C. A natural gas utility may account for eligible safety activity costs to be recovered pursuant to this section as deferred costs. The accumulated unrecovered balance of eligible safety activity costs deferred pursuant to this section shall not exceed four percent of the natural gas utility’s net plant investment that was utilized in establishing or confirming base rates in the natural gas utility’s most recent rate case using the cost of service methodology set forth in § 56-235.2 or performance-based regulation plan authorized by § 56-235.6. The eligible safety activity costs deferred hereunder shall be included in new base rates and charges instituted pursuant to a Commission order establishing or confirming customer rates in a rate case using the cost of service methodology set forth in § 56-235.2 or a performance-based regulation plan authorized by § 56-235.6. Such deferred costs shall not be subject to write-off or write-down by the Commission in an earnings test filing made pursuant to Commission rules governing utility rate increases and annual informational filings except as provided in this subsection. The natural gas utility shall be deemed to have recovered eligible safety activity costs to the extent that the return on equity earned by the natural gas utility in an earnings test filing for a given year, after consideration of the treatment of regulatory assets, is in excess of the mid-point of the rate of return on equity range specified or confirmed in the natural gas utility’s most recent rate case or performance-based regulation plan. Notwithstanding the foregoing, in the event that a utility’s base rates include eligible safety activity costs, the utility shall only be permitted to defer the level of eligible safety activity costs that are in excess of the level reflected in base rates.