The cover glass of a solar panel should be clear and not green-tinted, 3 to 4 mm thick and tempered if possible, because it must pass as much light as it can while resisting hail and debris. Tempered glass cannot be cut, so choose a salvaged pane whose size already fits, and prefer low-iron glass, which the carrier says transmits 2 to 3% more light than standard window glass.
Part of Solar Panel Construction
Choosing, finding and preparing the front glass of a solar panel.
The Glass Does Three Jobs: Pass Light, Resist Impact, Seal
The carrier states the front cover must be transparent (clear glass, not green-tinted), strong enough to resist hail and debris, and sealed against moisture. It recommends 3–4 mm and says tempered glass is ideal, with low-iron glass transmitting 2–3% more light than standard window glass. Green tint comes from the iron in common window glass, which absorbs light before it reaches the cell; looking along the edge of a pane shows a green cast if iron is high (general knowledge, not from the other articles).
Tempered Glass Cannot Be Cut
Glass Recovery states that tempered glass cannot be cut, shattering into small cubes instead of shards, and Salvaged Glass says to test a corner or check for the etched mark before trying to score it. So the size of a tempered cover is the size you find: design the panel around the glass. The best source is the cover of a broken commercial panel, which is already tempered, flat and sized for cells. Car side windows and shower doors are also tempered.
If tempered glass is not available, annealed window glass is easier to cut but breaks into large sharp pieces under hail, and hand-made flat glass from Flat Glass (crown glass panes 2–3 mm thick from a spun disc, or cylinder glass up to 60 × 90 cm) is thinner than the carrier’s advice.
What You Need
- A clear glass pane, salvaged, 3–4 mm thick if possible
- A way to check for temper — the etched corner mark, or the test in Salvaged Glass
- Cut-resistant gloves, goggles and boots — Glass Recovery
- Clean water, alcohol and a lint-free cloth
- A flat padded bench
Method: Prepare The Cover
Step 1: Choose the pane. Look for clear glass with no green cast along the edge, no deep scratches and no chips.
Step 2: Check for tempering with the etched mark, and do not attempt to cut a tempered pane.
Step 3: Handle on edge, never flat, and never under the arm (Glass Recovery).
Step 4: Clean both faces and let them dry fully; fingerprints and film cut light and weaken the seal.
Step 5: Plan the panel around it — cell layout, frame and junction box (Panel Encapsulation and Framing).
Worked Example: Weight And Light
Light lost at the glass: with a refractive index of 1.5 (general knowledge, not from the other articles), each air-to-glass surface reflects ((1.5 − 1.0) ÷ (1.5 + 1.0))² = 0.04, 4%. Two surfaces pass 0.96 × 0.96 = 0.92, a loss of 7.8% (Anti-Reflective Coating uses the same formula). The carrier’s low-iron advantage of 2–3% is a gain on top of this: on a panel making 9 W, 0.02 × 9 = 0.18 W to 0.03 × 9 = 0.27 W.
Weight: a 100 W array at 4% efficiency needs 100 ÷ (1,000 × 0.04) = 2.5 m² of cells (1,000 W/m² from Solar Energy Fundamentals). The glass is at least that area at 3.5 mm (middle of the carrier’s range): 2.5 m² × 0.0035 m = 0.00875 m³, which at a glass density of 2.5 g/cm³ (general knowledge) is 8.75 × 2.5 = 21.9 kg. That is why a roof of salvaged tempered glass is carried by a frame and not by hand-made panes.
Common Mistakes
- Trying to cut tempered glass — it bursts into cubes
- Using green window glass — it absorbs light the cells need
- Carrying a pane flat — it flexes and breaks
- Leaving fingerprints on the inside face — they cloud the optical contact with the sealant
- Sizing the panel before finding glass — tempered panes cannot be trimmed to fit
Safety
Glass cuts, and tempered glass bursts without warning
Tempered glass can shatter into a mass of small cubes if its edge is struck or stressed, and ordinary glass breaks into razor-edged shards. Wear cut-resistant gloves, eye protection and boots, never carry a pane under the arm or flat, and tape an X across a pane before lifting it (Glass Recovery). A panel in the sun can heat enough to burn bare skin on the glass.
What’s Next
- Learn next: Sealing and Backing — closing the other side
- Combine with: Panel Encapsulation and Framing — assembling the whole panel