delivered the opinion of the Court.

This case presents questions under Title II of the Federal Power Act, 49 Stat. 847, 16 U. S. C. § 824 et seq., which are in part similar to those we have decided today under the Natural Gas Act in United Gas Pipe Line Co. v. Mobile Gas Service Corp., ante, p. 332.[*350] The pertinent provisions of the Federal Power Act, set forth in the margin, ^[1] are §§ 205 (c), (d), and (e), and 206 (a), which are substantially identical to §§ 4 (c),[*351] (d), and (e), and 5 (a), respectively, of the Natural Gas Act. ^[2]

Respondent Sierra Pacific Power Company (Sierra) distributes electricity to consumers in northern Nevada and eastern California. For many years, it has purchased the major part of its electric power from petitioner Pacific Gas and Electric Company (PG&E), a “public utility” subject to regulation under Part II of the Federal Power Act. In 1947 Sierra, faced with increased postwar demands and consumer agitation for cheaper power, began[*352] negotiating for power from other sources, including the Federal Bureau of Reclamation, which at the time had unused capacity at Shasta Dam. To forestall the potential competition, PG&E offered Sierra a 15-year contract for power at a special low rate, which Offer Sierra finally accepted in June 1948. The contract was duly filed with the Federal Power Commission.

Early in 1953, when power from Shasta Dam was no longer available to Sierra, PG&E, without the consent of Sierra, filed with the Commission under § 205 (d) of the Federal Power Act a schedule purporting to increase its rate to Sierra by approximately 28%. The Commission, acting under § 205 (e), suspended the effective date of the new rate until September 6, 1953, and initiated a proceeding to determine its reasonableness. Sierra was permitted to intervene in the proceeding but its motion to reject the filing on the ground that PG&E could not thus unilaterally change the contract was denied. After completion of the hearings, the Commission, by order dated June 17, 1954, reaffirmed its refusal to reject the filing and held the new rate not to be “unjust, unreasonable, unduly discriminatory, or preferential.” 7 P. U. R. 3d 256. On Sierra's petition for review, the Court of Appeals for the District of Columbia, holding that the contract rate could be changed only upon a finding by the Commission that it was unreasonable, set aside the Commission’s order and remanded the case with instructions to the Commission to dismiss the § 205 (e) proceeding, but without prejudice to its instituting a new proceeding under § 206 (a) to determine the reasonableness of the contract rate. 96 U. S. App. D. C. 140, 223 F. 2d 605. We brought the case here because of the importance of the questions involved in the administration of the Federal Power Act. 349 U. S. 937.

The first question before us is whether PG&E’s unilateral filing of the new rate under § 205 (d), and the[*353] approval of the new rate by the Commission under § 205 (e), were effective to supersede PG&E’s contract with Sierra. We think not. As the parties concede, the provisions of the Federal Power Act relevant to this question are in all material respects substantially identical to the equivalent provisions of the Natural Gas Act. In United Gas Pipe Line Co. v. Mobile Gas Service Corp., supra, decided today, we construed the Natural Gas Act as not authorizing unilateral contract changes, and that interpretation is equally applicable to the Federal Power Act. Accordingly, for the reasons there given, we conclude that neither PG&E’s filing of the new rate nor the Commission’s finding that the new rate was not unlawful was effective to change PG&E’s contract with Sierra.

This case, however, raises a further question not present in the Mobile case. The Commission has undoubted power under § 206 (a) to prescribe a change in contract rates whenever it determines such rates to be unlawful. While this power is limited to prescribing the rate “to be thereafter observed” and thus can effect no change prior to the date of the order, the Commission’s order here, if based on the necessary findings, could have been effective to prescribe the proposed rate as the rate to be in effect prospectively from the date of the order, June 17, 1954. If the proceedings here satisfied in substance the requirements of § 206 (a), it would seem immaterial that the investigation was begun as one into the reasonableness of the proposed rate rather than the existing contract rate.

The condition precedent to the Commission’s exercise of its power under § 206 (a) is a finding that the existing rate is “unjust, unreasonable, unduly discriminatory or preferential.” Petitioners contend that the Commission did in fact make such a finding. It was stipulated in the proceedings before the Commission that 5.5% was normally a reasonable rate of return for PG&E’s operations,[*354] that the contract rate would produce a 2.6% rate of return, and that the proposed rate would produce a 4.75% rate of return. The Commission concluded that the proposed rate was not unreasonably high because it provided no more than a fair return and was not unreasonably low because the 0.75% deficiency of its yield from the stipulated reasonable rate of return was not being made up on other sales and was justified in order to retain business the loss of which by PG&E would result in idle facilities. It also concluded that the proposed rate was not unduly discriminatory or preferential, despite substantial differences between it and the rates being charged other customers. While no further findings were necessary in view of the Commission’s interpretation of the Act as permitting unilateral contract changes, the Commission went on to say:

“However, we may point out that if a finding on the lawfulness of the 1948 contract rate were necessary or appropriate, on the record before us that finding would have to be that the 1948 rate is unreasonably low and therefore unlawful. For none of the evidence in this record warrants a finding that any rate would be reasonable that would produce a return of substantially less than the 4.75% resulting from the proposed rate, which is the minimum PG&E is willing to accept.”

It is contended that by this statement the Commission in substance found that the existing contract rate was “unreasonable” and fixed the proposed rate as “the just and reasonable rate,” thereby satisfying the requirements of §206 (a).

But even accepting this statement as a finding of unreasonableness of the contract rate, the Commission’s conclusion appears on its face to be based on an erroneous standard. In short, the Commission holds that the[*355] contract rate is unreasonable solely because it yields less than a fair return on the net invested capital. But, while it may be that the Commission may not normally impose upon a public utility a rate which would produce less than a fair return, it does not follow that the public utility may not itself agree by contract to a rate affording less than a fair return or that, if it does so, it is entitled to be relieved of its improvident bargain. Cf. Arkansas Natural Gas Co. v. Railroad Comm’n, 261 U. S. 379. In such circumstances the sole concern of the Commission would seem to be whether the rate is so low as to adversely affect the public interest — as where it might impair the financial ability of the public utility to continue its service, cast upon other consumers an excessive burden, or be unduly discriminatory. That the purpose of the power given the Commission by § 206 (a) is the protection of the public interest, as distinguished from the private interests of the utilities, is evidenced by the recital in § 201 of the Act that the scheme of regulation imposed “is necessary in the public interest.” When § 206 (a) is read in the light of this purpose, it is clear that a contract may not be said to be either “unjust” or “unreasonable” simply because it is unprofitable to the public utility.

Whether under the facts of this case the contract rate is so low as to have an adverse effect on the public interest is of course a question to be determined in the first instance by the Commission. We shall therefore affirm the order of the Court of Appeals, with instructions to remand the case to the Federal Power Commission for such further proceedings, not inconsistent with this opinion, as the Commission may deem desirable.

It is so ordered.