The Maine Public Utilities Commission has ruled that Central Maine Power Co. shareholders, not ratepayers, should foot the bill for a $580,000 audit of the utility’s controversial SmartCare billing system.

In a ruling issued Wednesday, the PUC said that because the audit identified problems with the rollout of CMP’s billing system in fall 2017, the regulatory body has discretion under Maine statute to decide whether CMP or customers should pay for the audit.

“Given the circumstances that led to the audit and the magnitude of harm to ratepayers from the imprudence it uncovered, it would be unfair to expect ratepayers to absorb any of the costs of the audit,” the PUC said in its ruling.

In 2018, the PUC hired Liberty Consulting Group of Pennsylvania to conduct a so-called forensic audit, following customer complaints about high bills in late 2017.

Liberty reached its conclusions after months of exhaustive review of meter and billing operation and other facets of the operation. The findings informed two separate investigations at the PUC, leading to a $10 million financial penalty against CMP.

In March the PUC asked parties to the investigation and associated rate cases to comment on whether CMP or its customers should pay for the audit.

CMP argued that shareholders should only be held responsible for paying 28 percent of the audit’s cost, and that ratepayers should be required to cover the other 72 percent. Its rationale was that while the audit found “imprudence” related to the billing system’s implementation, it also said there was no systemic flaw in the billing system that caused it to erroneously inflate customer bills.

The Maine Public Advocate’s Office, which represents the interests of ratepayers before the PUC, argued that CMP’s comments were an attempt to deflect responsibility, and that it should be charged for the audit’s full cost.

“CMP’s mismanagement of SmartCare was the impetus for the Liberty audit,” the office said.