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effectiveness. If (to take a hypothetical example) regulating power plants would yield $5 million in benefits, the prospect of mitigating cost from $11 billion to $10 billion at later stages of the program would not by itself make regulation appropriate. In all events, we need not pursue these points, because EPA did not say that the parts of the regulatory program mentioned by the dissent prevent the imposition of costs far in excess of benefits. "[EPA’s] action must be measured by what [it] did, not by what it might have done." Chenery, supra, at 93–94.

Our reasoning so far establishes that it was unreasonable for EPA to read §7412(n)(1)(A) to mean that cost is irrelevant to the initial decision to regulate power plants. The Agency must consider cost—including, most importantly, cost of compliance—before deciding whether regulation is appropriate and necessary. We need not and do not hold that the law unambiguously required the Agency, when making this preliminary estimate, to conduct a formal cost-benefit analysis in which each advantage and disadvantage is assigned a monetary value. It will be up to the Agency to decide (as always, within the limits of reasonable interpretation) how to account for cost.

Some of the respondents supporting EPA ask us to uphold EPA’s action because the accompanying regulatory impact analysis shows that, once the rule’s ancillary benefits are considered, benefits plainly outweigh costs. The dissent similarly relies on these ancillary benefits when insisting that "the outcome here [was] a rule whose benefits exceed its costs." Post, at 16. As we have just explained, however, we may uphold agency action only upon the grounds on which the agency acted. Even if the Agency could have considered ancillary benefits when deciding whether regulation is appropriate and necessary—a point