No, it is not taxable income unless they reported it to you on a Form 1099-MISC.
It is most likely a return of a capital contribution ("deposit") you made when you first started getting service from a cooperative utility company ("member-owned"). The deposit wasn't deductible, so the refund isn't taxable.
Less likely, it was your allocated share of profits from a cooperative utility company ("member-owned"). Unless you were deducting utility costs (e.g. rental property or business), this allocation would not be taxable, either.
