I'm worried about several aspects of the offer. The very long term returns and the assumptions that upper wholesale prices will be achieved, as well as erosion by inflation.
100% of my energy would cost ÂŁ7800 and return potential savings of ÂŁ18000. If you put that in a cash ISA at today's rates it would be worth ÂŁ45,000 in 40 years as a conservative strategy. And you can withdraw it. And the interest isn't taxed.
I'm passionate about decarbonisation (have reduced home energy by more than 80%) but the deal seems poor value. Most people's incentive here seems to be savings. I would be interested to hear others' thoughts.