What is a hot spot?
A hot spot is a localised area of a solar cell or module that reaches significantly higher temperatures than the surrounding material — sometimes 30–80°C above the rest of the panel. Because solar panels are made of semiconductor material, this thermal stress degrades the cell's crystalline structure, accelerates glass delamination, and can in extreme cases cause discolouration, cracking, or even fire.
Unlike general degradation, which is slow and uniform across the panel, hot spot damage is concentrated and irreversible. A panel with a confirmed hot spot will typically need to be replaced, not repaired.
What causes hot spots?
The most common cause is partial shading or soiling of a single cell within a string. Solar cells in a string are connected in series. When one cell produces significantly less current than its neighbours — because it is shaded, dirty, or already degraded — it acts as a resistive load rather than a generator. The other cells effectively force current through it, dissipating energy as heat rather than electricity.
Soiling is a significant and underappreciated contributor. A patch of bird droppings, lichen, or accumulated grime covering part of a single cell is functionally identical to partial shading for hot spot purposes. Because droppings tend to be dense and opaque, a small patch can cause a disproportionate temperature rise in the affected cell.
Manufacturing defects (micro-cracks introduced during transport or installation) and mechanical damage can also cause localised current mismatch. But for installed panels with no history of physical trauma, sustained soiling is the most common preventable cause.
Bypass diodes — protection, not prevention
Most solar panels include bypass diodes, typically one per group of 20–24 cells (usually three diodes per 60-cell panel). When a group of cells is shaded or underperforming, the bypass diode activates and routes current around that group, preventing it from acting as a resistive load.
This significantly reduces the hot spot risk — but does not eliminate it. Bypass diodes have their own thermal limits and can fail under sustained stress. More importantly, bypass diodes protect at the group level: within a group of 20 cells, a single soiled cell still forces current through itself before the bypass diode threshold is reached.
How to detect hot spots
Hot spots are not visible to the naked eye under normal conditions. The most reliable detection method is thermal (infrared) imaging, which reveals temperature differentials across the panel surface. Drone-based thermal surveys are increasingly common in the UK, and some MCS installers offer them as a standalone service.
Monitoring data can provide an indirect signal: a string performing significantly below its expected output, particularly if the shortfall is worse on sunny days than overcast ones (when bypass diodes are less active), is worth investigating thermally.
If you suspect hot spotting, do not delay — continuing to operate a panel with a developing hot spot accelerates the damage.
Can cleaning help?
Yes, if the root cause is soiling. Professional cleaning that removes bird droppings and biological growth from individual cells restores uniform current production across the string and removes the partial-shading condition that drives hot spot formation. This is one reason why cleaning is not purely about output recovery — it is also preventive maintenance against a failure mode that voids most manufacturer warranties.
If hot spot damage has already occurred, cleaning does not reverse it. But for panels with no confirmed thermal damage, keeping the surface clean is the single most accessible intervention to reduce hot spot risk.