Solar panels normally continue producing electricity for many years, but output gradually changes and a lower annual total does not automatically mean the panels are degrading. This guide explains how to track solar panel performance in the UK, compare it with weather-adjusted expectations, interpret degradation warranties and decide whether maintenance, repair or replacement is the sensible next step.
Overview
Solar panel degradation is the gradual reduction in a panel’s ability to convert sunlight into electricity. It is usually measured as a percentage loss per year after the first year, although the warranty may describe the first-year reduction separately. The result is normally a slow change rather than a sudden failure.
A solar panel’s lifespan and its useful economic life are not exactly the same. Panels may still generate electricity after their warranty period, but their output, the condition of the mounting system, the age of the inverter and the cost of remedial work all affect whether continued operation makes financial sense.
In the UK, annual generation varies naturally. Cloud cover, daylight hours, high temperatures, shading, soiling, snow, export limitations and changes in household electricity use can all affect the figures shown in a solar monitoring app. A year-on-year comparison is therefore most useful when it is based on the same reporting period and interpreted alongside weather and system records.
Do not treat a manufacturer’s degradation warranty as a promise that every panel will produce the same amount of energy each year. It is a performance threshold under stated test and claim conditions. Keep the panel datasheet, warranty document, installation records and monitoring history together so that you can check the relevant terms if performance becomes a concern.
What to track
1. Annual and monthly generation
Record the electricity generated by the solar array, preferably from the inverter or an independent meter rather than relying only on an export reading. Save monthly figures as well as the annual total. Monthly records help identify when a fault began; annual totals are better for smoothing out seasonal variation.
Use a consistent measurement point. If the monitoring platform changes from inverter output to electricity exported to the grid, the figures are no longer directly comparable because exported energy excludes electricity used in the property or stored in a battery.
2. Weather and operating conditions
Note unusual conditions that could explain a change: prolonged cloud, nearby construction, new trees, roof work, persistent bird fouling, heavy soiling or a change in the roof’s shading pattern. If possible, compare your output with a reliable local or regional solar resource record, but use it as context rather than as a precise guarantee of what your roof should produce.
3. Inverter and panel-level data
Check whether your system reports array-level or panel-level performance. Panel-level monitoring can help identify one underperforming module, a disconnected optimiser or a problem affecting one roof section. Array-level monitoring is still useful, but it may hide a problem if other panels continue operating normally.
Also record inverter alarms, fault codes, periods when the system was offline and any firmware or configuration changes. A failing or ageing inverter can look like solar panel degradation, particularly if the drop in production is sudden.
4. Warranty information
Make a simple note of:
- the panel manufacturer and exact model;
- the warranty period for the product itself;
- the first-year performance allowance, if stated;
- the maximum annual degradation rate after the first year;
- the guaranteed output at the end of the warranty term;
- the evidence required for a claim and who is responsible for inspection or removal.
Warranty wording differs between product ranges. Compare the actual documents supplied with your panels rather than assuming that two panels with similar headline specifications have identical degradation terms.
Indicative degradation-rate comparison
The table below provides a screening framework, not a product ranking or a guarantee. The figure that applies to your system is the rate stated by the manufacturer for the exact panel model.
| Stated annual degradation after year one | How to view it | What to check |
|---|---|---|
| Lower than 0.3% | A comparatively strong warranty allowance on paper | Whether the first-year allowance, end-of-term output and test conditions are clearly defined |
| 0.3% to 0.5% | A common comparison band for product research | Whether the wider warranty, installer support and system design offer good value |
| Higher than 0.5% | Not necessarily unsuitable, but worth examining carefully | Panel age, purchase price, warranty claim process and expected period of ownership |
Cadence and checkpoints
A quarterly review is usually frequent enough for routine tracking. Check the monitoring app for missing data, unusual fault messages and major changes in generation. A full annual review is more useful for judging degradation because it includes a complete seasonal cycle.
Monthly or quarterly
- Confirm that the monitoring system is reporting normally.
- Look for a sudden drop rather than focusing on one low-generation day.
- Check inverter alarms, communication failures and periods of downtime.
- Visually inspect from ground level for obvious damage, new shading or heavy soiling.
Annually
Choose a fixed review date and compare the previous 12 months with earlier complete years. Record array generation, estimated irradiation or weather context, downtime, maintenance, shading changes and any battery or inverter alterations. If the roof has multiple orientations, review each array separately where the monitoring system allows it.
A useful annual record can follow this format:
| Period | Generation | Weather or shading notes | Faults or downtime | Action |
|---|---|---|---|---|
| January–December, year | ___ kWh | ___ | ___ | None / investigate / service |
| January–December, year | ___ kWh | ___ | ___ | None / investigate / service |
Before a warranty deadline
Start checking records well before a performance warranty expires. Gather invoices, commissioning documents, serial numbers, monitoring screenshots and any service reports. A claim may require evidence that the panels were installed, operated and maintained in line with the warranty conditions.
How to interpret changes
A gradual reduction that broadly follows the manufacturer’s stated allowance may be normal. A sudden or uneven reduction deserves investigation before anyone concludes that the panels have degraded.
- Check the data first. Confirm that the app is showing generation rather than export, that the meter has not been replaced and that there are no missing periods.
- Look for external causes. New shading from trees, chimneys or neighbouring development can reduce output without any panel defect. Soiling and physical obstruction can have a similar effect.
- Separate panel and inverter issues. A system-wide shutdown or repeated inverter fault points in a different direction from one low-performing panel. Ask a suitably qualified installer to test the system rather than opening electrical equipment yourself.
- Compare like with like. Use the same roof area, monitoring point and time period. A new battery, export control setting or inverter replacement may change recorded figures even when panel performance has not changed.
- Check for uneven output. If panel-level monitoring shows one module or string consistently below its neighbours under similar conditions, investigate connectors, optimisers, shading and the panel itself.
Replacement is more credible when there is a persistent, documented performance shortfall, a confirmed panel fault, extensive physical damage or a repair cost that is disproportionate to the remaining output. In many cases, cleaning, shade management, a connector repair or inverter work is more appropriate than replacing an otherwise serviceable array.
When comparing replacement options, assess the whole system. Check roof condition, mounting compatibility, inverter capacity, cable routes, generation limits, battery integration and any required approvals. A larger or higher-output array may require a revised design rather than a simple panel-for-panel exchange. For related decisions, see the UK compatibility checklist for adding a battery or EV charger and the guide to choosing a hybrid inverter.
When to revisit
Use this guide at least once a year, ideally after a complete generation year has been recorded. Revisit it sooner after roof work, tree growth, storm damage, an inverter replacement, a monitoring-system change or a noticeable fall in output. It is also worth reviewing before a panel or performance warranty deadline and when considering battery storage, an EV charger or a wider home energy upgrade.
Before deciding to repair or replace, complete this checklist:
- Have you saved at least one complete year of generation data?
- Are you comparing generation, not export, with earlier records?
- Have you checked shading, soiling, physical damage and monitoring errors?
- Have you reviewed inverter alarms and panel-level data, where available?
- Have you located the exact panel degradation warranty and claim requirements?
- Has a competent installer tested the system before replacement is considered?
- Would a repair, inverter upgrade or system extension solve the problem more effectively?
Keep the completed record with your solar installation documents. A consistent history turns a vague concern about solar panels losing efficiency into a measurable decision about maintenance, warranty action or replacement.
