Solar output dropped? Check the glass before you blame the system
Before you call the installer, work the list in order: compare your monitoring app against the same month last year, rule out new shading, check the inverter for fault codes — then look at the glass. Soiling typically costs about 5% of annual output, up to 25% in the dirtiest locations, according to NREL, and it is the cheapest fault on the list to fix.
Start with the app, not the ladder
Open the monitoring app before you touch anything else. Pull up this month's generation and set it against the same month last year — not last month, because solar output falls every autumn on schedule and rises every spring, and comparing April to January proves nothing. Cloudy fortnights happen too, so read the trend across several weeks rather than one bad Tuesday. What you want is a baseline: a like-for-like number that tells you whether the system is genuinely down, and by roughly how much.
The shape of the decline is half the diagnosis. A cliff — normal output one day, a fraction of it the next — points at hardware: an inverter fault, a tripped breaker, a failed string. A slow slide over months points at something accumulating: a tree putting on growth, or grime putting on layers. Write the percentage down. You will want it later, because it tells you whether whatever you eventually fix actually worked.
Rule out shade and the inverter next
Shading comes second because it costs nothing to check and is easy to miss. Look at the array from the street at different hours and ask what has changed since installation. Trees put on a metre a year. A neighbour's extension, a new aerial, even a pergola can throw a shadow that did not exist two summers ago. Winter sun sits lower, too — a shadow that clears the panels in December can sit across them all morning in June.
Then the inverter. Check the display or the app for fault codes, and compare strings against each other if your monitoring shows them separately — one string well below its twin is a wiring or hardware conversation, not a cleaning one. Inverter faults are a job for your installer or a solar electrician, and no amount of washing the glass will move that number. If the codes are clear and the shade has not changed, keep going down the list.
Then look at the glass
Which brings you to soiling — the film of dust, salt, pollen and bird mess that builds on the panels. It is the least dramatic item on the list and the most common. The US National Renewable Energy Laboratory, which runs soiling research across hundreds of monitored sites, puts the typical loss at about 5% of annual output, rising to 25% in the dirtiest locations. That range comfortably covers most of the mystery declines that get blamed on tired panels.
Sydney gives the film a head start. Salt drifts in off the harbour and dries to a haze the coastal suburbs know well. Bird mess bakes on through summer. Rain rinses the loose dust and misleads you into thinking the job is done — it does not shift the baked-on layer, and it leaves mineral spotting of its own behind. From the ground the panels look fine. From the roof, the film is obvious.
The bottom-edge strip does outsized damage
Look closely at the bottom edge of each panel — or ask whoever goes up to. Water runs down the glass, pools against the frame and dries there, leaving a concentrated band of grime a few centimetres wide. It looks trivial. It is not, because panels are wired in strings, and a soiled band across one edge can drag output well beyond its share of the surface area.
This is why a system can measure noticeably down while the panels look clean from the driveway. The dirty strip hides behind the frame line, below your sight angle, doing quiet damage across every panel in the row. It is also why a proper solar clean works the edges and checks every panel afterwards, rather than giving the middles a rinse and calling it done.
Why the clean goes first on the invoice
Run the list in order and the economics sort themselves out. The app costs nothing. Checking shade costs nothing. Reading a fault code costs nothing. If all three come back clear, the glass is the remaining suspect — and a professional clean costs a fraction of a solar electrician's call-out to diagnose. If the clean restores your baseline, you have your answer. If it does not, you have eliminated the cheapest variable and can ring the installer with actual data.
Schmick cleans panels the way the glass wants: soft brush, purified water, no pressure, no chemicals, every panel checked — edges included. Solar work is priced per job on size, pitch and access, and the instant quote returns a firm, bookable price in about 60 seconds. Keep your before-and-after numbers from the monitoring app. If the output comes back, the glass was your problem. If it does not, we will be the first to say so.