JERRE S. WILLIAMS, Circuit Judge:
This diversity case was brought by Mississippi Power & Light Co. (“MP&L”) and by plaintiff-intervenor, Mississippi Public Service Commission (the “Commission”) against gas supplier United Gas Pipeline Company (“United”) for breach of contract. The complaint charges United with violating the pricing provisions of a contract between MP&L and United dated December 8, 1967, and amended in 1969. MP&L sought to recover money damages in excess of thirty-one million dollars for alleged overcharges under the contract, and preliminary and permanent injunctive relief to prohibit future overcharges. MP&L alleged that future overcharges, if continued, would range from 1 to 2 million dollars per month, or up to 120 million dollars during the remaining life of the pricing provisions of the contract through December 31, 1987. After a total of six days of hearings between October and December 1983, the district court granted the preliminary injunction, entering its order on April 2, 1984. The order enjoined United from continuing to charge MP&L monthly rates which included the costs of certain gas purchased by United and certain transportation charges incurred by United. We affirm.
I.
On December 8, 1967, United and MP&L entered into a contract under which United agreed to sell and deliver, and MP&L agreed to purchase and receive, large volumes of natural gas. Article XIV of the contract set out various base rates MP&L would pay for different blocks of gas purchased each month. Such rates were to be adjusted up or down based on the “weighted average purchase price” per 1,000 cubic feet (mcf) of gas which United paid monthly for gas purchased in the Jackson (Mississippi) Area and in United’s five other “areas” in Louisiana and Texas described in the contract. On April 29, 1969, MP&L and United executed an amendment to the contract. The amendment provided that the price of gas sold to MP&L would be based on an average price United paid for the gas in only two areas — the “South Louisiana Area” and the “Jackson Area,” — instead of in all the “areas” in which United was purchasing gas.
The amendment specified that the price of gas sold by United to MP&L would be based upon United’s “weighted average purchase price of gas for the preceding billing month in the Jackson Area.” The “weighted average purchase price of gas in the Jackson Area”, would be derived by:
(1) adding together all the volumes of gas which Seller purchased in the Jackson area and transferred into the area during the preceding billing month;
(2) adding together the cost payable by Seller for the gas it purchased in the area and the cost assignable to the gas transferred into the area during the preceding billing month; and
[620]*620(3) dividing the total of said costs payable by the total of said volumes purchased in and transferred into the area. The resulting quotient shall be the weighted average purchase price of gas in the area.
The amendment specified how “cost of gas” was to be assessed. As for gas purchased by Seller in the Jackson Area, the amendment provided:
(a) [t]he cost of gas purchased by Seller shall be the amount payable by Seller to the producer, pipeline or other seller; except that the cost of gas purchased on the seaward side of the shorelines of Florida, Alabama, Mississippi, Louisiana, and Texas ... shall be the amount payable to the producer, pipeline or other seller thereof, plus the amount, if any, by Seller to others for delivery of the gas onshore____
Thus, for gas purchased in the Jackson Area and obtained from the Gulf of Mexico, United could include both the purchase price and the delivery cost for bringing that gas to shore. No provision was made, however, for any transportation or delivery charges for gas purchased in the Jackson Area that was obtained anywhere other than from the Gulf of Mexico.
Next, the amendment provided that the “cost assignable to gas transferred into the Jackson Area,” was to be the average price paid by United for gas which it purchased in only one specific area, the “South Louisiana Area.” Additionally, the cost of delivery onshore to the South Louisiana Area would be included in the weighted average for gas obtained in the Gulf of Mexico:
(c) Cost assignable to gas transferred into the Jackson Area shall be the result obtained by multiplying the weighted average price paid by Seller for gas purchased in its South Louisiana Area ..., and in the Gulf of Mexico for delivery onshore in the South Louisiana Area, by the volume of gas transferred into the Jackson Area.
In summary, the amendment based the price of gas to MP&L on the purchase price paid by United for gas which it purchased only in the “South Louisiana Area” and its “Jackson Area”. The only exception to this pricing system was the allowance of costs for delivery for gas purchased by United originating in the Gulf of Mexico — namely, the cost of getting the Gulf gas onshore.
Except for the Gulf of Mexico gas, United purchased all of its gas from these two “areas” until the mid-1970’s. At that time, United began to purchase from other areas and began to include such purchases in rates billed to MP&L. There is no evidence that United notified MP&L of its intent to include in the price of gas billed to MP&L the cost for gas purchased outside the areas specified in the contract.
In 1978, United entered into an agreement to purchase 450,000 mcf of Canadian gas per day from Northwest Alaskan Pipeline Company. The gas was purchased at the Canadian border and transported to Iowa by Northern Border Pipeline Company. It was there delivered to Northern Natural Gas Pipeline Company.1 Pursuant to an exchange agreement with Northern Natural, United was to exchange that gas in Iowa for equivalent volumes of gas that Northern Natural would deliver to points designated by United in Louisiana, Mississippi, and Texas. Canadian gas began to flow through the Northern Border Pipeline in September 1982. It was the most expensive gas in United’s system.2 Rates charged to MP&L began to include the cost of the Canadian gas as well as the transportation charges incurred by United in delivering the Canadian gas to Northern Natural.
[621]*621Of particular importance is the fact that ■in February 1979, shortly after United contracted to purchase Canadian gas from Northwest Alaskan Pipeline Company, United sought to amend its contract with MP&L so that the rates billed to MP&L would be calculated on a system-wide basis rather than on the limited geographic basis provided in the 1969 amendment. MP&L did not agree to the proposed amendment. In the summer of 1982, when MP&L learned of United’s Canadian Gas agreement, MP&L conducted an audit of United’s books pursuant to a provision in the agreement. MP&L conducted the audit primarily to ascertain whether United was including the cost of the Canadian gas in United’s calculation of MP&L rates. The audit revealed that United had included not only the cost of Canadian gas in its rates, but also the cost of gas purchased in several other states outside of the area specified in the amendment agreement.3
MP&L auditors projected overbillings by United to MP&L of $6,583,822 for the period of September 1983 through December 1983. The auditors further projected an average annual overbilling of $21,145,-205 for the years January 1, 1984 through December 31, 1987, when the current pricing provision expires. Assuming MP&L purchased only the minimum amount of gas required under the contract, the projected overbilling for the four year period, according to MP&L would be $84,-576,820.
The district court enjoined United from including in their calculation of rates billable to MP&L:
(a) the cost of gas and associated transportation charges for Canadian gas and other purchases made outside of the Jackson Area, the South Louisiana Area or the Gulf of Mexico for delivery onshore in the South Louisiana Area;
(b) the cost of gas and associated transportation charges for gas purchased by United in the Gulf of Mexico and delivered onshore outside of the South Louisiana Area; and
(c) transportation charges incurred by United for transporting gas from one point on land to another point on land.
United appeals from the preliminary injunction. 28 U.S.C. § 1292(a)(1). In this appeal we are not concerned with refunds for past overcharges. All that is before us is an injunction against future violations of the pricing provisions in the contract.
II.
The criteria for determining whether a preliminary injunction will be granted are set out in Canal Authority of State of Florida v. Callaway, 489 F.2d 567 (5th Cir.1974). Callaway established that the following requirements must be shown before a party will be entitled to preliminary injunctive relief:
(1) a substantial likelihood that plaintiff will prevail on the merits, (2) a substantial threat that irreparable injury will result if the injunction is not granted, (3) that the threatened injury outweighs the threatened harm to defendant, and (4) that granting the preliminary injunction will not disserve the public interest.
Id. at 572, The decision to grant or deny a preliminary injunction is discretionary with the district court. The standard we must apply in reviewing a grant of preliminary injunction, therefore, is whether the district court’s decision constitutes an abuse of discretion. See Doran v. Salem Inn, Inc., 422 U.S. 922, 932, 95 S.Ct. 2561, 2568, 45 L.Ed.2d 648 (1975); Apple Barrel Productions, Inc. v. Beard, 730 F.2d 384, 386 (5th Cir.1984); City of Meridian, Miss. v. Algernon Blair, Inc., 721 F.2d 525, 527 (5th Cir.1983). A preliminary injunction is an extraordinary remedy. It should only be granted if the movant has clearly carried the burden of persuasion on all four Calla-way prerequisites. The decision to grant a preliminary injunction is to be treated as the exception rather than the rule. State [622]*622of Texas v. Seatrain International, S.A., 518 F.2d 175, 179 (5th Cir.1975); Callaway, 489 F.2d at 576. We find that the district court was correct in holding that the Mississippi Power & Light satisfied its burden of meeting all four prerequisites to the preliminary injunction. We affirm.
Likelihood of Success.
We first consider the court’s finding of a substantial likelihood that MP&L will prevail on the merits of this case. To evaluate the plaintiff’s likelihood of success we determine what is the proper standard to be applied in evaluating plaintiff’s claims, and then we apply that standard to the facts presented in the record. Deerfield Medical Center v. City of Deerfield Beach, 661 F.2d 328, 334 (5th Cir.1981). This is a diversity case and the parties do not dispute that Mississippi law applies. See FMC Finance Corp. v. Reed, 592 F.2d 238, 241 (5th Cir.1979).
Under Mississippi law, words of a contract are to be given their ordinary meaning. Owen v. Gerity, 422 So.2d 284, 288 (Miss.1982); Continental Casualty Company v. Hester, 360 So.2d 695, 696 (Miss.1978). When a written instrument is clear, definite, explicit and free from ambiguity, a court in construing it will look solely to the language used in the instrument itself. Pfisterer v. Noble, 320 So.2d 383, 384 (Miss.1975). If, however, a careful reading of the instrument reveals it to be less than clear and free from ambiguity, the court is obligated to pursue the intent of the parties which must be determined by resort to extrinsic aid. Barnett v. Getty Oil Company, 266 So.2d 581, 586 (Miss. 1972). Applying these standards, the district court concluded that “[wjhile the pricing provisions of the Agreement are complex, at the present time it appears to the Court that there is a limitation on the charges which may be included in the computation of the price of gas to MP&L”.
The district court first found that the contract did not contemplate the inclusion of the price of the Canadian gas, or other gas purchased outside the areas specified in the contract, in the price calculation to be used in billing MP&L. Although the contract does refer specifically to pricing in relation to the Jackson and South Louisiana Areas, there is no mention of any costs associated with any other area, including the costs of the exchange gas which began to appear in MP&L’s billing in 1982 shortly after United began receiving the gas through its system from the Canadian border. MP&L never actually received any of the Canadian gas in the Jackson Area, yet the cost of the Canadian gas was reflected in MP&L’s billing.
United did not inform MP&L that it was including the exchange costs in MP&L’s billing structure, and it was not until MP&L conducted its own audit of United that the inclusion was discovered. Furthermore, United’s witnesses were unable to identify any other United contracts where the price of exchange gas was included in a customer’s billing, even though the witnesses were aware that other exchange agreements existed. Based on all of this evidence, the district court concluded that it was not the intention of United and MP &L to include the cost of exchange gas in the contract.
The district court also enjoined United from charging MP&L transportation costs for gas delivered from the Gulf of Mexico to locations other than the South Louisiana Area. The court concluded that the clear and unambiguous meaning of the words “onshore in the South Louisiana Area” meant that transportation charges should be included for gas obtained in the Gulf of Mexico and delivered onshore in that specific area. The court found United’s argument that “onshore to South Louisiana” meant “onshore to anywhere in United’s main Pipeline system” to be a gross distortion of the clear wording of the contract.
The billing system which United uses further supports the conclusion that the costs of neither the Canadian exchange gas nor the transportation costs for delivery outside the South Louisiana Area were to be included in MP&L’s billing under the contract. The majority of United’s indus[623]*623trial customers are billed for gas using a “system-wide” computation. This means that the cost of gas is calculated based on all gas purchased throughout United’s system. Under the system-wide approach, no differentiation by location of the gas purchases is made.
By United’s own admission, the contract with MP&L is not based on a system-wide computation for determining the price of gas. Rather, the contract provides that the price of gas will be based on an area-specific computation. This means that regardless of where the gas ultimately delivered to MP&L is purchased, the price charged will be based on the areas specified in the contract. Prior to 1982, MP&L’s billing reflected computations based on the area-specified calculation. Shortly before United concluded its Canadian deal with Alaskan, United approached MP&L to discuss the possibility of amending the contract to reflect a system-wide computation as opposed to the area-specific computation. MP&L declined to amend the contract. Shortly thereafter and without MP&L’s knowledge the cost of the Canadian exchange was included in MP&L’s price computation.
United claims that its inclusion of those costs in its computation was justified under the terms of the contract and that the district court, by granting the preliminary injunction against United, merely accepted MP&L’s interpretation of the contract over its own. In light of the evidence before the district court, however, we agree that under the language of the contract and evidence relating to the intent of the parties, there is a substantial likelihood that MP&L will prevail. After MP&L refused to alter the terms of its contract with United, United took it upon itself to include the additional exchange and transportation costs anyway. Once this action was questioned by MP&L, United attempted to justify its position with an unconvincing interpretation of the existing terms of the contract.
We stress that the district court’s conclusion as to the likelihood of success does not represent a final determination on the merits. It is true that the pricing terms of the contract are complex, and protracted litigation over the interpretation of the contract terms may well ensue. In the meantime, we find that the district court properly concluded that the probability of success weighed substantially in MP&L’s favor.
Irreparable Injury and the Public Interest.
Having determined that there is a substantial likelihood that MP&L will prevail on the merits in this dispute, the district court next found a substantial threat that irreparable injury would result if the preliminary injunction did not issue. The court also found that granting the injunction was in the public interest, stating that “the dominant presence of the public interest in this case convinces the Court that a preliminary injunction is warranted”. We believe that the district court has correctly identified the public interest as the central issue in this case. The irreparable harm asserted is the adverse impact that the overcharges would have on the public. Thus, the irreparable harm and the public interest inquiries are intertwined, and we consider them jointly. See Treasure Salvors, Inc. v. Unidentified Wrecked and Abandoned Sailing Vessel, 640 F.2d 560, 568 (5th Cir.1981).
As all parties to this dispute acknowledge, pecuniary loss to MP&L is not at issue in this appeal. MP&L concedes that monetary damages would be an adequate remedy to it at the end of a trial on the merits. MP&L passes all fuel charges on to its customers, so it is the public that is ultimately affected by any overcharges. The Mississippi Public Service Commission intervened in this action to protect consumers from overcharges. Indeed, the public interest is substantial in this case in that fuel charges comprise between fifty and sixty percent of the total cost charged to the consumers.
On the questions of irreparable harm and public interest, the district court granted injunctive relief based primarily on the tes[624]*624timony of Mr. Marvin Havens, Chairman of the Mississippi Public Service Commission. As the Commissioner pointed out, if the injunction were denied and MP&L ultimately prevailed on the merits of this case, the consumers of MP&L would have to be compensated by a refund system for the overcharges. The Commissioner testified that previous experience with refunds to the public shows that they create serious administrative problems and ultimately prove to be an inadequate remedy. Citing as an example a 1980 rate case where MP &L was ordered to make refunds to its customers, he explained that some 20,000 refund checks marked “undeliverable” were eventually returned to MP&L, representing approximately 1 million dollars in refunds lost to their intended recipients. In addition, the cost of processing each check was approximately five dollars, which substantially reduced the pool of funds available for consumer refund. Based on this testimony, the district court concluded that a refund would be an inadequate remedy.
The case of Federal Power Commission v. Tennessee Gas Transmission Co., 371 U.S. 145, 83 S.Ct. 211, 9 L.Ed.2d 199 (1962), supports the conclusion that refunds in a utility rate dispute are not adequate to compensate the consumers who have borne the burden of overcharges pending the resolution of litigation. In that case, the Court was called upon to review an interim order of the Federal Power Commission to reduce rates pending final determination of proceedings with respect to the allocation of rates by a natural gas company. The company argued that ultimately it could be decided that a higher rate was reasonable, so the interim order might result in irretrievable loss to the company. The Court, however, upheld the order. It said that use of such an order was in keeping with the purpose of the Natural Gas Act, 15 U.S.C. § 717, “to protect consumers against exploitation at the hands of natural gas companies” and “to underwrite just and reasonable rates to the consumers of natural gas.” Tennessee Gas Transmission, 371 U.S. at 154, 83 S.Ct. at 216 (citations omitted). The Supreme Court projected that had the gas company been allowed to continue its collection of illegal rates:
the exaction would have been subject to refund, but experience has shown this to be somewhat illusory in view of the trickling down process necessary to be followed, the incidental cost of which is often borne by the consumer, and in view of the transient nature of our society which often prevents refunds from reaching those to whom they are due.
Id. (footnote omitted).4 Here the district court considered testimony that indicated the likelihood of similar problems if a refund were ultimately required. The illusory nature of the remedy of a refund under the circumstances of this case supports the conclusion that irreparable harm would result if overcharges continued. Many of the consumers injured would never be compensated adequately.5
Two other considerations have been raised which underscore the importance and magnitude of the public interest at stake. First, the demographics of the area served by MP&L are of particular concern in relation to the threatened harm to the public. Mississippi has the lowest per capita income of the fifty states. Forty-four of the forty-five counties served by MP&L had an average per capita income of only $6,723 in 1981, as compared to the low [625]*625state average of $7,409. Statewide, approximately twenty-two percent of the families in Mississippi lived below the poverty level in 1979.6 A refund of overcharges sometime in the future could never adequately compensate families living at or close to the poverty line for hardships they would endure as a result of overcharges they would have to pay at present and during the course of litigation. This is especially true in light of evidence presented to the court that low income bracket families frequently consume large amounts of fuel.
Second, in our consideration of the public interest at stake we note that the Mississippi Legislature recently enacted the Utility Reform Act of 19837 which was a substantial remodeling of the previous Utility Act of 1956. In its declaration of policy, the Act announces the strong public interest involved in the determination and regulation of utility rates. Miss.Code Ann. § 77-3-2 (Supp.1984). The Chairman of the Commission, who has testified concerning the Act on numerous occasions, stated that one of the primary obligations of the new legislation was to prevent charges from being passed on to any consumers before some judicial or administrative determination on the lawfulness of the charges. Compliance with the obligation could be met, for example, in a Commission hearing on the fairness and lawfulness of the charges.
But the Commission is without jurisdiction to regulate the contract between MP&L and United, and may only regulate the rates ultimately charged by MP&L to its customers. Thus, in a sense, the district court was fulfilling a role analogous to that of an administrative rate regulation body in deciding whether to issue the preliminary injunction. In this dispute, the Commission urged that United’s insistence on billing certain charges conflicted with the legislative intent of the Act by seeking the continuation of charges before the lawfulness of the charges had been determined. The Commission urged the district court to consider the Commission’s legislative interest and regulatory role in protecting the public.
There is an issue of the propriety of considering harm to the public when the injunctive relief sought concerns a private contract. In this respect, we first note that although the consumers of MP&L are not parties to this suit, the Mississippi Public Service Commission has intervened on their behalf. Indeed, it would be anomalous to conclude that the Commission’s intervention was proper, but that its interests, i.e., the interests of the consumers of MP&L, cannot be considered because the Commission was not a party to the disputed contract. More significant is a reminder that the standard for determining the propriety of granting a preliminary injunction is the same regardless of the nature of the dispute. The prerequisites set out in Calla-way remain the proper standard for granting a preliminary injunction in all instances where a statute is not otherwise controlling. This means that even in a private contract dispute we are required by Calla-way to take into account the vital public interest which may be in jeopardy. The requirement that we consider the public interest would be meaningless if we were to identify potential injury to the public but were to conclude that in the context of a contract dispute we were constrained to consider only the immediate interests of the parties to the contract.8
In recent years, federal courts have given increasing attention to the importance to the public interest of prelimi[626]*626nary injunctive relief. In the opinion of one comméntator, such a shift in emphasis is perhaps a demonstration of judicial concern about the impact of legal decisions on society as a whole as opposed to the more limited interests of private litigants. See Leubsdorf, “The Standard for Preliminary Injunctions”, 91 Harv.L.Rev. 525, 549 (1978); see also 7 J. Moore, Moore’s Federal Practice, § 65.04[1] (2d ed. 1984). We conclude that the public interest involved in this dispute compels us to look beyond the immediate interests of the named litigants and to consider the situation of the consumers upon whose behalf the Mississippi Public Service Commission has intervened. The vital public interest involved in protecting the consumers of MP&L against the harmful effect of overcharges establishes the requisite apprehension of irreparable injury and warranted the granting of preliminary injunctive relief.
Balance of Harm.
The final task under Callaway is to balance the harm that would be suffered by the public if the preliminary injunction were denied against the possible harm that would result to United if the injunction were granted. United contends that if it is to be enjoined from collecting the alleged overcharges, its revenues will be substantially reduced, seriously affecting its borrowing capacity. United estimates that enjoining it from collecting approximately 21.1 million in revenues from MP&L during 1984 could result in a loss to United of 60 million dollars in available credit. This loss of revenues and any concomitant loss in borrowing capacity, it is urged, could affect United’s ability to settle the take-or-pay disputes that United is currently negotiating with several natural gas producers.9 United estimates its take-or-pay exposure for 1984 at a possible one billion dollars. United fears that if its borrowing ability is diminished due to reduced revenues from MP&L, producers might lose confidence in United and begin taking their claims to court, seriously jeopardizing United’s viability as a gas supplier.
United’s potential exposure to liability, take-or-pay or otherwise, is not a new state of affairs. In past years, United has managed to preserve its viability in the face of substantial liability. United’s annual report for 1982 indicates that revenues were nearly 3.5 billion in that year. In the same year, United listed on its consolidated “Statement of Income and Retaining Earnings” as an extraordinary expense item nearly 60.5 million in litigation costs.10 Despite its liability exposure, United still had end of the year retained earnings of $216,-369,000, and paid out 66 million dollars in dividends. A loss of any revenues would inevitably have some impact on operations, but the district court found that United’s projection of harm did not depict a likely scenario given the healthy state of United in 1982 when the company was already facing considerable liability. The court thus concluded that the balance of harm tipped in favor of granting the injunction because the harm threatened to the public was perceived as being greater than any speculative harm that might be suffered by United. We find no reason to disturb this finding.11
[627]*627
Conclusion.
The purpose of a preliminary injunction is to prevent irreparable injury so as to preserve the court’s ability to render a meaningful decision on the merits. Callaway, 489 F.2d at 576. The district court found that this purpose would best be served by enjoining United from charging certain costs because of the substantial likelihood they were overcharges and because of the irreparable harm such overcharges would inflict upon consumers who purchased their fuel from MP&L. We conclude that there is ample evidence in the record to justify the district court’s decision.
AFFIRMED.