Solar panels lose about 0.7% of their power output per year, according to baseline figures from the National Renewable Energy Laboratory. That means after 25 years, a typical panel still produces roughly 80% of its original capacity. But that steady decline isn’t caused by one thing. It’s the result of several overlapping processes, some triggered by sunlight itself, others by heat, moisture, voltage, and physical stress.
Light-Induced Degradation
The most immediate form of degradation happens within the first hours and days of operation. When sunlight first hits a silicon solar cell, it activates defects in the material, particularly involving boron and oxygen atoms in the silicon crystal. These defects act as tiny traps that capture electrical charge before it can be collected as usable power. This process, called light-induced degradation (LID), typically causes a quick initial drop in efficiency that then stabilizes.
A related but slower process occurs when panels operate under both light and heat simultaneously. This form of degradation can reduce the relative performance of multicrystalline silicon cells by up to 16%, and it affects nearly all types of silicon wafers to some degree. Even newer cell designs experience losses up to about 1%. The root cause appears to be linked to hydrogen atoms and trace metal impurities (like titanium, nickel, and cobalt) embedded in the silicon during manufacturing. During high-temperature processing steps, a protective film on the cell releases hydrogen into the silicon bulk, where it interacts with these impurities to create performance-sapping defects. Thinner wafers below 120 micrometers seem to resist this effect, likely because there’s simply less bulk material for these defects to form in.
UV Damage to the Encapsulant
Solar cells are sandwiched between layers of a clear plastic encapsulant, most commonly ethylene-vinyl acetate (EVA). Over years of UV exposure, this material undergoes chemical changes. The UV-blocking additive mixed into the EVA gradually breaks down and disappears entirely. Without that protection, the encapsulant itself starts forming chains of carbon double bonds that act like a tint, turning the once-clear plastic yellow and eventually brown.
The impact on performance is dramatic. A light yellow-brown encapsulant reduces cell efficiency by about 9%. A dark brown encapsulant, the kind seen on severely aged panels, cuts efficiency by roughly 50%. The degraded EVA also releases acetic acid and other volatile compounds, which can accelerate corrosion of the metal contacts inside the module.
Moisture Ingress and Internal Corrosion
Moisture is at the core of most degradation mechanisms in solar panels. Water vapor slowly works its way through the panel’s backsheet and edges over time, reaching the encapsulant and the delicate metal components inside. Once there, it triggers a cascade of problems: corrosion of the thin metal grid lines that collect electricity from each cell, delamination where internal layers separate from each other, discoloration of the encapsulant, and adhesion losses between layers. Panels with breathable backsheets are designed to let trapped moisture escape, but non-breathable designs can trap humidity inside, accelerating damage.
Voltage Leakage Between Cells and Frame
In a solar array, panels are wired together in strings that can reach high voltages, sometimes 600 to 1,500 volts relative to the grounded metal frame. This voltage difference can push sodium ions from the glass into the solar cells, creating electrical short circuits within the cell. This process, known as potential-induced degradation (PID), comes in three forms: shunting (where ions create pathways that bypass the cell’s electrical junction), polarization (where charge builds up and reduces voltage), and corrosion (where the leakage current chemically attacks cell materials). PID can cause rapid, severe power losses but is largely preventable through module design choices and proper grounding configurations.
Thermal Cycling and Solder Fatigue
Every day, solar panels heat up and cool down. Over thousands of cycles, the different materials inside the panel, including silicon wafers, silver contact strips, copper ribbon connectors, and solder joints, expand and contract at different rates. This mismatch in thermal expansion creates mechanical stress at the joints where these materials meet. The solder absorbs this stress through tiny amounts of deformation each cycle, gradually accumulating fatigue damage in the form of creep strain. Eventually, cracks form in the solder joints, increasing electrical resistance or breaking connections entirely. This is one of the primary reasons older panels develop “hot spots,” areas where poor electrical contact generates excess heat instead of electricity.
Microcracks From Mechanical Stress
Silicon solar cells are fragile. Manufacturers have reduced wafer thickness from 300 micrometers to as thin as 100 micrometers to cut costs, making cells increasingly prone to cracking. Microcracks can form at almost any stage: during soldering (from thermal stress at the junction of silicon and metal), during transport, and during operation from wind loads, snow weight, or hail impacts.
These cracks aren’t always visible or immediately harmful. A hairline fracture might not affect performance at all initially. But over time, thermal cycling and moisture can widen cracks, isolating sections of a cell from the electrical circuit. Depending on how the module is wired internally, a crack that disconnects even a small area of one cell can reduce the output of an entire string of cells within the panel, causing power losses far larger than the cracked area alone would suggest. Cracks that form during lamination can also serve as starting points for further fracture under stress.
What This Means for Panel Lifespan
Despite all these degradation pathways, modern panels are engineered to last decades. Most manufacturers guarantee at least 80% of original power output at 25 years, with some extending warranties to 30 or even 40 years. A few premium brands guarantee over 92% output at the 25-year mark. The 0.7% per year average degradation rate is a planning figure, not a death sentence. NREL projects that ongoing improvements could bring that rate down to 0.5% or even 0.2% per year by 2035.
The panels on your roof are being slowly worn down by all of these forces at once. Sunlight reshuffles atoms in the silicon, UV rays yellow the encapsulant, moisture creeps in through the edges, temperature swings fatigue the solder, and occasional hailstorms test the structural limits of the cells. No single factor dominates in every installation. A panel in a hot, humid climate will degrade differently than one in a cold, dry region. But understanding these causes helps explain why output dips over the years, and why panel quality, installation choices, and local climate all influence how gracefully your system ages.