If any public service company earns in the period of five consecutive years immediately preceding the commission order fixing rates for such company a net utility operating income in excess of a reasonable rate of return upon the fair value of its property used and useful in the public service, the commission shall take official notice of such fact and of whether any such excess earnings shall have been invested in such company’s plant or otherwise used for purposes beneficial to the consumers of such company and may consider such facts in fixing rates for such company.