Del. Code tit. 26 § 312
If the public utility fails to make refund within 90 days after the final determination by the Commission or by the court on appeal from the Commission's order that the rate is excessive, any person entitled to such refund may sue therefor in any court of this State of competent jurisdiction and shall be entitled to recover, in addition to the amount of the refund due, all court costs and reasonable attorney's fees, but no action may be maintained for that purpose unless instituted within 2 years after such final determination. Any number of persons entitled to such refund may join as plaintiffs and recover their several claims in a single action, and in such action the court shall render a judgment severally for each plaintiff as each plaintiff's interest may appear.
26 Del. C. § 312