First check what you are comparing
Output varies with the season, the time of day and cloud. Winter days in Gauteng are clear but short, with the sun lower. A cloudy week in summer can give less than a bright winter week. Compare the same month with last year, or look at your inverter's daily figures on a clear day around midday.
Dirt on the panels
Gauteng has a long dry season, and dust builds up. One industry guide puts typical soiling loss at about 5% in areas with long dry seasons, with extra where dust is frequent. Bird droppings are worse: they can block a cell and may not wash off in the rain.
Cleaning with clean water and a soft sponge or brush is fine if you can reach the panels safely from the ground. Do not climb onto a roof or use harsh chemicals. Panels should not be cleaned when hot.
Shade, even a little
Shade hurts more than its size suggests. A shaded cell reduces the current of the whole string it sits in. A new tree, a neighbour's building, an aerial, a vent pipe or a water tank can cast a moving shadow that costs you power at certain hours.
Watch the array through the day and note when shade appears. Share what you see with your installer.
Orientation, tilt and heat
Panels facing north, with a suitable tilt, usually give the best yearly output in South Africa. East and west roofs produce well in the morning and afternoon but peak lower. That is a design choice, not a fault.
Hot panels make less power. Output drops for each degree above 25 degrees Celsius, and panels laid flat on a hot roof run hotter.
String faults and inverter limits
Panels are wired in strings. If one panel, connector or bypass diode fails, the string can drop sharply, or an inverter input can show no power. Mismatched or damaged panels can also drag a string down. You cannot find this safely by touch. Panels produce DC voltage whenever there is light, so leave the cabling alone and let a qualified installer test it.
Some systems are designed with more panel power than the inverter can pass. On sunny middays the inverter caps the output. This is called clipping, and it can be deliberate. Your inverter may also reduce its output when it overheats.
Age and what to record
New panels lose a small amount in the first hours and then degrade slowly over the years. Manufacturer guarantees set the expected rate.
Before you call, record the date, the weather, the inverter's daily and peak figures and any warnings. Compare them with a clear day. A drop of a few percent is normal. A sudden large drop, or one string at zero, needs a test.
Also check whether your own habits changed. If the house now uses more power in the morning or evening, the panels may be producing as before while the grid or battery covers more of the load. The inverter app usually shows production and consumption separately, which makes this easy to see.
The job behind this: Repairs and servicing.