Good for the Earth and Good for Your Wallet : Environment: Products that reduce pollution and conserve power and water result in significant long-term savings.

Psssst! Want to earn a guaranteed 40% on your investment? Four words: energy efficient light bulb.

Check out this cash flow--a water-conserving shower head that yields up to 275%.

In these days of rock-bottom interest rates, environmentally sound versions of everyday household items can provide investors tantalizingly high returns.

Purchasing earth-friendly products is not new. But considering them investments is. Amid rising interest in the environment and declining yields on traditional investments, there are some extraordinarily simple ways to soothe your conscience and pad your wallet.

Consider the light bulb. An incandescent version costs about 75 cents for 75 watts and lasts 750 hours. Light for more than one year, or about 9,000 hours, cost you about $52--$43 in electricity (the national average is 8 cents per kilowatt-hour) and $9 for 12 bulbs.

But for about $18, you can buy an 18-watt compact fluorescent bulb. It lasts 9,000 hours and sheds comparable light, but uses about $13 in electricity. Your cost: $31.

So the extra $17.25 initial investment saves $21 over a little more than one year of continuous usage. If that usage stretched over three years, you theoretically “earn” $7 a year on your $18 original investment--a roughly 40% annual return.

Not bad, considering the same $18 earns less than 70 cents a year sitting in a 30-month certificate of deposit with an annual yield of about 3.75 percent. Even if that $18 were in a stock mutual fund returning a fat 15% in 1992, it would have earned just $2.70.

But don’t rush to foresake conventional investments just to save the earth. A fluorescent bulb costs far less than the minimum required by most CDs and mutual funds, and few people need thousands of dollars in light fixtures.

Moreover, it’s hard to rely on returns you really don’t see. While you can apply gains from a savings bond toward your retirement, the impact of lower electric bills on your nest egg is far less direct.

Spreading little investments throughout your home, though, can add up.

“You could take a portfolio of $500 or $1,000 and invest it in a variety of things. Some make 18%,some 62%, some make 3%. But overall, you start to treat it as any marketbasket of investments,” said Joel Makower, editor of The Green Consumer Letter and The Green Business Letter.

Makower, a Washington-based environmentalist who supplied the figures for this article, said thinking of money saved as a “return on investment” helps incorporate environmental activism into everyday living.

So, screwing on that water-conserving shower head first will set you back around $20. But for homes with gas-heated water, the savings in energy and water bills over conventional shower heads is about $20 a year--a 100% return. With electric heat, you save about $55, or a 275% annual return.

Better yet, consider mining the hidden value in your lunchbox. Chances are it’s carrying America’s newest mealtime staple: the disposable drink box.

They’re nonbreakable, light and equipped with straws. They’re made of layers of plastic, paper and foil--difficult to recycle and costlier than earth-friendly alternatives.

Assume you drink a quart of Hi-C fruit drink a week. At $3.49 for a three-pack of 8.45-ounce boxes (at a Washington Safeway), the cost is around $4.40 a week, or $228.80 for a year’s supply. Your cost for three years of Hi-C: $686.40.

Enter the Servin’ Saver Litterless reusable juice box from Rubbermaid. It’s also nonbreakable, it looks like the disposable kind and holds the same amount. It costs about $1.99, is dishwasher safe and has a lifetime warranty.

You can fill it each day from a 64-ounce bottle of Hi-C made of recyclable plastic. That creates less waste. It costs about $2.09, or $54.34 a year. Including the $1.99 investment in Rubbermaid’s container, your cost over 3 years totals $164.99.

So investing $1.99 saves you $521.41 over three years, or $173.80 a year. That’s an annual return on investment of 8,734%.

For aficionados of exotic investments, here’s a soapless product that claims to clean your clothes.

Tri-Clean Laundry Discs resemble hockey pucks, filled with “electrically active” ceramic beads that help water lift off dirt. They cost $72.50, including shipping and handling. (They’re available via mail order: Schweitzer Enterprises, 834 Spindle Hill Road, Wolcott CT 06716).

The discs could be great news for reducing water pollution and packaging waste. Start with the premise that the investment isn’t akin to some junk bond that promises steep yields but has a high risk of default.

Makower said his editorial staff and family in “highly unscientific blind tests” couldn’t tell the difference between clothes washed with the device vs. detergent.

Say you do around three loads of laundry per week, or 156 loads a year. You usually add bleach once a week and liquid fabric softener twice a week. Total cost by conventional method: $80 a year.

Tri-Clean is supposed to last 1,460 loads, or about nine years and four months. The directions allow for a pinch of Borax to whiten. That’s about $7.80 a year for a total cost of $80.30 the first year, including the $72.50 initial investment.

Thereafter, the disc method costs $7.80 a year. So over the expected life of the discs, the method costs $145.50.

During the same period of roughly nine years, four months, using detergent powder, bleach and liquid fabric softener will set you back up to $745.

The bottom line: a $145.50 investment over nine years, four months saves you up to around $600--for an average annual return of up to 40%.

Environmentally sound products, like any investment, come with caveats. A prospectus detailing risks of light bulbs and shower heads might mention that actual returns can vary with your electric and water bill rates. In New York City, where rates are among the nation’s steepest, returns will be higher. But returns are lower in low-cost regions like the Midwest.

Then there are the less tangible “comfort” factors. On the plus side, the light given off by a modern compact fluorescent bulb is softer than many conventional bulbs, yet it is equally powerful. Some bathers, however, complain that the flow of a water-conserving shower head is too stingy.

Packaged goods manufacturers will be quick to point out that metal thermoses can be heavy and clunky. Even the Rubbermaid juice box may have drawbacks. Many people don’t want to be bothered with filling reusable containers.