If you're one of the tens of thousands of Americans who own a rental home or condominium -- or who plan to buy one in 1985 -- you might be interested in some words of wisdom from Eugene Isaacs.

Isaacs is the undisputed colossus of the American rental-home industry. The firm he heads, Epic Realty Services, owns or manages more than 15,500 single-family houses, town houses, duplexes and condominiums of nearly every shape and size from the East Coast to the West Coast. They range from modest $50,000 Texas tract units to sprawling $400,000 homes in the suburbs of New York and Los Angeles.

Epic just completed a massive end-of-the-year internal financial analysis of its rental investments, and Isaacs agreed to share some of the findings.

One major conclusion: Think small as a rental-home purchaser and you'll pocket more rent. Epic's study of its houses and condos in 12 of the largest metropolitan markets documented dramatic differences in the relative earning power of lower-priced versus higher-valued properties.

Nationwide, detached houses in the $70,000 range and condominiums or town houses in the $50,000-to-$60,000 range turned out to be the most efficient rental-income producers. For example, the typical $70,000 detached single-family home in Epic's portfolio pulled in $6,300 a year in rent -- 9 percent of its market resale value as of December. Houses above $100,000, by contrast, earned steadily less. Their yields ranged from 8 percent (on $110,000 homes) to a rock-bottom 5 percent on big-ticket luxury models.

Rents for town houses and condos showed the same correlation. Modest units worth $50,000 pulled in an average 9 percent of their resale value during 1984, whereas top-of-the-line condos in the $200,000 class barely earned 5 to 6 percent.

Modest-priced houses and condos in what Isaacs calls "high-demand, low-supply" markets performed even better. For instance, rental homes worth $60,000 in the New Jersey suburbs of New York and Philadelphia were among Epic's star producers of 1984. They pulled in an average 14 percent of resale value, about $700 a month. (That's not a bad return for properties that were sometimes acquired with little cash.)

Houses in South Florida, Los Angeles, Dallas, Tampa, San Antonio, Denver, Washington, D.C., and Tampa in the $60,000-to-$70,000 price range were also well above the national average as rent-earners, yielding from 10 to 14 percent of value. Bigger homes in the same markets generally were nowhere near as efficient, however.

Why so sizable a gap in earning power between economy-model houses and the luxury end? Why will a $60,000 home near Denver pull in $7,200 a year in rent (l2 percent of its cash worth) while a $110,000 house with more bedrooms, bigger yard and more amenities in the same community go for $7,700 (7 percent of market value)?

According to Isaacs, part of the answer has to do with market variations in the depth of demand for rental housing in any form. For example, in the economically booming New York suburbs, where rentals for apartments are among the costliest in the country, families will pay premium rents for detached housing that gets them out of apartment living altogether.

Inevitably, the detached rental houses that are affordable for people fleeing apartments are at the bottom end of the sale-price scale, Isaacs said. There's tremendous demand for homes or condos in the $600-to-$800-a-month range, but an insufficient supply, he added. As a result, the rent-to-sale-price ratio has been bid up sharply.

The reverse pattern tends to be true in the swank, upper reaches of the market. Families who have $1,500 to $2,000 a month to spend on shelter want to put it into equity ownership, not into their landlord's bank account. Houses valued at $250,000 and more in California, the Northeast and elsewhere sometimes pull in as little as 4 percent of their value a year. They are the biggest bargains in the national rental-housing bazaar by far, but they still go begging.

Isaacs cautions, however, that investors in rental homes shouldn't blindly assume that smaller and cheaper will always be better in every local market in 1985. To the contrary, he argues. Appreciation rates may be significantly higher in your particular community this year for mid-price-range rental homes that can be sold to regular home buyers. High-bracket homes may be puny rental-income producers, in other words, but could jump in resale value at 10 to 15 percent a year in hot markets.