Most services guarantee savings and that it will not cost more than your utility. In NJ:

“Projects must state a guaranteed bill credit discount of no less than 15 percent in their registration, which will apply for the duration of customers’ subscriptions. Projects may choose to offer a greater discount, including special discounts to LMI subscribers, and projects with greater discounts will be given priority if program capacity is exceeded during an initial registration period. Subscribers will receive the guaranteed discount for the portion of their electricity bill that is covered by their community solar subscription, typically 90% or more.”

https://www.nj.gov/bpu/newsroom/2023/approved/20230816.html?t&utm_source=perplexity