I am not sure what legally binding stuff there is, but the advertised promise was that I will never pay more than RIE charges.

I don't think there's any real way for it to cost more...even if the solar farm output is 0 kWh, then I'm just getting supplied with the default RIE generation mix at the RIE rates.

Which might be something I should clarify...Arcadia is not actually my energy supplier. They are an intermediary that manages community solar projects that buys cheaper solar power from farms, sells it to RIE at market rates, and passes a portion of that savings to the consumer. On paper, my supplier is still RIE.

They are distinct from alternative suppliers like you'd find on this list: https://www.ri.gov/app/dpuc/empowerri/rate_card

edit: it looks like the "smart energy" supplier fits your earlier description...slightly lower advertised rate right now, but a 6 month commitment that will extend into next quarter's RIE rate update which will almost certainly be lower than current...so if you signed up with them, you'd have a slightly-lower rate for now, then be stuck with a higher rate later.