The city of Springfield (city) owns a hydroelectric plant in the town of Granville (town), and for forty years paid a property tax to the town based on an assessed valuation of $1,100,000. The city challenged the 1973 assessment on grounds of overvaluation and dispro*160portionate assessment. We hold that the fair cash value of the plant is fixed at $1,100,000 by St. 1928, c. 267, § 4, and that the statute has not been shown to be unconstitutional, but that the city is entitled to an abatement by reason of disproportionate assessment.
The case was tried in the Appellate Tax Board (board) on the pleadings, exhibits, testimony and a stipulation that all real and personal property in the town, other than property owned by the city, was assessed at 31% of fair cash value. The board denied a motion of the assessors of the town to dismiss for lack of jurisdiction. It then ruled that the 1928 statute established or fixed the value of the property, contrary to the view of both parties that that statute created a presumption of value only. Further, the board ruled that the governing statutes afforded no relief on the basis of disproportionate assessment. The city appealed to this court pursuant to G. L. c. 58A, § 13.
We summarize the board’s findings. Under a contract dated June 21, 1928, the city agreed to pay a private utility $1,100,000 for constructing the plant and to lease the plant to the utility for thirty years at a stated annual rental of $270,000. The city also agreed to construct, at its own expense, additional facilities to be used for water supply purposes as well as in the generation of electric power. The plant was completed in 1932, and was assessed at $1,100,000 in that year and each succeeding year through 1973. The city’s total expenditure on the project was over $5 million. As of January 1, 1973, the value of the plant, exclusive of facilities also used for water supply purposes, was $821,000.
1. Jurisdiction of the board. The authority for the project is found in St. 1912, c. 607; Spec. St. 1918, c. 163; and St. 1928, c. 267. Under § 3 of the 1912 statute, property used for the generation of electricity for sale "may be valued for the purpose of taxation at the fair cash value thereof, and the tax thereon may be collected, by the cities or towns within which the same are located. But nothing herein contained shall be construed as authoriz*161ing the taxation of dams, reservoirs and other structures used and appropriated for any other purpose than the generating of electricity for sale.” A similar provision, extending to property used "for the generation or transmission of electricity for sale or for the use of said city within its own limits,” subject to an identical reservation for structures used for another purpose, was included in § 5 of the 1918 statute.
The 1918 statute also provided that the city might appeal from the valuation to the county commissioners or to the Superior Court, subject to St. 1909, c. 490, Part I, §§ 77, 78, and "any amendments thereof, so far as the same are applicable.” The city was not required to bring in to the assessors the list of property required by § 41 of Part I. The 1928 statute, § 4, provided, "For the purposes of’ St. 1912, c. 607, § 3, and Spec. St. 1918, c. 163, § 5, "the value of the property to be taxed thereunder shall be assumed to be as follows: — in Granville, one million one hundred thousand dollars ....”
The 1909 statute providing for appeals to the county commissioners or to the Superior Court was codified as G. L. c. 59, §§ 64-66 (1921). By St. 1930, c. 416, § 14, the Board of Tax Appeals was substituted for the Superior Court, and the Appellate Tax Board was substituted by St. 1937, c. 400, § 1. Thus the board now has jurisdiction of valuation appeals formerly heard by the Superior Court. G. L. c. 58A, § 6; c. 59, § 65. The board correctly traced its jurisdiction. See P. Nichols, Taxation in Massachusetts 199-220 (3d ed. 1938).
2. Interpretation of "assumed” value. The board read the 1928 provision that the "value” of the property shall be "assumed” to be $1,100,000 as establishing its fair cash value at that figure. We agree. There was evidence before the board warranting a conclusion that the parties, in drafting the 1928 statute, understood the difficulties inherent in determining value and fixed the value of the plant to avoid yearly estimations. Late in 1927 the city’s engineer suggested that "a definite figure for assessed *162valuation should be put into the bill.” The selectmen and assessors of the town informed him by letter that a valuation of $1,100,000 was acceptable, “subject to a proper provision in the bill for increased valuation if and when the plant is enlarged.” The statute did not contain such a provision for increased valuation, and there was no evidence that the plant was ever enlarged. The $1,100,-000 figure was the price to be paid by the city to the constructing utility.
Subsequent events tended to confirm this interpretation. The same value was used each year for forty years, and no abatement was sought by the city until 1973. The town requested State authority to increase the valuation, and the request was refused, as was a similar request from another town subject to a $3,000 value fixed in the 1928 statute. 1934 Senate Doc. No. 35. As to the effect of such practical construction, see Ace Heating Serv., Inc. v. State Tax Comm’n, 371 Mass. 254, 255-256 (1976).
3. Constitutionality. The assessors now argue that the arbitrary valuation in the 1928 statute and the exemption of water supply property in the 1912 and 1918 statutes violate the equal protection clause of the Fourteenth Amendment to the United States Constitution and the requirement of the Massachusetts Constitution that taxes be proportional and reasonable. Declaration of Rights art. 10; Part II, c. 1, § 1, art. 4. It is not clear that these contentions were presented to the board. See G. L. c. 58A, § 13. Certainly the board did not discuss them in its opinion, and the record before us does not permit us to deal with them definitively.
Since the board proceeded on a legal theory different from that asserted by both parties, however, we think it proper to comment briefly on the facial validity of the statute as construed. Since power plants are rarely bought and sold, determination of their “fair cash value” necessarily requires some estimation. See J.C. Bonbright, Valuation of Property 511-521 (1937). The difficulties are compounded when the taxable property derives some of *163its value from exempt property. See Blackstone Mfg. Co. v. Blackstone, 200 Mass. 82, 89-92 (1908). We presume that a statute "is based on adequate facts justifying its adoption in the public interest.” Thompson v. Chelsea, 358 Mass. 1, 6 (1970). Here there is no showing either that the scheme of fixed valuation and exemption was unreasonable when enacted, or that circumstances have since rendered it unfair. See Holbrook v. Randolph, 374 Mass. 437, 441 (1978).
The principle of proportional taxation leaves room for the Legislature to grant exemptions reasonably related to the public interest. Weinstock v. Hull, 367 Mass. 66, 68-70, appeal dismissed for want of a substantial Federal question, 423 U.S. 805 (1975). Assessors of W. Springfield v. Eastern States Exposition, 326 Mass. 167, 170 (1950). Opinion of the Justices, 195 Mass. 607, 608-610 (1908). Cf. Tax Collector of N. Reading v. Reading, 366 Mass. 438, 440-441 (1974) (implied exemption for municipal electric utility); Gas & Elec. Comm’rs of Middleborough v. Assessors of Lakeville, 355 Mass. 387, 389-390 (1969) (same); Assessors of Quincy v. Cunningham Foundation, 305 Mass. 411, 416-417 (1940) (exemption despite lack of benefit to municipality where property located), and cases cited. Property may be exempted "in whole or in part.” Cabot v. Assessors of Boston, 335 Mass. 53, 60 (1956).
The Legislature has broad powers in allocating financial burdens among municipalities. Thompson v. Chelsea, 358 Mass. 1, 8 (1970). Where the taxpayer is a municipality, "the strict rule of proportionality in taxing individuals need not be satisfied.” Brookline v. County Comm’rs of Norfolk, 367 Mass. 345, 350 (1975). On the present record, the town has not shown that the statutory scheme is unconstitutional.
4. Disproportionate assessment. The 1912, 1918 and 1928 statutes were enacted during the regime of Lowell v. County Comm’rs of Middlesex, 152 Mass. 372 (1890), under which no relief for disproportionate assessment was available in a tax abatement proceeding. The board *164thought, therefore, that the Legislature could not have intended that such relief would be available under our decision in Shoppers’ World, Inc. v. Assessors of Framingham, 348 Mass. 366 (1965), and its progeny.
We think, however, that the 1918 statute discloses a general intent to treat the city like other taxpayers with respect to the conduct of abatement proceedings, giving it the benefit of statutory and other developments. The 1928 statute fixed the "value” of the property for the purposes of the 1918 statute at $1,100,000. Thus the case is in the same posture as if the assessors had correctly determined by appraisal, instead of by statutory assumption, that the "fair cash value” of the property was $1,-100,000. Since all other property was assessed at 31% of fair cash value, the city was entitled to be assessed at the same percentage. Tregor v. Assessors of Boston, 377 Mass. 602, 609 (1979), and cases cited.
5. Disposition. The decision of the board is reversed and the case is remanded to the board for further proceedings consistent with this opinion.
So ordered.