We had initially estimated that it would take around 10 years to pay off the £12k cost of the panels. Assuming we keep getting £1,500 per year, that takes it down to ~8 years.

If we are lucky enough to get that amount every year for 25 years, that would net us £37k. Which is roughly the equivalent of a 4.7% ROI (assuming I've done my sums correctly). A little better than the top ISA rate.