It can be better to just invest. £10k upfront saving you £750 per year would break even after 15 years. £10k invested getting 7% return (above inflation) would have you with £8,750 after 10 years (due to your extra £750 of energy each year taken off your returns). It would take 23 years to be better off with the panels and from then on the panels would be better assuming there was no additional cost and energy prices were static.
Based on
@Ex Footy Legs quote of £6k up front, £650 saving per year it would take 10 years to break even, your £6k investment would be worth £2.8k at that point. From year 12 the panels are better than the investment (assuming they are still working and there is no maintenance cost, energy prices are static and there is no financing costs for installation).
According to Martin Lewis' site the typical installation cost is £7k and the typical saving is about £400 (scale up or down for other values). That would take 18 years to pay back at which time your £7k investment will have become £10k and will keep rising. Solar will never beat it. That is also based on our current high energy costs which are expected to fall.
They still seem to me to be too big a risk to expect a good return and you are better off just not spending the money. They are a nice to have and make you feel like you are helping the environment but they aren't a good investment. There is also the risk that subsidies and just cost plain old cost reduction due to competition/production costs could bring the costs down to hundreds instead of thousands and then they will be good value.
