What the Realtor probably won't tell you. Or, why you should check energy costs before signing on the dotted line

YOUR chosen house has the requisite number of bedrooms, bathrooms on both floors, a cozy den, and a patio and private yard that leave you dreaming of summer entertaining. Even the view from the living room is great. In short, everything about the place speaks of ``home.'' But before you sign up and commit your family to sizable mortgage payments in the decades ahead, check into something that isn't immediately obvious when the real estate salesman shows you around: the cost of keeping the house acceptably warm in winter and pleasantly cool in summer.

In brief, is it energy efficient?

To do this, you will need the previous year's utility bills. When you're just looking over the home for the first time, the agent's word on energy costs is acceptable. But as soon as you get serious about the place, ask to see the electric, oil, and or gas bills for the previous 12 months. They may tell you a lot about how affordable your new home will be.

Ask for the bills for a full 12 months, not just for the heating or cooling seasons. If the owner hasn't kept a complete record, he can give you a letter requesting copies of last year's bills from the utility company. Utilities will not release them without the owner's written permission.

With the bills in hand, simple arithmetic will give you the seller's annual energy costs. If these are similar to or below the energy costs of your previous home, you will probably be satisfied, but you will still need to take more facts into consideration.

Your energy costs could vary considerably from the previous occupants' if:

The previous family was much larger or smaller than yours. Also, a family of teen-age sons tends to use a lot less hot water than a family of teen-age daughters.

The home was unoccupied for long periods of time, particularly in winter. Modest January and February heating costs, compared with say April's, might indicate that the family wintered in the Bahamas.

The previous family used a thermostat setting that was unusually high (above 74 degrees F.) or unusually low (below 65 degrees F.).

The previous winter was unusually severe or mild.

While the cardinal rule is, don't buy a used home until you have seen an accurate, documented record of past energy bills, don't automatically reject a house that appears disappointingly costly to keep comfortable. Ways of trimming energy bills without sacrificing comfort are many and inexpensive relative to the money saved. This is particularly true of most older homes.

You might also take energy-improvement options into consideration in comparing two homes with similar heating costs.

The one with obvious opportunities to improve its energy efficiency (the addition of caulking, weatherstripping, storm windows and doors, for example) would then be the better buy.