Under the Federal Power Act, federally regulated electric utilities must obtain FERC’s approval before changing the rates they charge to wholesale and retail consumers This case presents the question whether a different provision of the Act requires utilities to obtain similar “pre-approval” before modifying the depreciation rates they use for accounting purposes Petitioners contended unsuccessfully before the Commission that the statutory provision does not impose such an obligation. We agree with petitioners that the Commission unreasonably read the statute, and accordingly we vacate the orders under review.
I.
Electric utilities incur expenses in producing power, but regulators do not permit every sort of expense to be recovered immediately — nor should they, it would make no sense to allow a utility that constructs a power plant for $100 million to set rates high enough to recover that expense over the course of one year. Although the plant will deteriorate from use from year to year, the plant might last for 20 years or more, and its cost should be recovered over the course of that useful life.
Depreciation charges are the means by which a utility recovers over time the capital invested in the facilities or plant used in producing power. Because a higher depredation charge implies that the utility should be permitted a higher rate (and conversely a lower depredation charge implies that the utility should be allowed a lower rate),1 one would expect both the Commission and the utilities it regulates to have a keen interest in the accurate measurement of depredation. It is not surprising, then, that as a by-produd of its authority under §§ 205 and 206 of the Federal Power Ad (FPA) (codified as amended at 16 U.S.C. §§ 824d, 824e (1994)) to regulate the rates utilities can charge to consumers, the Commission has regulated depredation accounting by utilities. See, e.g., Cities of Aitkin v. FERC, 704 F.2d 1254, 1255-56 (D.C.Cir.1982) (rejeding challenge to FERC order granting a utility’s proposed rate increase in part because the utility had adequately justified increasing the rate of depredation daimed for steam and hydraulic generating equipment); South Carolina Elec. & Gas Co., 76 F.E.R.C. ¶ 61,338 (1996) (rejeding proposed rate change because the utility had improperly shifted depredation reserves from one asset to another).
Where depredation accounting is nd implicated in a ratemaking proceeding, however, it appears that the Commission has not attempted to regulate it. That is not to say that the Commission lacks the statutory authority to do so. Apart from its duties regarding the rates utilities may charge for their service, the Commission has been delegated authority over accounting procedures per se. Section 301 of the Ad addresses record keeping generally, requiring utilities to keep “such accounts ... as the Commission may by rules and regulations prescribe as necessary or appropriate for the purposes of the administration of this chapter” and authorizing the Commission to insped such accounts at all times 16 U.S.C. § 825(a)-(b) (1994). Pursuant to this authority, the Commission adopted in 1960 the Uniform System of Accounts, 18 C.F.R. pt. 101 (1998), a comprehensive dassification and enumeration of revenue and expense items that utilities must use in keeping their books Yet although the Uniform System of Accounts defines “depredation,” 18 C.F.R. pt. 101, Definition 12, and sets forth the components of the depredation account, id. at Item 108, nowhere does it prescribe a depredation method or fix depredation rates See Office of Chief Accountant, Federal Energy Regulatory Commission, Electric Utility Depreciation Practices, 1976, at 1 (1980) (“The Uniform System of Accounts requires depredation accounting but prescribes no particular depredation method.”).
*79Section 302, unlike section 301, reflects a specific locus on depreciation and depredation rates. 16 U.S.C. § 825a (1994). Section 302(a) grants the Commission the authority, “after hearing,” to prescribe “rules) regulations, and forms of account” to govern the depredation accounting of regulated utilities Id. at § 825a(a). The Commission is also authorized to “ascertain and determine, and by order fix, the proper and adequate rates of depredation of the several dasses of property of each licensee and public utility.” Id. Section 302(b) obliges the Commission to notify State commissions having jurisdidian with resped to any public utility invdved “before prescribing any rules or requirements as to accounts, records) or memoranda, or as to depredation rates” Id. at § 825a(b). Yet while § 302 would seem to authorize the Commission to promulgate rules and regulations governing depredation accounting, it has lain dormant in the United States Code since Congress added it to the FPA in the Public Utility Ad of 1935, ch. 687, Title 11, sec. 213, § 302, 49 Stat. 803, 855 (1935).
Section 302’s dormancy ended in 1994, however, when the Commission, responding to Midwest Power’s unsolidted letter informing the Commission of its intention to change its depredation rates, dedared that it was “inappropriate for [Midwest Power] to reduce its depredation rates for accounting purposes without a corresponding change in the depredation rates embedded in its wholesale and retail eledric rates” Midwest Power Sys. Inc., 67 F.E.R.C. ¶ 61,0746, at 61,207 (April 19, 1994) (quoting 1994 Letter Order from FERC’s Chief Accountant) (internal quotation marks omitted) (alteration in original).2 The Commission reasoned that the plain language of § 302 — espedally the part of § 302(a) providing that public utilities shall not charge “with resped to any dass of property a percentage of depredation other than that prescribed therefor by the Commission,’’ 16 U.S.C. § 825a(a) (emphasis added) — implies that a utility may not unilaterally choose its depredation rates, for rates so chosen would not be “prescribed by the Commission.” Midwest Power, 67 F.E.R.C. at 61,208, 61,209. The Commission conducted that utilities must henceforth seek prior approval from the Commission before changing their depredation rates. See id. at 61,209-10.
When Midwest Power obeyed this order and filed a request for authorization to reduce its annual composite depredation rate, the Commission dismissed the request as moot — changes made before April 19, 1994 (the date of FERC’s Midwest Power order) were retroadively accepted if based on sound accounting pradices — but did nd offer further justification for the new pre-approval system. MidAmerican Energy Co., 79 F.E.R.C. ¶61,169, at 61,794 (May 15, 1997).3 Because Midwest Power’s request for a rate change was granted, it did not seek rehearing of the May 1997 order. But Southern Company Services, I ncx, ading as agent for its subsidiary companies induding Alabama Power Company, was granted permission to intervene and requested rehearing. MidAmerican Energy Co., 81 F.E.R.C. ¶61,081 (October 22, 1997). Southern argued that § 302 is an enabling statute that is not self-executing, and therefore does not require public utilities to seek pre-approval from the Commission but only to conform to rates that the Commission has already fixed. Id. at 61,326. Southern also asserted that the May *801997 order was procedurally invalid because the Commission had neither given notice to the State commissions as § 302(b) requires nor followed the notice and comment procedure of 5 U.S.C. § 553 (1994) in promulgating what Southern claimed was a substantive rule Id. at 61,327.
The Commission rejected Southern’s arguments and denied the request for rehearing. Pointing to the same language in § 302(a) that it had emphasized in its April 1994 order, the Commission concluded that § 302 is not merely an enabling provision, but rather expressly imposes a pre-approval requirement. Id. at 61,328. The Commission responded to Southern’s APA objection by characterizing the May 1997 order as an “interpretative” rule exempt from the APA’s notice and comment procedures; in the Commission’s view, the May 1997 order “did little more than reiterate the statutory obligation imposed on public utilities and licensees by Congress in 1935.” Id. As for Southern’s claim that the Commission violated § 302(b) by not providing notice to the State commissions prior to issuing the May 1997 order, the Commission maintained that it did send the May 1997 order to the State commissions after issuance and that no State commission had responded or indicated any objection to the order. Id. at 61,327 n. 11. Southern and the other intervenors before the Commission now petition us for review of the May 1997 and October 1997 orders.
II.
Before deciding whether § 302(a) requires utilities to gain the Commission’s approval before changing their depreciation rates for accounting purposes, we see an initial weakness in the Commission’s position: its apparent failure to comply with § 302(b). Section 302(b) provides:
The Commission, before prescribing any rules or requirements as to accounts, records, or memoranda, or as to depredation rates, shall notify each State commission having jurisdiction with respect to any public utility involved, and shall give reasonable opportunity to each such commission to present its views, and shall receive and consider such views and recommendations.
16 U.S.C. § 825a(b) (emphasis added). Petitioners argue that the Commission neglected to provide the requisite notice to the State commissions before issuing its May 1997 order, and that the May 1997 order and the October 1997 order justifying the May 1997 order are therefore invalid. The Commission responds that it complied with § 302(b) by publishing the May 1997 order in the Federal Register after it was issued, see 62 Fed.Reg. 28,015 (May 22, 1997), and by sending the May 1997 order to the State commissions before issuing the October 1997 order.
We think it obvious that the Commission did not comply with § 302(b). The Commission identified its May 1997 order as a “rule” in its October 1997 order, in its brief, and at oral argument. See, e.g., MidAmerican Energy Co., ¶ 61,081, at 61,328 (emphasis added) (“[W]e believe that the May 15 order properly may be characterized as an ‘interpretative rule.’ ”). I n doing so, the Commission has hoisted itself on its own petard, for § 302(b) requires the Commission to notify the State commissions “before prescribing any rules.’’ 16 U.S.C. § 825(a)(b) (emphasis added). Publishing the May 1997 order in the Federal Register and notifying the State commissions after issuing the May 1997 order do not satisfy that requirement. The Commission’s neglect of § 302(b) suffices for us to hold the May 1997 and October 1997 orders invalid.
Alternatively, the Commission contends, rather obliquely, that any failure to comply with § 302(b) was harmless in light of the absence of response or objection from a single State commission since the State commissions received notice — albeit tardy — of the May 1997 order. We say “oblique” because the Commission neither died the APA’s harmless error rule nor explained why a State commission’s failure to object after receiving a belated notice necessarily implies that no harm was done.4 I n any event, we *81think it impossible to conclude that no State commission would have had objections, or at least comments, if they had been presented with a proposal rather than a fait accompli.
Finally, we should note that the Commission’s characterization of its May 1997 order as an “interpretative rule”5 for purposes of the APA’s notice and comment requirement — assuming the characterization were correct — would not release the Commission from its obligations under the organic statute, § 302(b), which draws no distinction between interpretative rules and other sorts of rules. See 5 U.S.C. §§ 553(b), (b)(A) (emphasis added) (“Except when notice or hearing is required by statute, this subsection does not apply ... to interpretative rules”); Union of Concerned Scientists v. Nuclear Regulatory Comm’n, 711 F.2d 370, 380-81 (D.C.Cir.1983) (holding that 5 U.S.C. § 553(b)(B)’s “good cause” exemption from notice and comment requirements does not dispense with procedural requirements imposed by the organic statute).
III.
Although we could rest on our determination that the Commission’s May 1997 order violates § 302(b), we think it appropriate to consider alternatively petitioners’ more substantive argument — that the order is simply not authorized under § 302(a). Petitioners and the Commission, not atypically, both claim that the plain language of the section supports their respective readings and therefore ends our analysis See Chevron U.S.A. Inc. v. Natural Resources Defense Council, Inc., 467 U.S. 837, 842-43, 104 S.Ct. 2778, 81 L.Ed.2d 694 (1984). We set out § 302(a) in full:
The Commission may, after hearing, require licensees and public utilities to carry a proper and adequate depreciation account in accordance with such rules, regulations, and forms of account as the Commission may prescribe. The Commission may, from time to time, ascertain and determine, and by order fix, the proper and adequate rates of depreciation of the several classes of property of each licensee and public utility. Each licensee and public utility shall conform its depreciation accounts to the rates so ascertained, determined, and fixed. The licensees and public utilities subject to the jurisdiction of the Commission shall not charge to operating expenses any depreciation charges on classes of property other than those prescribed by the Commission, or charge with respect to any class of property a percentage of depreciation other than that prescribed therefor by the Commission. No such licensee or public utility shall in any case include in any form under its operating or other expenses any depreciation or other charge or expenditure included elsewhere as a depreciation charge or otherwise under its operating or other expenses. Nothing in this section shall limit the power of a State commission to determine in the exercise of its jurisdiction, with respect to any public utility, the percentage rate of depreciation to be allowed, as to any class of property of such public utility, or the composite depreciation rate, for the purpose of determining rates or charges
16 U.S.C. § 825a(a).
As if did in its October 1997 order, the Commission emphasizes the portion of *82§ 302(a) that states that “utilities ... shall not charge to operating expenses any depredation charges on dasses of property other than those prescribed by the Commission, or charge with resped to any dass of property a percentage of depredation other than that prescribed therefor by the Commission.” Id. (emphasis added). The Commission asserts that this language “unambiguously requires public utilities to employ as depredation charges and rates only those charges and rates that have been prescribed by the Commission,” and therefore that “common sense didates that the changes in depredation rates must also be approved by the Commission before they are used.” Petitioners disagree, contending that the Commission has opportunistically focused on selected portions of the provision. Petitioners daim that a reading of the provision as a whole — espedally the repeated use of the word “may” rather than “shall” — indicates that § 302 is merely an enabling provision that grants the Commission authority to regulate depredation accounts; in other words, § 302(a) does nd itself impose any obligation but merely lays the groundwork for the Commission to do so.
Although the language d § 302(a) is nd pelludd in all respeds, we agree with petitioners that it is dear on the discrete issue presented in this case — whether § 302(a) is a self-executing statute or merely an enabling statute. A careful reading reveals that Congress did nd establish any self-executing requirements for utilities to fdlow. Rather, Congress intended to grant the Commission the authority to regulate depredation accounting by utilities
We think the Commission’s paraphrase d the sedion that a utility may only use depredation rates prescribed by the Commission may be misleading. The strudure d the sedion seems to speak in terms d prescribing general rules and fixing the rates d individual utilities The first sentence d § 302(a) authorizes the Commission, after hdding a hearing, to “require ... public utilities to carry a proper and adequate depredation account in accordance with such rules, regulations, and forms d account as the Commission may prescribe.” 16 U.S.C. § 825a(a) (emphasis added). Here Congress spoke in permissive terms — using “may” rather than “shall” — and established the procedural prerequisite d a “hearing,” thereby contemplating that positive adion by the Commission, rather than the language d § 302 itself, would underlie the adual regulation d utilities’ depredation accounting. Although we need nd dedde the issue, it appears that Congress here contemplated a general rulemaking authority for the Commission. The verb “prescribe” — meaning “to lay down ... [a] rule,” Webster’s New International Dictionary 1954 (2d ed.1934)— as well as the words “rules?’ and “regulations” illustrate that Congress intended to authorize the Commission to promulgate rules and requirements generally applicable to all utilities. F or example, the Commisa on might promulgate a rule that utilities must fdlow generally accepted accounting prindples, or the Commission might set a rate d depredation (for a dass d property) applicable to all utilities.
The second sentence d § 302(a) provides that “[t]he Commission may, from time to time, ascertain and determine, and by order fix, the proper and adequate rates d depredation d the several dasses d property d each licensee and public utility.” 16 U.S.C. § 825a(a) (emphasis added). Like the first sentence, it is phrased to enable the Commission to regulate rather than to impose self-executing requirements Although, again, we do nd say definitively, Congress here seems to have contemplated a more spedfic type d regulation: case-by-case, utility-by-utility adjudication d rates d depredation as opposed to generally applicable rules governing accounting practices or generally applicable rates This is suggested by Congress? use d the verb “fix” — meaning “to assign predsely,” Webster’s New International Dictionary 958 (2d ed.1934) — rather than “prescribe,” and the words “order” and “each licensee and public utility.”6
*83The third sentence explains the relationship between the first and the second sentences* stating that “[e]ach licensee and public utility shall conform its depreciation accounts to the rates so ascertained, determined, and fixed.” 16 U.S.C. § 825a(a). In other words, where the Commission has specified general rules of depreciation accounting and has fixed an individual utility’s rates of depreciation in an adjudication, that utility must employ the fixed rates in complying with the more general accounting rulea Notably, as with the first two sentences* the third sentence reflects Congress!’ understanding that the Commission, not § 302 itself, would carry out the fixing of rates. Only after the Commission had “ascertained, determined, and fixed” a utility’s depreciation rates — an obvious reference to the adjudication procedure described in the second sentence — would that utility face any legal obligation.
Nor does the fourth sentence — on which the Commission stakes its argument — support the notion that § 302 itself establishes a pre-approval requirement. It provides that “[t]he licensees and public utilities subject to the jurisdiction of the Commission shall not charge to operating expenses any depreciation charges on dasses of property other than those prescribed by the Commission, or charge with respect to any dass of property a percentage of depredation other than that prescribed therefor by the Commission.” 16 U.S.C. § 825a(a).7 The Commission infers from the words “depredation charges on dasses of property other than those prescribed by the Commission” and “a percentage of depredation other than that prescribed therefor by the Commission” that changes in depredation rates must be approved by the Commission before they are used, for otherwise, utilities would be using rates established not by the Commission but by the utilities themselves. We think the Commission takes this sentence out of context. Read in light of the preceding sentences, it seems dear to us that Congress, by using the past tense of the verb “prescribe,” was referring back to the first sentence, which concerns the Commission’s basic authority to “prescribe” generally applicable rules and generally applicable rates Thus, if the Commission adheres to the procedural requirements of the first sentence and prescribes which dasses of property are subject to depredation, utilities may not thereafter take depredation charges from other dasses of property. Similarly, if the Commission follows the procedural requirements of the first sentence and generally prescribes a “percentage” (i.e., a rate) of depredation with resped to a dass of property, utilities may not thereafter employ a different rate with resped to that dass of property. In short, the fourth sentence lacks bite until the Commission regulates by the rulemaking procedure authorized by the first sentence.
In any event, whether or not the sedion mandatorily divides Commission regulation into discrete rulemaking and adjudication spheres, we think petitioners are corred that it is nd self-executing. Even if the term “prescribe” in the fourth sentence could be read as applying to the Commission’s setting of an individual utility’s depredation rates, that sentence dearly calls for the Commission to take some action with resped to rates of depredation before a utility is under an obligation nd to alter those rates.
Andher provision d the statute supports our reading of § 302. When Congress amended the FPA in the Public Utility Ad d 1935, it added nd only § 302, but several other provisions, among which was § 205(d). Public Utility Ad d 1935, ch. 687, Title 11, sec. 213, § 205(d), 49 Stat. 803, 851-52 (1935) (codified at 16 U.S.C. § 824d(d)). Sedion 205(d), part d a provision dealing with the *84regulation of rates “made ... in connection with the transmission or sale of electric energy,” 16 U.S.C. § 824d(a), provides:
Unless the Commission otherwise orders, no change shall be made by any public utility in any such rate, charge, classification, or service, or in any rule, regulation, or contract relating thereto, except after sixty days’ notice to the Commission and to the public. Such notice shall be given by filing with the Commission and keeping open for public inspection new schedules stating plainly the change or changes to be made in the schedule or schedules then in force and the time when the changes will go into effect.
16 U.S.C. § 824d(d) (emphasis added). In contrast to § 302(a), § 205(d) does impose a self-executing requirement on utilities: utilities must file their proposed rate schedules with the Commission and may nrt implement those changes until at least 60 days after filing.8 We can assume, therefore, that Congress knew how to impose a pre-change filing requirement and intentionally chose not to do so in § 302. See Russello v. United States, 464 U.S. 16, 23, 104 S.Ct. 296, 78 L.Ed.2d 17 (1983) (“ ‘[W]here Congress includes particular language in one section of a statute but omits it in another section of the same Act, it is generally presumed that Congress acts intentionally and purposely in the disparate inclusion or exclusion.’”) (quoting United States v. Wong Kim Bo, 472 F.2d 720, 722 (5th Cir.1972)) (alteration in original); Halverson v. Slater, 129 F.3d 180, 185 (D.C.Cir.1997) (noting that the Russello rule has force where, as here, “the two provisions in question are included within the same legislative enactment”).
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We should make dear the limitations of our holding. It may be that the Commission could, pursuant to its authority under the first sentence of § 302(a) to prescribe generally applicable rules relating to depredation accounting, promulgate a substantive rule requiring utilities to obtain approval from the Commission before changing their depredation rates for accounting purposes. I n promulgating such a rule, the Commission would of course have to comply with the “hearing” prerequisite of the first sentence of § 302(a), with § 3Q2(b)’s requirement of advance notification to State commissions, and with the notice and comment procedures set forth in 5 U.S.C. § 553. And the Commission’s rule would still have to withstand scrutiny under the Chevron framework. But the “predse question at issue,” Chevron, 467 U .S. at 842, 104 S.Ct. 2778, would be different from what it was in this case. I nstead of asking whether § 302 itself requires utilities to gain the Commission’s approval before changing their depredation rates, we would ask whether § 302 forbids the Commission from imposing such an obligation on utilities — or at least whether § 302 is ambiguous in that resped. We do not dedde that question.
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For the foregoing reasons, we grant the petition for review, vacate the Commission’s May 1997 and Odober 1997 orders, and remand for proceedings not inconsistent with this opinion.
So ordered.