Depends on your energy requirements and future technology and energy costs. At the moment, one should value this outlay as a fixed income equivalent investment [1].

The panels have a ~25 year warranty though [2] (at which point, they should still produce ~80% of rated output), so it’s entirely possible to just leave them in place. At a certain age (~55-60), these are the last PV panels you’ll need to buy, as they’ll potentially outlive you (assuming developed country life expectancy).

[1] https://magnifina.com/articles/rooftop-solar-yield/

[2] https://www.energysage.com/solar/solar-panel-warranties/

Why would you replace it if doing so is uneconomic?

Panel lifetime is very high. The scope for efficiency improvement is not huge (unless there is a cost breakthrough in multi band photon capture). It's not a car, phone, or computer. It's more like the rest of the house electric infrastructure.

I had my rooftop solar over 10 years ago and basically intend to leave it until some maintenance issue forces action.

(Also, the kit secondhand value is hard to determine but far from zero; 30-50% maybe?)

The technology is unlikely to improve meaningfully in 7 years. And you'd only upgrade if it was a financial improvement so it makes complete sense to give an estimate based on keeping it for 20 years.

I don't see what your issue is.

Then you are unlikely to be the person I'd be taking advice from.

Let me rephrase: the thing that you said clearly isn't an issue so it doesn't make sense for you to have an issue with their advice.

These calculations often fail to account for present vs future value of money.

If you’re financing the system you have no big cash outlay, but returns are further out, possibly never when accounting for the useful like of the system.

With cash up front all the returns are yours, but they are much lower than what that cash would net you in an average investment.

The financial math on small solar systems can be complex. If the system is sufficient to provide power to major appliances in a power outage (assuming you have a power outage risk in your area), it can make more sense to tie money up in these systems.

I got the exact same values.

I'd like it if it would actually show me how much sun it thinks I'd get at the postcode I put in. I've got about a third of an acre of garden in a 6 acre field to play with, before I start having to dig up roads. I can afford to be quite free and easy with placement ;-)

Notice that the pricing will come down. In Germany (where the market is mature), I can buy a 2kWp system for 500-600 EUR. Then your payback time basically halves...

To me, 7.5 years is not worth it. and the 1K in 20 years is nothing. This should be the standard and it should be free. Until then we'll keep paying through the nose for energy...

Maybe you guys wouldn't be paying through the nose for energy if you had more solar.

It wouldn't be England if the sun shone.

Not on those terms. Nobody wants to pay through the nose and see a return in a decade. We wouldn't be paying through the nose if we used our own gas instead of importing it. Crazy idea, I know...