One of the hottest new trends in solar design is the solar greenhouse, sometimes called the "sun space." Unlike conventional solar panels, a solar greenhouse adds good looks and extra living space to your home. It even gives you a place to grow flowers and food all year round.

But the best part is that, properly designed and situated, it can not only heat itself, but can harvest energy to help heat your home.

Just how much home heat can it provide depends on a lot of variables, but as an example at least one maker of these greenhouses offers a computerized energy study to help its customers project potential savings. The study is based on a computer program developed by Los Alamos Labs. I asked the company for one of these studies for a hypothetical house of 1,500 square feet, heated with electricity at seven cents a Kwh, and located in Hartford, Connecticut.

According to the study, installing a 24-foot long greenhouse on the south wall of this home would cut fuel use by 22 percent, giving a saving of $346 for the first year.

That's not bad. But performance like that demands proper greenhouse design and location. To perform well, a solar greenhouse must be attached to your house, so it can feed the heat it collects into your living space. To receive the maximum amount of solar radiation, the greenhouse must face south, or within about 30 degrees east or west of south. In addition, it will perform best if its south face is slanted at about a 60 degree angle, to put the face roughly perpendicular to the rays of the winter sun.

Finally, since little solar energy will enter through the roof or ends of the greenhouse (at least during the critical winter months) the roof and ends of the greenhouse should be opaque and heavily insulated so they won't lose the energy being collected by the south face.

If you are interested in such a greenhouse, there are now several companies offering them in kit form. Many of these makers are pushing the fact that a solar greenhouse may be eligible for a 40 percent federal tax credit as a source of renewable energy. If that's so, it's the same as taking 40 percent off the cost of any such greenhouse you install, up to a maximum of $4,000.

That's a hefty chunk to save on your taxes, but actually getting this credit may not be as easy as some of the kit-makers say it is.

Most makers agree that to quality, the greenhouse has to have a collection area (that south face), plus some form of energy-absorbing surface such as a dark stone floor. It must also have a heat-storage medium such as a mass of rock or water, some kind of heat-distribution system such as a thermostat-controlled fan, and heat-retaining features such as double glazing, insulation and thermal curtains.

At least one greenhouse designer I have talked to agrees that all these features are necessary to get a tax credit. But he also says that in addition, the greenhouse must be used as a solar collector exclusively. If you use it as an actual greenhouse, or equip it with furniture, it no longer qualifies.

At present, the solar greenhouse is in a gray area with respect to federal tax credits, and you can't tell in advance whether you'll get a credit or not. All you can do is apply for it after installing the greenhouse, and hope IRS allows it. That makes it a bit difficult to determine the final cost of one of these installations ahead of time. Without a tax credit, it may cost you $5,000. With a credit, the same greenhouse would cost just $3,000.

If that kind of cost uncertainty doesn't turn you off, here's a list of some of the major kit-makers: Brandy & Sun, 12 Jacques Street, Worchester, Massachusetts 01603; Garden Way Greenhouses, 101 Ferry Road, Charlotte, Vermont 05445; Habitat, 1232 Elm Street, Deerfield, Massachusetts 01373; Abundant Energy, 116 Newport Bridge Road, Warwick, New York 10990; Weather Energy Systems, 39 Barlow Landing Road, Pocasset, Massachusetts 02559; Vegetable Factory, 100 Court Street, Copiague, New York 11726; Solar Resources, P.O. Box 1848, Taos, New Mexico 87571.