Good news. I installed 14 last year and expected if all went well to have them paid off in 12-14 years. Going by the last 10 months it will be less than 6. That's from a combination of savings (1/3 of the energy bill) and also selling the excess to the grid (£164 worth so far) and all of Ttat's at 55-ish degrees North too on a not ideal roof. Anywhere south of me should absolutely have them covering their roofs, there really isn't a good reason not to.
Next up is top up insulation and replace the boiler with a heat pump. Possibly a second battery.