AND considering that replacing only the panels has to be far cheaper than the initial installation, if they do drop too much in 25 years. So long as your roof is still okay, I suppose.

Just bake new panels into the cost of a new roof. It's probably time for both after 25 years.

Eh? My crappy roof was installed in 1945 and has some life left.

What sort of roofing lasts 25 years?

Afaik shingle roofs (common in US) need replacing every 10-20 years.

I actually wonder how solar panels reduce the rate at which a roof deteriorates. The panels themselves should bear the brunt of the light and rain that the shingles below them would otherwise endure, but on the other hand the panels don't cover the entire roof, so maybe at best you'd need to replace less of the roof but on the same schedule.

Yeah, that makes sense to me. I don't know the specifics of shingle deterioration, but just having them physically shielded from light and precipitation has got to drastically improve their lifespan. That's a good point about only part of the roof being protected though.

The president of the roofer's lobby has entered the chat!

Composition roofs need to be replaced when they are worn out and not before that. Good materials competently installed could last 50 years, more in mild climates. If you have the kind of weather that will wear down a roof in 10 years you probably cannot have solar panels.

Asphalt singles

Is there a reason that corrugated iron, concrete tile or similar aren’t used? They last near indefinitely if looked after, and a hell of a long time if not looked after.

They are very cheap.

My parents neighborhood is coming up on 25 years old and they are having to replace the layers under the roof tiles. The tiles themselves will probably last a thousand years but the under layer has a lifetime. They pull off the tiles, replace the waterproof layer and put the tiles back on — apparently takes just a couple days and they get better nails so the tiles don’t fall off during the windy monsoons.

Sorry, life after 25 years? That's WAY higher than I thought. That's incredible.

This is standard now. My REC Alpha Pure Black 400 panels are warrantied for 92% at 25 years.

The degradation is about 0.25%/yr. Assuming no exponential drop-off it would take >50 years to hit 80%.

Essentially no one has had to replace their panels due to degradation losses as of yet (since the vast majority of panels have been installed in the past ~30 years).

Generally you can find the degradation curve in their datasheet, and yes, 80% after 25 years sounds possible. IIRC they fade faster with time, so it might be 50% after 50, but ... that's still pretty good.

I don't see anything in the NREL data that suggests panel degradation accelerates. Most of the panels in their data have higher initial degradation, then less in subsequent years. This LG panel seems to be getting stronger as it ages: https://www.nrel.gov/docs/fy22osti/81172.pdf#page=28

you'll need to supplement with some new panels to account for the loss, but each new panel is more efficient than the one you're "replacing"

The idea that PV panels become trash in a decade is a lie promulgated by oil companies. According to NREL, reputable panels degrade as little as 0.1% per year, which means they will still be above 90% of rated output after 100 years. The trashiest panels NREL has tested degrade at 1.1% per year which would put them at 75% rated capacity in 25 years.

https://www.nrel.gov/pv/lifetime.html

This is very exciting. Does this mean the biggest continuing expense would be batteries if you wanted to self-sustain with solar?