“Underperforming” gets used loosely when it comes to solar panels, and that’s part of the problem. Some drops in generation are completely normal and nothing to worry about. Others are a genuine fault that’s costing you real money the longer it goes unaddressed. Telling the two apart starts with knowing what your system was actually supposed to generate in the first place.
How do you know what “should” even looks like?
This is the part most homeowners skip entirely. As part of the MCS installation standard, your installer is required to provide an estimated annual generation figure at handover, calculated from location-specific irradiance data rather than a rough guess. That figure, based on your roof’s orientation, pitch, and postcode, is your actual benchmark, not a number you remember reading somewhere online or a friend’s system in a different part of the country.
MCS’s own guidance recommends reviewing your monitoring platform annually against that original estimate and contacting your installer if generation is significantly below it. If you’ve never been given that figure, or you’ve lost track of it, that’s worth chasing down before anything else, because without it, “underperforming” is really just a guess.
It’s also worth comparing like with like when you do this. A fair comparison looks at the same month or season across different years, or your actual generation against the estimate adjusted for the time of year, rather than comparing this month’s figure to last month’s and assuming any drop is a fault. Weather varies meaningfully year to year too, a cloudier than average summer will show up as lower generation on a perfectly healthy system, which is exactly why a single unusual reading isn’t cause for alarm on its own, but a sustained, unexplained gap against your benchmark is worth looking into properly.
Things that look like a problem but usually aren’t
Before assuming a fault, it’s worth ruling out a few things that are completely normal and get mistaken for a problem more often than genuine faults do. Seasonal variation is the big one, generation naturally drops through autumn and winter as daylight hours shrink, and comparing a December reading to a June one will always look alarming even on a perfectly healthy system.
Inverter clipping is another one worth understanding, because it sounds like a fault but usually isn’t. Most systems are deliberately designed with a DC to AC ratio somewhere around 1.2 to 1.3, meaning the panels can technically produce more DC power than the inverter is rated to convert to AC at its absolute peak. On the very brightest days, the inverter caps, or “clips”, that excess rather than converting all of it. According to Solar Power World’s explainer on inverter clipping and DC to AC ratios, this is standard, deliberate system design, not a malfunction, and the resulting annual loss is typically small, often just one to three percent of total output, far less than the headline numbers might suggest.
Extreme heat is a similarly counterintuitive one. It feels logical that the hottest, sunniest days would produce the most electricity, but solar cells actually become slightly less efficient as their temperature rises, so a very hot, still summer afternoon can sometimes generate a touch less than a bright but cooler day earlier in the year. It’s a small effect, but worth knowing about before assuming something’s wrong simply because a scorching August day didn’t beat your best spring reading.
Things that genuinely are worth investigating
That said, plenty of causes behind reduced generation are real and worth chasing down. New shading from trees or a neighbouring extension that’s grown up since installation, soiling that’s built up faster than usual, a single failed panel or bypass diode dragging down an entire string on a traditional string inverter system, or a loose or corroded connection are all genuine, fixable problems rather than normal variation.
It’s also worth checking the monitoring itself before assuming the panels are the problem. Occasionally the fault is in the meter or monitoring sensor rather than the generation, meaning the panels are performing fine but you’re not actually seeing accurate figures. Ruling that out early can save a lot of unnecessary worry, or conversely, catch a genuine issue that a faulty monitor has been masking.
How the fault presents also depends on your system’s design. On a traditional string inverter setup, a single failed panel or a shaded section can drag down the output of the whole string it’s wired into, sometimes making the problem look bigger than it actually is. On a system using power optimisers or microinverters, that same fault tends to stay contained to the affected panel, showing up clearly in the monitoring data as one underperforming unit among otherwise healthy ones, which is often easier to spot and considerably quicker to diagnose.
How a professional actually diagnoses a real fault
A proper diagnosis goes well beyond a visual inspection or comparing two numbers on an app. Trained installers use dedicated equipment such as an I-V curve tracer, which, as explained by test equipment manufacturer Fluke, sweeps a panel or string through its full range of electrical conditions and produces a detailed curve that can reveal cell damage, a failed bypass diode, connection resistance, or panel mismatch that a simple voltage check would miss entirely. Thermal imaging is often used alongside this, picking up hotspots on individual cells or connections that indicate a developing fault before it becomes visible or measurable through generation figures alone.
This kind of testing is precisely why a proper diagnosis, rather than a guess based on how the panels look from the ground, matters. Two systems can show an identical drop in generation for completely different underlying reasons, and only proper testing tells you which one you’re actually dealing with.
Why a qualified, insured electrician matters here
GU Solutions is a NICEIC Approved Contractor and Domestic Installer, and a genuine underperformance investigation from us starts with your original MCS estimate, not a generic assumption, and uses the same kind of proper diagnostic testing described above rather than a quick look and a shrug.
What to check before booking a diagnosis
Before booking an investigation, dig out your original MCS handover documentation if you still have it, since your installer’s estimated generation figure is the single most useful piece of information for a proper comparison. Ask whoever you call whether they’ll be comparing your generation against that original estimate, adjusted for the time of year, rather than an arbitrary average. And ask what diagnostic equipment they’ll actually use, since a visual check alone won’t catch most of the faults that cause a genuine, meaningful drop in output. If you can’t find your original documentation, don’t let that stop you getting a proper investigation, a good installer can still work out a reasonable expected figure for your roof using the same irradiance data your original installer would have used, even without the paperwork to hand.
Getting a proper answer, not a guess
A drop in generation isn’t automatically bad news, and it isn’t automatically nothing to worry about either, for homes across Basingstoke, Old Basing, Chineham, Oakley and Sherborne St John just as much as anywhere else. The only way to know which one you’re dealing with is a proper comparison against your actual benchmark, followed by real diagnostic testing if something doesn’t add up.
If your system’s output doesn’t look right, GU Solutions is the first port of call. As NICEIC Approved Contractors and Domestic Installers, we’ll investigate it properly rather than guessing, and tell you honestly whether it’s a fault worth fixing or simply normal variation.
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