I signed up with Ecotricity a few years back.
There are no shareholders. So, instead of paying dividends they invest your bills in building new forms of green energy.
They may be more expensive but I have peace of mind that they only have one rate whether you're a new customer or been with them for years.
In their words
"All our customers start and stay on our latest best price, and there are no contracts or exit fees to tie you in.
Unlike most energy companies, we don’t hook you in with a discount and jack up your tariff after a year or two. There’s no need to worry, haggle or switch."