<!– –>{ “@context”: “https://schema.org”, “@graph”: [ { “@type”: “Article”, “headline”: “Solar System Underperforming — Low Output Causes and How to Diagnose Them”, “description”: “Your solar system is generating, but not as much as it should. This guide covers shading, panel degradation, inverter efficiency losses, CT clamp issues, and how to assess whether your system is actually underperforming.”, “url”: “https://solar-tech-support.co.uk/problems/system-underperforming/”, “datePublished”: “2026-03-18”, “dateModified”: “2026-03-18”, “author”: { “@type”: “Organization”, “name”: “STS Solar Tech Support”, “url”: “https://solar-tech-support.co.uk” }, “publisher”: { “@type”: “Organization”, “name”: “STS Solar Tech Support”, “url”: “https://solar-tech-support.co.uk” } }, { “@type”: “HowTo”, “name”: “How to assess whether your solar system is underperforming”, “description”: “A step-by-step method to measure actual vs expected generation, identify the cause of underperformance, and determine whether investigation is worthwhile.”, “url”: “https://solar-tech-support.co.uk/problems/system-underperforming/”, “step”: [ { “@type”: “HowToStep”, “name”: “Establish a realistic expected output figure”, “text”: “Before concluding the system is underperforming, establish what it should actually produce. Use the PVGIS tool (pvgis.ec.europa.eu) with your system’s panel size, orientation, and tilt angle to calculate expected annual yield. UK solar systems typically produce 850–1,000 kWh per kWp per year depending on location and orientation. A 4kWp south-facing system in the Midlands should produce around 3,400–3,800 kWh/year.” }, { “@type”: “HowToStep”, “name”: “Compare actual generation to expected using monitoring data”, “text”: “Download or view your annual generation history from the monitoring portal. Compare total kWh generated against the PVGIS estimate. A system generating more than 80% of the PVGIS estimate is broadly within normal range. If generation is significantly below 80% — particularly if it was higher in previous years and has declined — there is likely a genuine fault.” }, { “@type”: “HowToStep”, “name”: “Check for shading that wasn’t present at installation”, “text”: “Trees, new buildings, extensions, and satellite dishes can create new shading that didn’t exist when the system was installed. Even partial shading on a single panel in a string system significantly reduces output from all panels in that string. Walk around the property at different times of day to check whether any new obstruction is casting shadow onto the panels.” }, { “@type”: “HowToStep”, “name”: “Check for a failed string or inverter MPPT channel”, “text”: “Multi-string inverters have separate MPPT (Maximum Power Point Tracking) channels for each string. If one channel has failed or a string has an open circuit, half the system stops contributing. In the monitoring portal, check whether string voltages and currents are balanced. An imbalance where one string shows significantly lower current than others indicates a string-level fault.” }, { “@type”: “HowToStep”, “name”: “Check whether export limiting is reducing apparent generation”, “text”: “If your system has a DNO export limit applied, the inverter will curtail generation whenever the house consumption plus battery is not sufficient to absorb all the solar power. This doesn’t reduce actual generation — it reduces exported generation. Check whether the portal shows curtailment events, or whether the generation figure on sunny low-consumption days appears to hit a ceiling at the export limit value.” } ] }, { “@type”: “FAQPage”, “mainEntity”: [ { “@type”: “Question”, “name”: “How do I know if my solar system is underperforming?”, “acceptedAnswer”: { “@type”: “Answer”, “text”: “Compare your actual annual generation (from the monitoring portal) against the expected yield from PVGIS, using your system’s panel size, orientation, and tilt. A system generating below 75–80% of its PVGIS estimate warrants investigation. Also look at whether generation has declined year-on-year — some decline is normal (panel degradation of 0.5–0.7% per year) but a sharp drop suggests a fault.” } }, { “@type”: “Question”, “name”: “What is normal panel degradation?”, “acceptedAnswer”: { “@type”: “Answer”, “text”: “Most modern solar panels degrade at around 0.5–0.7% per year. A system that is 10 years old might produce 5–7% less than when new — this is within the manufacturer’s warranty performance guarantee. Degradation above 1% per year, or a sudden drop in output, is beyond normal and should be investigated.” } }, { “@type”: “Question”, “name”: “Can a dirty panel significantly reduce output?”, “acceptedAnswer”: { “@type”: “Answer”, “text”: “In typical UK conditions, soiling loss from rain-washed panels is relatively minor — usually 2–5% annually. In unusual situations (near a construction site, bird roost, or under trees) contamination can be much higher. Professional cleaning is cost-effective if panels are visibly soiled, but cleaning alone rarely resolves significant underperformance — check for faults first.” } }, { “@type”: “Question”, “name”: “What is an MPPT fault?”, “acceptedAnswer”: { “@type”: “Answer”, “text”: “MPPT (Maximum Power Point Tracking) is the algorithm inverters use to extract maximum power from each solar string. Most multi-string inverters have two or more MPPT channels, one per string. An MPPT channel failure means one string stops contributing to generation — typically halving output on a two-string system. The monitoring portal will usually show one string with near-zero current.” } }, { “@type”: “Question”, “name”: “My system hasn’t met its predicted output since installation — is that a fault?”, “acceptedAnswer”: { “@type”: “Answer”, “text”: “Not necessarily. Installer predictions are often made using optimistic irradiance assumptions. Check the PVGIS estimate for your specific postcode, orientation, and tilt — this is a more accurate benchmark than most installer projections. If the PVGIS estimate and your actual generation are broadly aligned, the system is performing correctly even if it doesn’t match the installer’s sales figure.” } } ] }, { “@type”: “BreadcrumbList”, “itemListElement”: [ { “@type”: “ListItem”, “position”: 1, “name”: “Home”, “item”: “https://solar-tech-support.co.uk/” }, { “@type”: “ListItem”, “position”: 2, “name”: “Problems”, “item”: “https://solar-tech-support.co.uk/problems/” }, { “@type”: “ListItem”, “position”: 3, “name”: “System Underperforming”, “item”: “https://solar-tech-support.co.uk/problems/system-underperforming/” } ] } ]}
Performance fault · All setups
Solar system underperforming
The system is generating — but not as much as it should be. Output has dropped, or it never produced what you were told to expect. Before calling an engineer, it is worth establishing whether the system is actually underperforming against a realistic benchmark, or whether the expectations were wrong to begin with.
This guide helps you measure it properly — then identify the cause if there is a genuine shortfall.
PVGIS gives the honest benchmarkString faults are diagnosable remotelyNew shading is a common hidden cause
Confirmed underperformance against PVGIS?
A solar system health check includes a full generation audit, string performance analysis, and a written report identifying the cause and quantifying the generation loss.
Installer projections are often optimistic. The honest benchmark is PVGIS — a tool from the EU Joint Research Centre that calculates expected solar yield for any location in Europe using decades of irradiance data.
Go to pvgis.ec.europa.eu, enter your postcode, set your panel rated power (in kWp), orientation (azimuth — 0° is south), and tilt angle. The tool gives you an annual kWh estimate that accounts for your specific location and mounting.
As a rough guide, UK systems produce approximately:
950–1,000
kWh/kWp — South England, south-facing, 30–35° tilt
A system generating 80–100% of its PVGIS estimate is performing well. Below 75% warrants investigation.
Common causes of genuine underperformance
If the system is confirmed below 75–80% of PVGIS, check these.
New shading
Trees that have grown, a new extension, a neighbouring building, or a satellite dish. Even a small shadow on a single panel in a string system can halve that string’s output through the “Christmas lights effect”.
Failed string or MPPT channel
One inverter MPPT channel or string has failed — from a blown fuse, open-circuit cable, or inverter fault. On a two-string system this typically reduces generation by around 50%.
Panel fault or degradation
Individual panel failure (from hotspot damage, delamination, or PID degradation) reducing output below the expected rate. More common on systems over 8 years old.
Export limiting curtailment
A DNO export limit is actively curtailing generation on sunny low-load days. The system isn’t underperforming — it is deliberately limited. Check whether the generation figure hits a ceiling on sunny days.
Inverter efficiency degradation
An ageing inverter operating below its rated efficiency. More common on string inverters over 10 years old where capacitors and switching components have degraded.
Soiling
Heavy panel contamination from bird droppings, dust from construction, or a nearby industrial source. Rarely the primary cause in the UK — but worth checking visually if significant soiling is visible from the ground.
How to identify the cause
Using monitoring data to narrow it down.
1
Calculate PVGIS estimate and compare
Run your system through PVGIS and compare the estimate against your last 12 months of generation data from the portal. If the shortfall is above 20%, the investigation is worthwhile. If it is within 15–20%, check whether the PVGIS inputs accurately reflect your actual orientation and shading.
2
Check string voltage and current data
In the monitoring portal, check whether your inverter shows separate data for each string or MPPT channel. On a sunny day both strings should show similar current levels. If one string shows near-zero current while the other is normal, you have a string fault — blown fuse, open circuit cable, or disconnected panel.
3
Look for a generation ceiling on sunny days
On the sunniest days in summer, does the generation figure plateau at a specific number — say 3.0kW on a 5kWp system — rather than climbing towards peak rated output? If yes, check whether an export limit is set at that figure in your inverter configuration.
4
Walk around and check for new shading
Go outside at solar noon (midday in winter, 1–2pm BST in summer) and look at the panels from the ground. Check whether any trees, structures, or objects are now casting shadows onto the array. Even a shadow on one panel can drag down an entire string.
5
Compare year-on-year generation totals
If the monitoring portal shows multiple years of history, compare annual totals year-on-year. A gradual decline of 0.5–1% per year is normal degradation. A drop of 5% or more in a single year indicates a fault event — check the event log for that year for fault codes or equipment changes.
Brand-specific underperformance checks
Each monitoring platform exposes generation data differently. Use the note for your brand to find the right figures for your PVGIS comparison.
GivEnergy
GivEnergy portals show daily PV generation on the Generation page. Compare the daily kWh figure against your PVGIS estimate for the same calendar month. Note that some older firmware versions report generation from the CT clamp rather than the PV input — if your daily figures fluctuate wildly with load, check which measurement source is configured. See the GivEnergy hub for portal navigation help.
Growatt
Growatt’s ShineServer portal provides per-MPPT yield data for multi-string hybrid systems. Navigate to Device > Details to compare MPPT1 and MPPT2 daily yield — a significant divergence between strings isolates the underperformance to one set of panels or one MPPT channel, making diagnosis much faster.
Sunsynk / Deye
Use the Sunsynk Connect app History view to compare daily yield across recent weeks. If a single week shows a step-change drop with no weather explanation, correlate with the inverter event log for that date — a firmware update or grid event on that day often explains the change. Sunsynk inverters can also experience MPPT derating in partial shade conditions; a soft reset of MPPT tracking can help.
Compare your actual annual generation against the PVGIS estimate for your location, panel size, orientation, and tilt. A system generating below 75–80% of PVGIS warrants investigation. Also look at year-on-year trends — a sudden drop in output compared to previous years suggests a fault rather than normal degradation.
Most modern panels degrade at 0.5–0.7% per year. A 10-year-old system might produce 5–7% less than when new — this is within warranty performance. Degradation above 1% per year, or a sudden drop, is beyond normal and should be investigated.
In typical UK conditions, soiling loss is usually 2–5% per year on rain-washed panels. Near construction sites, bird roosts, or under trees it can be higher. Cleaning alone rarely explains major underperformance — check for faults first.
Not necessarily. Check your installer’s projection against the PVGIS estimate — if PVGIS agrees with your actual generation, the system is likely performing correctly and the installer’s figure was optimistic. If PVGIS is significantly above actual generation, there may be a genuine fault or shading issue that existed from installation.
MPPT (Maximum Power Point Tracking) is how inverters extract maximum power from each solar string. A failed MPPT channel means one string stops contributing — typically halving output on a two-string system. The portal will show one string with near-zero current.
Confirmed underperformance — get it investigated
A solar system health check includes a full generation audit against PVGIS, string-level performance analysis, and a written report identifying causes and quantifying the generation loss in kWh and £ per year.
