Your inverter or monitoring app records every hour your panels work. Read it on clear days, compare like with like, and it will tell you about dirt, shade and faults long before a bill does.
How do I read it?
Most apps from Enphase, SolarEdge and others offer the same three views.
Find clear days
Pick days with a smooth, rounded daily curve. Cloudy days are jagged and tell you little.
Set a baseline
Note the peak and the daily total for three clear days. That is your reference.
Compare same months
Hold this June against last June, not against December. The sun angle changes everything.
Scan each panel
With microinverters or optimizers, look for one panel or row darker than its neighbors.
Watch the trend
A slow slide over weeks of clear days points to film. A sudden drop points to a fault.
How low is too low?
A Solar Energy method from UTS, UNSW and Diagno Energy flags major underperformance when output stays below 60% of expected for three non-cloudy days, and minor below 80% for seven days. It was tested on 1,089 Australian systems. Use it at home: compare clear days with your baseline.
What do I do at 60%?
Below 60%, three days
Treat it as a fault. Call your installer; soiling rarely costs that much.
Below 80%, seven days
Look for film, new shade, a nest or one weak string. A visual inspection helps.
Above 80%, slight slide
Usually light soiling or the season. Run the output check before paying for a wash.
What does this pattern mean?
A flat top on a sunny curve
Clipping: the inverter has reached its limit on a bright day. Normal, and not a loss you can wash away.
A notch at the same hour daily
Shade from a chimney, tree or vent moving across the array. It shifts with the seasons.
One panel darker than its row
Something local: a dropping, a leaf pile, a crack or a failing microinverter. Look from the ground first. See where it matters
A whole row lower
Often dust or snow on the bottom edge, or a string issue. A wash fixes the first; your installer fixes the second.
Zero all day
A tripped breaker, a fault, or the app lost Wi-Fi. Check the app’s status page, then call your installer.
A slow slide over weeks
Classic soiling, especially in dry stretches from July to October. Rain may not clear it. See where it matters
A sudden drop after a storm
Possible hail damage or debris. Follow the hailstorm checklist from the ground. See where it matters
A hazy week, all panels down
Likely smoke. A Colorado State University study found transported wildfire smoke cut solar resources by only about 5%, more near active fires. See where it matters
Want someone reading it monthly?
Output watch reads your app with read-only access and flags slides and faults, from $12 a month.