PET vs ETFE vs Composite Flexible Solar Panels for Boats

Flexible solar panels let you put power where a rigid panel never could: on a bimini, a dodger, a curved cabin top, or a deck you walk on every day. But two flexible panels with the same wattage on the label can have very different lives on a boat. The biggest difference is usually the clear top layer that protects the cells, and it almost always comes down to one of two plastics: PET or ETFE.

This post breaks down what those layers are, how they hold up in salt, sun and heat, and where a newer third option, fiberglass composite panels like those from Open Waters Solar, fits in. For the bigger picture on system design, see our Marine Solar Design Guide.

What PET and ETFE Actually Are

A semi-flexible panel is a sandwich. Thin monocrystalline cells sit in the middle, an encapsulant bonds them in place, a backsheet sits underneath, and a clear front film faces the sun. That front film is the part you can see and touch, and it does most of the work keeping UV, water and grime away from the cells.

• PET (polyethylene terephthalate) is the same family of plastic used in drink bottles. It is cheap, easy to laminate and shows up on most budget flexible panels. It usually has a glossy, slick surface.
• ETFE (ethylene tetrafluoroethylene) is a fluoropolymer, a chemical cousin of Teflon. It costs more, but it is far more resistant to UV, chemicals and heat. ETFE panels typically have a textured or dimpled surface rather than a mirror finish.

PET vs ETFE: Side by Side

Lifespans are field ranges, not guarantees. Mounting method and the quality of the cells and lamination matter as much as the top film.

Why the Top Layer Matters So Much on a Boat

On a house roof, a flexible panel would just sit there. On a boat it lives with constant UV, salt spray, heat, vibration and flexing. The front film is the first line of defense, and when it gives up, the failures follow a predictable pattern.

UV Haze and Yellowing

PET breaks down under sunlight, turning cloudy or yellow. Every bit of haze blocks light before it reaches the cells, so output drops slowly year after year. ETFE resists this far better, which is the single biggest reason we steer customers toward it.

Heat and Hot Spots

Flexible panels have no air gap behind them, so they run hot. Heat speeds up aging of the film, the encapsulant and the solder joints. Add a shaded or soiled cell, which can turn into a hot spot, and the damage compounds. PET's lower heat tolerance makes it the weaker link here.

A Third Option: Composite Panels from Open Waters Solar

PET and ETFE are both plastic films glued over fragile cells. Open Waters Solar, a manufacturer based in Prince George, British Columbia, takes a different approach: it encapsulates the cells inside a fiberglass composite laminate. The idea is to stop the cells from cracking in the first place rather than just keeping weather off them.

According to the manufacturer, their Flex Series panels:

• Are about 1.3 mm thick and weigh roughly 3.5 lbs per 100 watts
• Use high-efficiency Maxeon back-contact cells, with a stated efficiency of about 24%
• Are rated walkable for light foot traffic and built to resist saltwater, UV and impact
• Use more inline bypass diodes than typical flexible panels to reduce shading losses
• Are designed for full-surface adhesive mounting directly to the deck, which they say limits vibration damage
• Can be built in custom shapes, including their SolaDek solar decking product with a non-slip finish
• Carry a 5-year warranty

Their 115W Flex Series panel measures 41.5" x 21.5" and weighs about 4 lbs, and was listed at $1,115 CAD at the time of writing (shown as currently unavailable on their site). That puts it well above even top-tier ETFE panels on a cost-per-watt basis.

Our Take on Composite Panels

The construction addresses the failure we see most often on flexible panels: cracked cells. That is a real step forward, especially for deck-mounted installs where people walk. A few things to weigh before choosing them:

• Cost. You are paying a larger premium per watt. It makes the most sense where a panel will be walked on, needs a custom shape, or where replacing failed panels would be expensive.
• Permanent mounting. Full-surface adhesive bonding is great for durability but means the panel is effectively part of the deck. Plan wire penetrations and removal carefully.
• Track record. The company was founded in 2022, so long-term field data is still building. The 5-year warranty is the same length as many quality ETFE panels.
• Not for canvas. These are designed to be bonded to a solid surface, not sewn or zipped into a bimini.

Which Should You Choose?

• Choose PET only for short-term, low-cost projects, such as a panel on aging canvas you expect to replace in a few seasons anyway.
• Choose ETFE for nearly every flexible install on a boat. It is our default recommendation for canvas and most deck mounts, and it is worth pairing with a reputable manufacturer and good cells.
• Consider composite when the panel will be walked on, needs to follow a complex shape, or when you want the best possible resistance to cracking and are willing to pay for it.
• Consider rigid panels if you have an arch, davits or rail space. Glass and aluminum still win on cost per watt and lifespan.

Installation Tips That Matter More Than the Film

• Never bend a panel past the manufacturer's limit, and avoid bending it inward.
• Do not sew through panels. Use adhesive-backed Velcro or zippers on canvas installs.
• Keep canvas taut to reduce flexing and vibration.
• Rinse panels with fresh water to keep salt and droppings off.
• Use an MPPT charge controller and proper PV wire, MC4 connectors and fusing.
• Inspect panels each season for bubbles, haze, discoloration or burn marks.

Need Help Choosing?

Shop our selection of marine solar panels and charge controllers at Sea & Land Supply, or reach out to Sea and Land Yacht Works at 401-753-7479 for help designing and installing a solar system on your boat.