catches shape changes.
Record the four monthly numbers — production, irradiance, performance ratio, availability — outside the monitoring platform.
Plot the PR annually, all years on one chart.
And compare strings once a year, on a clear day, whether or not anything looks wrong.
Total: perhaps two hours a year, and it is the entire operational discipline a solar plant needs from its owner.
What to keep and why
The data outlasts the platform. Monitoring subscriptions lapse and suppliers change, and the multi-year series is what makes any future claim, sale or expansion decision possible.
A spreadsheet is enough. Date, production, irradiance, PR, availability, notes.
And the notes column matters more than it looks — "cleaned", "inverter fault, three days", "new building to the south". In year eight it is the only explanation of a step in the chart.
The short version
- Four views: the daily curve, the monthly total, the annual performance ratio plot, and the string comparison
- The daily curve finds problems earliest; the monthly total is what most people look at and is nearly useless for diagnosis
- On a daily curve: a notch is shading, a flat top is clipping, a late start is horizon or threshold, asymmetry points at orientation or a partial fault
- On an annual PR plot: a gentle slope is degradation, a steeper one is a claim, a step is a discrete event, a sawtooth is soiling recovering after cleaning
- The habit is one daily curve a month, four numbers recorded, an annual PR plot, and a yearly string comparison — about two hours a year
- Keep it in a spreadsheet outside the platform, and keep the notes column, because in year eight it is the only explanation of a step