If you've been thinking about making some energy-conserving improvements this fall to the home you own or rent, don't overlook the possibility of getting federal tax credits for your good sense.

New regulations issued by the Internal Revenue Service spell out what qualified for residential energy tax breaks -- on everything from insulation in your attic to solar panels and windmills -- and what doesn't. The rules get tricky in a few areas, and they're written in bureaucratese, but here's how they work:

Picture two categories of energy-related expenditures involving your real estate.

The first category includes a wide range of conservation devices that help you cut fuel usage but don't actually produce additional energy themselves. The most common of these are insulation (batts, foams, strips, blown-in materials), storm windows or thermal glazing, weatherstripping, and caulking, furnance efficiency controls, thermostats that control heating cycles, and the like.

For expenditures on these conservation devices, you can get a credit on your next tax return of 15 percent of the first $2,000 of the costs of purchase and installation, or a maximum of $300. The credit is subtracted directly from the amount you would otherwise owe the Treasury.

The second category of expenditures is for "renewable" energy resources: passive and active solar heating sytems, wind-powered and geothermal systems. The credits here go much higher. For qualified installations this year and through 1985, you can get a 40 percent credit on the first $10,000 of expenditures ($4,000 maximum). For expenditures you made prior to Jan. 1, 1980 but after April 19, 1977, you may qualify for a maximum credit of $2,200 (30 percent of the first $2,000 of costs, 20 percent of the next $8,000).

The tricky parts of these two categories -- and the places where the IRS's rules can snag you at tax time -- are in the definitions of what qualifies as worthy of an energy credit.

If you're hoping to qualify for an insulation credit, make sure the materials you list have as their "sole" purpose that of insulation. The IRS won't allow you a cent on anything with a "dual function," only one of which happens to be insulation. That rules out the carpeting, walls, wall coverings, drapes, wood paneling, glass fireplace screens and awnings that taxpayers have sought to list in this category on their returns in past years.

Insulation can be made of a variety of substances -- fiber glass, vermiculite, rock wool, polystyrene and cellulose, among others -- but it can't have any other function for your house.

Ditto on "dual functions" for anything related to the increasingly popular passive or active solar improvements. Forget it if you're looking for a tax credit for those insulated skylights, big windows or greenhouses you've installed on the sun-facing South side of your house.

If your "solar" installations serve any structural function whatsoever, or if they aren't part of a complete solar system as defined by the IRS, they're not going to pass muster. In the case of a passive solar heating system, for instance, it's got to have at least three components: a solar collector surface, an "absorber" of the heat (such as a floor), and a "storage mass" (such as masonry) that holds the later transfers the heat to the interior of the house.

Any expenses you had on such a system that have basic structural purposes -- such as materials and labor associated with the roof, clerestory windows or mass walls -- won't qualify for a credit.

In the case of an active solar system, you've got to have all of the following, in addition to collectors: a storage tank, rockbed, a thermostat to activate a circulator pump or fan and a heat exchanger.

Energy-conserving devices that qualifty for credits must have an expected life of at least three years, and have to be put into first use by the taxpayer (they can't be used equipment). Any of the following can qualify: furnance replacement burners for oil or gas-fired boilers or furnaces that improve fuel efficiency; flue modifiers that reduce heated or cooled air losses through chimneys; automatic furnance ignition systems (but only those "that replace a gas pilot light" on your existing unit); automatic setback thermostats that regulate demand on heating or cooling systems; and energy usage display meters that show the dollar cost of electricity, gas or oil consumption on a daily or cumulative basic.

A copy of the IRS rules, or more information on who and what qualifies for residential tax credits, can be obtained from Walter H. Woo, Office of the Chief Counsel, Legislation and Regulations Division, IRS, 1111 Constitution Ave. NW, Washington 20224 (Attention: CC: LR: T).