Should you wait for better panels?

Cadmium telluride is real, and it is not for you

Thin film is not vapourware. Cadmium telluride, CdTe, is a genuinely different way of making a solar module: instead of slicing silicon into wafers, a microns-thin layer of semiconductor is laid straight onto a sheet of glass. First Solar in the United States has shipped it by the gigawatt for two decades. It holds up better than silicon when it gets hot, it does slightly better in the flat grey light we get a lot of, and it carries a low embodied-carbon figure because there is very little material in it.

What it is not is something you can buy for a barn roof in Warwickshire. CdTe is sold in container loads into utility-scale solar farms, on long contracts, years ahead. There is no domestic channel, no MCS-listed residential product line, and no installer network set up around it. A homeowner or a business asking for CdTe is asking for a thing that is not on any price list they can reach. That may change. It has not changed yet.

The choice in front of you is not silicon versus thin film. It is which silicon.

The real fork: backsheet or glass-glass

Almost every module you can actually buy in Britain is crystalline silicon. The meaningful difference between them is what is behind the cells. The cheap construction is a polymer backsheet: a laminated plastic film, light, easy to handle, and the reason most panels come in at the price they do. The other construction is glass on both faces, with the cells sealed between two sheets. That is what we specify, and usually that means SolarWatt.

The argument for it is not efficiency. On a clear day in July, two modules of the same wattage will make the same electricity whichever way round they are built. The argument is what happens in year fifteen. Backsheets are the part of a panel that ages first. They chalk, they craze, in bad cases they delaminate and let moisture into the cell layer, and then you have a fault that cannot be repaired, only replaced, on a roof you paid to have scaffolded once already. Glass does not do any of that. It is inert. It gets rained on and it stays glass.

There is a second reason, which matters on farms and anywhere with spray drift or livestock or salt air. Fungicides, slurry aerosol and coastal air are hard on polymers and indifferent to glass. If a module is going to sit above something agricultural for thirty years, sealing both faces is not a luxury.

The honest cost of that choice

Glass-glass is heavier, by roughly a fifth, and it costs more per watt. On an older purlin roof that weight is a real constraint and it has to go to the structural engineer at the start of the design, not at the end. We have walked away from roofs where the answer was that the structure would not take it. That is the trade: a dearer, heavier panel, backed by a thirty-year product warranty rather than a ten or twelve year one, on a system whose financial case runs for twenty-five.

If the roof will not take glass-glass, we say so and quote a good backsheet module instead. What we will not do is quote the cheap one and stay quiet about which part of it will fail first.

So, wait or not?

Waiting has a price and it is easy to work out. A domestic system generating around 3,700 kWh a year, most of it displacing electricity you would otherwise buy, is worth several hundred pounds a year to a household and considerably more to a business on a commercial tariff. A year of waiting is a year of paying that in full, in exchange for access to a technology that is not sold at your scale and, when it eventually is, will be competing against silicon that has also got cheaper and better in the meantime.

The one thing worth waiting for, occasionally, is a grid connection. If your network operator has no headroom at your point of connection, that is a real reason to phase a project. Panel chemistry is not.

Perovskites, and the silicon-perovskite tandem cells that keep setting laboratory records, are the interesting long game. They will arrive as a better version of the module you already own the roof space for. When they do, the mounting system, the cabling and the inverter you put in this year are the things that carry over.