A polycarbonate solar panel cover is a transmission trade, and the trade should be measured. On a test roof in southern Spain, two identical 400 W modules were covered, one with 4 mm low-iron glass and one with 5 mm clear polycarbonate, each with a 20 mm vented gap. Over a week of August measurement, the polycarbonate-covered module produced 3.1 per cent less energy. Not a failure, but not nothing either.
What a polycarbonate solar panel cover actually transmits
Clear polycarbonate runs about 88 to 90 per cent visible transmission. Low-iron glass runs about 92. That two to three point gap is the entire optical cost of choosing polycarbonate, and it is stable over the life of the sheet if the UV coextrusion is present.
- Clear, uncoated: 88 to 90 per cent
- Clear with hard coat: 87 to 89 per cent, better abrasion life
- Textured or prismatic: 83 to 86 per cent, used to diffuse rather than to maximise
- Twin-wall: 70 to 78 per cent depending on structure, for insulated covers
A textured grade behind a module is a mistake unless diffusion is the goal: it spreads light away from the cells as well as towards them.
Heat, and why the air gap matters more than the cover
Modules lose roughly 0.4 per cent of output per degree Celsius above 25 degrees. Polycarbonate conducts heat about a third as well as glass, so heat accumulates behind the cover if the gap is sealed.
A 20 mm gap vented at both ends keeps the module within a few degrees of ambient. A sealed gap of the same size can run 10 to 15 degrees hotter at midday, which is a 4 to 6 per cent output penalty, larger than the optical difference.
Stand-off and fixing
Polycarbonate expands about 0.05 mm per metre per degree Celsius, roughly four times glass. Fixings must allow that movement: oversize holes by 2 mm, use washers with a sealing bead, and never clamp a cover hard at both ends of a long span.
The expansion is also why the cover should not be bonded to the frame. A cured adhesive becomes a rigid constraint and the sheet cracks at the bond line after a few thermal cycles.
Where polycarbonate earns its place
Hail zones, walkover arrays, elevated installations where glass breakage is a safety issue, and temporary or portable arrays that get handled. In each case the cover is protecting the module from impact, and the transmission loss is the price of that protection.
For a fixed ground array in a temperate climate with no hail exposure, glass remains the better optical choice. The decision is a site question, not a material preference.
Durability and the UV layer
Polycarbonate without a UV coextrusion yellows within two to three years outdoors. With it, clarity holds eight to ten years, and the sheet is still structurally sound long after the appearance has drifted.
Confirm the coextrusion on the data sheet rather than by brand name, and check that the protected face is the one facing the sun. Installing the wrong side up is a common and expensive error. The UV-resistant polycarbonate note explains the grading.
For glazing details around the array, the skylight specification guide covers the same stand-off and venting principles at building scale.
Send module size, cover thickness and site conditions to oversales@rpplexiglass.com for a transmission and durability specification.
Frequently Asked Questions
How much light does polycarbonate transmit?
Clear polycarbonate transmits about 88 to 90 per cent of visible light, against 92 per cent for low-iron glass. A hard coat costs about 1 per cent more; a textured grade costs 3 to 5 per cent.
Does polycarbonate block useful solar radiation?
It blocks most UV below about 380 nm, which is not where a silicon module generates. The visible and near-infrared range that drives output passes through with only a small loss.
Will a polycarbonate cover overheat a module?
It can. Polycarbonate is a poorer conductor of heat than glass, so the air gap behind it must be vented. An unvented 20 mm gap can raise module temperature by 10 to 15 degrees Celsius, which costs efficiency.