A coalition of Prince George's County unions opposed to County Executive Lawrence J. Hogan's proposed budget for 1980 released a study yesterday that said showed Hogan had underestimated next year's revenues by about $20 million.

However, county officials who prepared Hogans's budget discounted this estimate, saying it was based on inaccurate and incomplete information.

The study was done by Robert Pellicoro, a professional consultant and budget analyst hired by a coalition that includes the county's teachers, blue-collar and clerical employes. The coalition has been campaigning against Hogan's proposal to limit county workers cost-of-living pay increases to 3 percent next year.

Pellicoro concluded that the estimates of the county's tax collections and other revenues included in Hogan's budget were far below what they should have been, based on study of the collections for other years.

In releasing Pellicoro's work, Toby Rich, the president of the county teachers' union and a leader of the coalition, charger that Hogan had deliberately underestimated next year's revenues in an attempt to have a surplus on hand for the 1981 budget.

Hogan, Rich said, wants to force the County Council to make cuts in employe salaries and other services by insisting that the funds for further spending were not available.

The charges came hours before the council's second public hearing on the 1980 budget. For his part. Hogan sent a memorandum yesterday to council members that argued that "unlike New York City, we cannot create revenues that do not exist."

Hogan argued the council members to discount the arguments of union leaders for higher cost-of-living increases, saying the unions were "shortsighted." Hogan also noted that "a cursory examination" of the school's telephone directory showed him that "one-third of the central and area office staff reside outside Prince George's County.

"I am sure this percentage would prevail for all school board employes," Hogan wrote. "It is not surprising, then, that they are not worried about the impact . . . their demands (for higher wage increases) will have on our budget for other services."

School officials said yesterday that they had never tabulated what percentage of school employes live outside the county, and therefore could not gauge the accuracy of Hogan's statements.

The study released by the county unions criticized Hoganhs estimates of tax collections in several areas because the 1980 revenue estimates fell below anticipated 1979 revenue collections.

County budget officials said, however, that they expected the county to collect less in taxes related to financing and home building next year because they thought high interest rates and rising costs would soon slow down new development in Prince George's. CAPTION: Picture, LAWRENCE J. HOGAN . . . denies funds are there