Your system is producing less than last year: when to worry

Sunday morning, coffee, the monthly look at the app. And there it is: the month closed below last year's. The question arrives on its own: is something deteriorating?

Almost always the answer is no. But "almost always" is useless to you without a method for telling whether your case is the "almost" or the "always". Here it is, in order: first the free eliminations, then the clues, and at the end the phone call.

Before looking for culprits, check the yardstick:

Are you comparing the same month? June against June, not June against March. Generation changes a lot across the year and only the same month is comparable.

Was the weather similar? An exceptionally cloudy June generates less than a bright one, and nothing is broken. Weather years aren't identical to each other: normal variation can be several points, and a single month can deviate considerably more. Think back over the month that just passed. Did it rain for fifteen days? There's your answer.

How big is the deviation? A few points down in a mediocre-weather month: normal noise. 20 or 30% down with similar weather: keep reading. A zero, or nearly: go straight to step 3.

If the deviation is small and the weather explains it, you're done. A system is judged by the year, not the month.

A real, sustained deviation the weather doesn't explain. Before panicking or calling anyone, three checks you can do yourself:

Dirt. When were the panels last cleaned? Has there been a long drought, building work nearby, a harvest? Look at them: if they're clearly dirty, clean them (or have them cleaned) and compare the following week against the previous one. If generation recovers, case closed. And you've incidentally learned the real cleaning frequency for your area.

New shade. Shadows grow quietly: the tree added two metres, an aerial appeared, a building, a pergola. Shade has a recognisable signature in the app: the day's curve shows a dip always at the same hour, day after day. Compare today's clear-day curve against one from the same period last year: if a new trough appears, you've found your culprit.

A string of the system down. Sometimes it isn't that everything generates less: it's that one part generates nothing and the total disguises it. If your app shows detail per panel or per group, look for the one sitting at zero or notably below the rest. If it only shows the total, the clue is a step: generation was running normally and on one specific day dropped sharply and stayed there. Steps aren't weather. Weather doesn't create permanent steps. Something disconnected or failed that day.

And the one I rule out quickly: "the panels are wearing out". Degradation is real but extremely slow, on the order of a fraction of a point per year. From one year to the next it's imperceptible. If your system is three years old and generates 15% less, it isn't wear: it's one of the above.

Call the installer when:

• Generation is zero or near zero on sunny days. First check the inverter is on with no errors, and that no breaker is switched off. If something looks odd, all the better: you already have something to tell the technician.
• There's an identifiable step that persists.
• The deviation is large, sustained, and you've already ruled out dirt and shade.
• The inverter shows recurring errors or behaves differently.

And here everything you did right beforehand pays off: you arrive with data (since when, how much, the curve that shows it, what you've ruled out) and with your folder of warranties and serial numbers. That call takes ten minutes and ends in a visit with a probable diagnosis. The version without data, "hi, I think it's generating a bit low", ends in a blind visit: more expensive and slower.

• Did I compare the same month against the same month?
• Does the weather explain the deviation?
• Are the panels visibly dirty?
• Is there a new trough in the curve, always at the same hour?
• Is there a step from one specific day?
• If none of the above: phone call, with the data written down

The underlying lesson: a system isn't looked after by watching it anxiously. It's looked after with an honest monthly comparison and by knowing how to tell three things apart: the weather, which isn't a problem; the cheap stuff, which you solve yourself; and the real stuff, which gets called in on time. A deviation dealt with in month one is a repair. The same deviation discovered a year later is a year of lost generation. And that doesn't come back.