Why Is My Solar Output Dropping, and Can I Check It Myself?

The most expensive problem a solar system can have isn't the one that shuts it down — you notice that right away and call a technician. It's the one that makes it produce a little less, bit by bit, with nothing to warn you.

Check it yourself before calling a technician

Every inverter in common use today comes with an app that shows your generation history. It's the best tool you have for a first diagnosis.

1. 1Open the app and look at this month's daily graph. Compare it with the same month last year, if you have the data.
2. 2Only compare days that were sunny in both periods — don't compare a rainy day with a sunny one.
3. 3Look at the shape of the graph, not just the total, because the shape points to the cause.

Reading the shape of the graph

Causes, from most to least common

1. Dirty panels

The most common and the easiest to fix — especially bird droppings and algae that rain won't wash off. If the panels haven't been cleaned for over a year, start here.

2. New shade

Trees growing taller, a new building next door, even a newly installed satellite dish — all cast shade that wasn't there when the system was designed. This is often overlooked because the shadow lengthens a little each year.

3. An overheating inverter

Inverters reduce their own output when they get hotter than their rated limit, to protect themselves. If yours is installed somewhere enclosed, without airflow, or in direct sun, this happens only in the afternoon on very hot days. Sometimes the fix is simply moving it or adding a sun shade.

4. Loose connections or aging cables

A loose connection increases resistance, so output drops and heat builds up at that point. This is more dangerous than the other causes because it can overheat enough to cause damage. Have a technician check it — don't try to fix it yourself.

5. A few faulty panels

A panel with internal damage drags down the output of the whole string, because the cells are wired in series. Finding which panel it is requires measuring equipment, so this is a job for a technician.

6. Normal aging

Every brand of panel produces less over time, as stated in its performance warranty — but the decline is less than one percent a year. If output has dropped noticeably within a few years, this is almost never the reason.

When to call a technician

• Output is down more than 20% compared with the same period last year, with similar weather.
• The inverter shows a red light or a warning code that won't clear.
• The graph cuts out repeatedly, every day.
• You've cleaned the panels but the numbers haven't come back.
• You smell burning, see scorch marks or hear unusual noises from the control box — in that case, shut the system down and call a technician immediately.