Quick definition
Solar cells in a panel are wired in series, so current has to pass through every cell. If one cell can't keep up with the others, because it's shaded, damaged, or defective, it can end up absorbing the current the rest of the string is generating instead of contributing its own share. That absorbed energy comes out as heat, concentrated in a small area: a hotspot.
Bypass diodes exist specifically to prevent this in most normal shading situations, which is why genuine sustained hotspots are relatively uncommon on well-functioning panels.
At a glance: thermal imaging cameras are the standard diagnostic tool for spotting hotspots, since a genuine hotspot shows up as a clear temperature anomaly even when the panel looks perfectly normal to the eye.
Why it matters
A hotspot isn't just a performance issue costing a small amount of lost output, it's a real physical stress point on the panel. Sustained overheating in a concentrated area can degrade the cell material over time, damage the panel's laminate or backsheet, and in severe or prolonged cases create an actual fire risk. This is why hotspots get treated as a genuine maintenance and safety concern, not just a minor efficiency loss.
Bypass diodes handle most everyday causes of hotspots, like a passing shadow from a tree branch or a bird sitting on part of the array. What typically produces a persistent, dangerous hotspot is something the bypass diode isn't fully compensating for: a failed diode itself, a manufacturing defect in a cell, physical damage like a crack from hail or debris, or a persistent shading source that's severe enough to overwhelm the panel's normal protection. A trickier case is a diode that's missing or disconnected from the factory: the module tests and looks completely normal under the uniform light of a flash test or a clear-sky thermal scan, since nothing is shading it yet to trigger the fault. The risk only shows up once real-world shading finally occurs and the string has no functioning diode to reroute current around it, which is why some inspection programs now call for deliberately shading a section during an infrared scan rather than relying on passive thermography alone.
This is exactly why thermal imaging has become a standard part of professional solar system inspections, especially for larger commercial and utility-scale installations. Outdoor infrared inspection of PV modules is formalized enough to have its own technical standard, IEC TS 62446-3, which lays out equipment, ambient conditions, and procedure specifically because thermography supports fire-safety preventive maintenance, not just performance troubleshooting. A drone-mounted thermal camera can scan an entire array quickly, flagging any panel running hotter than its neighbors for closer inspection, catching a developing hotspot well before it causes lasting damage. The Department of Energy's homeowner guide explains what to look for in panel quality and warranties before you buy.
Where you'll see this
The output loss from hotspots estimator shows the performance impact when a hotspot forms on part of an array.
Since thermal inspections are how hotspots typically get caught, the annual inspection cost estimator helps budget for the kind of routine service visit that includes this check.
This term comes up in thermal inspection reports, in warranty claims involving panel damage, and any troubleshooting conversation about a panel or string producing noticeably less than expected. The U.S. Energy Information Administration's solar explainer describes how a photovoltaic cell turns sunlight into electricity, and why a shaded cell behaves differently.
Related terms
Frequently asked
Genuine sustained hotspots are relatively uncommon on properly functioning panels, since bypass diodes are specifically designed to prevent them under normal partial-shading conditions. They're more likely when a bypass diode has failed, a cell is physically damaged, or a manufacturing defect is present.
Thermal imaging cameras, sometimes handheld, sometimes drone-mounted for larger arrays, are the standard tool for detecting hotspots. They show temperature differences across a panel's surface, making an abnormally hot cell or section visually obvious even though it looks identical to surrounding panels in normal light.
Yes, if it persists. Sustained overheating in a small area can degrade the cell material, damage the panel's backsheet, or in severe cases create a real fire risk. This is exactly why hotspot detection is part of routine solar system inspections.
Not necessarily. Sometimes the underlying cause, like a failed bypass diode or a loose connection, can be repaired without replacing the entire panel. A qualified technician needs to diagnose the specific cause before deciding whether repair or replacement is the right fix.