Solar panels gradually lose power as their cells and protective packaging age outdoors. Some technologies also show early light- or heat-related degradation. But a drop in your system’s electricity output does not, on its own, prove that the panels have permanently degraded: weather, shading, dirt, downtime, damage, or equipment faults can also reduce generation.
What causes solar panels to lose power?
Gradual aging outdoors
Long-term exposure gradually changes module performance. The National Laboratory of the Rockies (NLR) says photovoltaic (PV) modules often lose less than 1% of performance per year. A gradual decline is generally identified as a trend under controlled test conditions, not inferred from one low-production day. It may be difficult to distinguish from measurement uncertainty in a system’s first operating years. NLR’s PV Lifetime Project overview describes typical module loss; its 2025 annual report, issued in 2026, describes periodic indoor current-voltage testing at standard test conditions (STC).
Early light- and heat-related effects
Some module technologies can experience early performance changes after light exposure, known as light-induced degradation (LID). Light- and elevated-temperature-induced degradation (LeTID) is another technology-dependent mechanism. In the 2025 report, NLR describes seasonal changes in tested modules consistent with LeTID susceptibility, including lower indoor measurements after prolonged high-temperature exposure and recovery during cooler periods. Results from particular tested samples should not be treated as a prediction for every panel.
Soiling, operating conditions, and downtime
Dirt and other deposits can block some light from reaching the module surface. Output also changes with available sunlight and temperature, as well as shading and whether the system is operating. NLR distinguishes soiling and AC system availability from intrinsic module DC degradation; these losses affect what a whole system delivers without necessarily indicating permanent cell aging.
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Damage or equipment faults
Physical damage and component problems can cause sharper or more sudden losses than normal aging. The U.S. Department of Energy’s Federal Energy Management Program (FEMP) identifies hot spots, cell cracks, water intrusion, and delamination as module conditions that may require repair. Inverter downtime and wiring faults can also affect system output. FEMP notes that “PV modules have no moving parts and require very little maintenance,” but this does not mean a PV system never needs inspection or repair. See its PV performance and longevity guidance.
How much efficiency do solar panels lose each year?
Published estimates vary because studies measure different things: individual modules or whole systems, over different periods, using different methods and samples. These figures are useful context, not a single guaranteed rate for a particular home.
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| Estimate | What it measures | Important qualification |
|---|---|---|
| Often less than 1% of performance per year | PV module performance | NLR’s current PV Lifetime Project overview; a general description, not a guarantee for every module. |
| 0.5% per year median | Reported degradation rates | NREL’s 2012 review of nearly 2,000 reported rates; study definitions and samples vary. |
| Around 0.75% per year median performance loss | Whole-system performance | DOE’s 2023 summary of NREL’s PV Fleet analysis using data from nearly 19,000 inverters. This is a fleet result, not a residential-module guarantee; the analysis reported slower degradation in cooler climates. |
| Less than 0.5% per year maximum-power decline for most units | Modules in an outdoor study | NREL’s study of a limited sample of 12 mono- and polycrystalline modules installed at NREL; it does not represent the entire market. |
These results are not a controlled head-to-head comparison. For example, NLR’s 2025 work measures module DC loss using periodic indoor STC tests, while the PV Fleet estimate concerns system performance. System-level figures can include losses beyond module aging, such as soiling and downtime. The relevant sources are NLR’s project overview, NREL’s 2012 degradation-rate review, DOE’s 2023 PV Fleet summary, and NREL’s outdoor module study.
How can you tell whether your system is underperforming?
Field generation is not a direct measurement of intrinsic panel efficiency. To interpret it, compare production with what would be expected under comparable sunlight and temperature conditions, and account for whether equipment was available to run.
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- Look for a sustained trend. Compare meaningful periods, such as month to month or year to year, rather than treating a cloudy day or a seasonal dip as proof of aging.
- Check for changed conditions. Consider sunlight, temperature, shading, soiling, and system downtime. These can affect expected output without permanent module degradation.
- Compare actual and modeled production. DOE FEMP defines the performance ratio as actual production divided by modeled production based on the same measured solar resource and temperature. Also check system availability—the share of time the system is operational and able to deliver power—and any recorded outages.
- Review monitoring alerts. Persistent low output, inverter alerts, or outages can help narrow down whether the issue is system operation rather than gradual panel aging.
- Escalate persistent problems safely. If performance remains low, have a qualified service provider assess possible module, inverter, or wiring problems. Check your system documentation, warranties, and manufacturer maintenance instructions before attempting cleaning or repairs or buying parts.
FEMP’s O&M guidance covers performance monitoring, availability, maintenance schedules, records, warranties, and diagnostics. It provides general guidance, not a remote diagnosis of an individual array.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Can cleaning restore lost solar-panel efficiency?
Cleaning may help when removable soiling is blocking light, but it cannot reverse intrinsic cell aging or repair a damaged module. FEMP includes routine cleaning in PV operations and maintenance plans. Whether cleaning is useful depends on whether deposits are present and can be addressed safely; follow the manufacturer’s instructions and avoid attempting repairs yourself.
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